Home

NABCEP Study Guide - Brooks Engineering

1. ARTE EE 33 3 6 INSTALLING SUBSYSTEMS AND COMPONENTS THE SITE TASKS 6 1 THRU 6 10 THE NABCEP TASK JANABYSIS erred t Reed A ette en abe tiet eiui Re 34 3 7 PERFORMING A SYSTEM CHECKOUT AND INSPECTION TASKS 7 1 THRU 7 8 OF THE NABCEP TASK ANALYSIS 37 3 8 MAINTAINING AND TROUBLESHOOTING A SYSTEM TASKS 8 1 THRU 8 7 OF THE NABCEP TASK ANALYSIS 40 STUDY GUIDE REVIEW QUESTIONS eese eee eese eese testen 42 4 1 WORKING SAFELY WITH PHOTOVOLTAIC SYSTEMS eene nennen ener ener innen enn 42 4 2 CONDUCTING A SITE ASSESSMENT e P ID n Tr renean 43 4 3 SELECTING A S YSTEM etie eene tropa doeet say Nee E eee pee tuexs 45 4 4 ADAPTING THE MECHANICAL DESIGN rrr n t ber ar ie t t t Pte ree 46 4 5 ADAPTING THE ELECTRICAL DESIGN rr ROT t e RO EHE PH NP 47 4 6 INSTALLING SYSTEMS AND SUBSYSTEMS AT THE SITE eene nennen nennen nennen enne enin 52 4 7 PERFORMING A SYSTEM CHECKOUT AND INSPECTION eese nennen nennen nein enne nennen enn 53 4 8 MAINTAINING AND TROUBLESHOOTING A 54 ANSWER KEY TO STUDY GUIDE REVIEW QUESTIONS recessisse eene 56 61 6 1 IRRADIATION FORMERLY CALLED INSOLATION DATA FOR DENVER 61 Disclaimer I
2. o Voltage Crystalline STC STC STC STC Temperature Dimensions Weight Silicon Coefficient 21 0V 7 2A 17 1V T OA 120W 0 5 66 cm x 142 cm 24 Ib 3 5 1 Wire Fuse Circuit Breaker and Disconnect Sizing The required ampacity of the wiring or conductors from modules to source circuit combiner box generally depends upon the maximum PV module short circuit current from the module Under certain atmospheric conditions it 1s possible for the modules to operate at 125 of the STC short circuit current rating for 3 hours or more The example shown in Figure 7 has a module short circuit rating of 7 2 A Thus the continuous current rating of that module is 125 of the STC short circuit current rating 1 25 x 7 2 9 amps Because conductors for continuous duty operation must be sized at 125 of the continuous duty current the source circuit wiring and overcurrent protection must be sized at an additional 125 of continuous current 11 23 A 1 25 x 9A 11 23 A This is the same as sizing for 156 1 25 x 1 25 of the short circuit current As an example using the specification in Table 2 the required source circuit conductor ampacity is 7 2A x 1 56 11 23 A 3 5 1 1 Temperature and Conduit Fill Corrections for Ampacity of Conductors In almost all cases wiring behind modules will be exposed to elevated temperatures sometimes as high as 75 C The 2005
3. eee ee aaa gy OEE E E EEE AREER 13 eer dees ioo eee 13 3 2 4 Array Mounting Methods eese eene ener ene 15 3 2 9 BOS LOCAUONS ig 15 3 3 SELECTING A SYSTEM DESIGN TASKS 3 1 THRU 3 4 OF THE NABCEP TASK 5 15 3 4 ADAPTING THE MECHANICAL DESIGN TASKS 4 1 AND 4 2 OF THE NABCEP TASK 5 16 3 5 ADAPTING THE ELECTRICAL DESIGN TASKS 5 1 THRU 5 8 OF THE NABCEP TASK 19 3 5 1 Wire Fuse Circuit Breaker and Disconnect Sizing sese 20 3 5 1 1 Temperature and Conduit Fill Corrections for Ampacity of Conductors eee 20 35 L2 Voltage Drop for Circuits ntt eee rr tte te qe ie o o o ec shes Re HIE HE OL R2 21 3 5 2 Charge Controllers and Linear Current Boosters eene reete 24 3 25 2 Batteries Battery Wiring iet e tiet eere reb oet ie gend 27 3 3 4 iecit verre e eee fe ede ose 27 35 9 Pomit of Utility Connection 3 eei vend eee e tee ber ble dte b ont edges 31 3 56 Optional Standby System Panels iti e dete ie endete Hle daro 32 3 9 47 GEOUndIng er at eot Rt ete usta e 32 3 5 6
4. 0 5 the array power 1s further degraded by another 1596 It is also reasonable to assume that there will be a 3 loss in the wiring Calculations Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 38 1 Change all loss figures to consistent numbers For example if a loss of 10 is expected then an efficiency factor of 90 applies to that loss So for a 10 loss the number 0 9 represents the 90 efficiency factor 2 Multiply all the numbers obtained in step 1 together to get the overall efficiency factor expressed as a decimal 3 Multiply the starting amount by the overall efficiency factor to obtain the remaining amount or output The procedure for the example gives the following results e Efficiency factor for module tolerance array mismatch and dust degradation 0 9 e Loss from array temperature degradation 55 25 x 0 005 0 15 which represents a percent loss expressed as a decimal Efficiency factor 0 85 e Efficiency factor from 3 wiring losses is 0 97 e Efficiency factor from 6 inverter losses is 0 94 e The output taking into account all four loss mechanisms 2000 0 9 0 85 0 97 0 94 1395 watts This result is consistent with field observations So a general rule of thumb is that the output of a utility interactive system with no storage can be expected to be about 70 of the sum of the rated module powers on a summer day with ambie
5. The purpose of the LVD 1n the system in Figure 7 is to prevent the batteries from becoming overdischarged This might occur if the array does not provide sufficient daily charge to meet the daily load requirements If more than 80 of full charge is removed from deep discharge lead acid batteries and the batteries are left in that state for several weeks or months the batteries may sustain permanent damage The LVD monitors the battery state of charge SOC and disconnects the loads from the batteries if the battery SOC decreases below the 20 level Some smaller charge controllers incorporate the overcharge and overdischarge functions within a single controller Generally for larger currents separate charge controllers must be used for each function If two charge controllers are used it is possible that they may be the same model but simply installed with different settings for different purposes Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 26 Low voltage disconnects must be capable of handling the maximum load current as well as the maximum system voltage Under normal operation these charge controllers will have the battery voltage as their input voltage but if the batteries are disconnected then the charge controller input voltage may rise to the maximum system voltage defined in 690 7 A It is also possible to employ multiple LVD controllers on the load side of the ba
6. 100 008 092 066 oo Dean _ Sou 97 199 097 060 sEsw 094 095 090 083 059 Ew 083 957 082 075 066 945 rit oo Bean _ South 003 0909 199 006 066 057 sEsw 093 096 096 090 082 057 The above tables were produced using Clean Power Estimator which is available free of charge on the web in several locations e g http www consumerenergycenter org renewable estimator index html Several cities were used to generate these tables providing an average of these slightly different regions The tables illustrate that array orientations near 30 tilt 7 12 pitch roof and facing South produce the most energy on an annual basis for much of the continental U S However many orientations up to Southeast and Southwest produce very close to the same energy within 7 as a perfectly oriented array Compared with the losses often associated with shading that can be much higher the losses due to fairly significant variations in orientation tend to be smaller than many expect When tradeoffs are being made between orientation and aesthetics having this information available can help the prospective owner and installer make the right decision for their situation Even horizontally mounted flat arrays will outperform systems that are mounted facing east or west when seeking to maximize annual energy production These tab
7. Because the bypass breakers must be rated at a minimum of 125 of the inverter output current the inverter bypass switch also serves as the main ac disconnect for the inverter output circuit breakers that can handle inverter output currents should be sized at 12596 of the inverter output current rating The circuit breaker in the main panel provides overcurrent protection from utility supplied currents for the conductors between the main panel and the inverter bypass switch The circuit breakers in the inverter bypass switch provide overcurrent protection from utility supplied currents for the conductors between the bypass switch and the ac input of the inverter and the conductors from the bypass switch to the subpanel Normally these circuit breakers do not trip on faults involving the inverter output because they are sized at 12596 of the inverter output A short circuit on the inverter output generally will cause the inverter to shut down before the circuit breakers trip The input circuit breaker on the subpanels serves only as a disconnect switch for the subpanel 3 5 5 Point of Utility Connection The NEC allows a utility interactive PV system to be connected on either the line side or the load side of a customer s service disconnecting means If the PV system is owned by the utility it will probably be connected on the line side upstream of the meter Line side connections must meet the requirements of NEC 230 82 5 If the customer ow
8. Examining Wire Table 8 in Chapter 9 of the shows that if a 12 AWG copper conductor is adequate for a 3 drop for 24V dc operation then 12Vdc operation will require a 6 AWG conductor for the same load power consumption Hence except for very small systems that require only a few modules a 12V system may not the best choice for low cost and reasonable wire sizes If a system can be configured as a 12V 24V or 48V system it is almost always best to use the 48V configuration Systems operating at higher voltages have fewer source circuits resulting in less wiring and often lower installation costs The installer should always be prepared to answer questions about system selections and the alternative choices This includes knowing the functions of all the components that may be used in either a stand alone or a utility interactive system It is important for the installer to understand how each PV system component works For example the differences between a square wave inverter and a pure sine wave inverter often needs be explained if certain loads malfunction with poor power quality The concepts of electrical and mechanical maximum power tracking should be understood so a mechanical tracking device can be distinguished from a maximum power tracking charge controller or inverter The installer should also know when and where to consult an experienced system designer when design issues extend beyond the installer s capabilities 3 4 Adapting t
9. Typical battery bulk and float voltage set points and equalization process Sealed Gel AGM Maintenance Free Deep Cycle Liquid NiCad or NiFe Battery type Lead Acid Lead Acid RVI Marine Electrolyte Lead Antimony Alkaline Bulk volts 28 2 28 8 28 8 29 2 32 0 Float volts 27 2 26 8 26 8 26 8 29 0 Not Charge to Not Not Equalization recommended 31V Recommended Charge to 31V recommended An additional charging stage provided in many controllers is the equalization stage During the equalization stage the charging voltage is raised above the bulk voltage level for a period of time after the battery is fully charged When this is done all cells are brought up to the highest state of charge that they can accept the battery 1s equalized the cell voltages continue to rise and more of the electrolyte electrolyzes into gases This is typically called gassing The gassing process produces a boiling type action in the electrolyte which helps to scrub the battery plates and destratifies the electrolyte Some charge controllers automatically incorporate an equalization charge at intervals of approximately 30 days With some controllers it is also possible to manually control the equalization stage Note that equalization is not recommended for many sealed or valve regulated lead acid VRLA battery types or for nickel based batteries Alway
10. as long as the installer knows the proper and safe installation procedures Before beginning the installation it is important to get the paperwork in place Agreements with the owner utility and permit agencies must be completed before work begins Whether or not an inspection is required all work should be done in a manner that meets all code requirements and would pass inspection It is important to remember that the last word on Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 34 all installations belongs to the local code official and perhaps the local utility There may be instances where the installation is fully code compliant but the utility has additional requirements for interconnection For example in a utility interactive system the NEC does not require a lockable disconnect at a location accessible to utility personnel but this may be a local utility requirement A system with batteries 1s generally considered an optional standby system by the electrical code NEC 702 and the local fire inspector will need to inspect the battery disconnect NEC 702 8 requires that sign shall be placed at the service entrance equipment that indicates the type and location of on site optional standby power sources utility and code officials who may have an interest in the system must be familiar with and understand the plans for the system before beginning the installation
11. availability an auxiliary electric generator may be incorporated into the system Generators may be used in stand alone systems hybrid or with utility interactive systems with batteries Utility interactive systems that utilize a generator but no batteries are completely separate systems and require separate installations If the utility interactive system has a battery backup the system schematic diagram may look nearly the same as the system of Figure 10 The only difference is that the system likely will use an inverter that has an input for an additional ac source Other system configurations are possible using different equipment such as separate transfer switch or relay The inverter may also be programmable so that it will send a starting signal to the generator whenever the batteries reach a voltage Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 33 considered to be low enough for the generator to take over It may also be possible to control the time at which the generator starts so for example the generator can be kept off when the PV system is providing charging and turned on when charging from the PV system stops When the batteries have reached an adequate state of charge defined by the inverter programming the inverter will shut down the generator Generators used with utility interactive systems that do not have battery backup are not designed to interact with the invert
12. it is useful for the installer to understand each of the three stages The first charging stage is the bulk stage where the charger delivers all available PV current to the batteries During this stage the battery terminal voltage increases as the battery charges When the battery terminal voltage reaches the bulk voltage setting the charging mode changes to the absorption mode During the absorption mode the battery terminal voltage 1s maintained constant at the bulk voltage setting and the charging current gradually decreases as the battery state of charge continues to increase The absorption mode is maintained for a time period called the absorption time which is controlled by an internal timer or a device that monitors battery current in the charge controller At the end of the absorption period the battery terminal voltage is reduced to the float voltage setting which is lower than the bulk voltage setting During this stage the batteries continue to charge at a reduced current level If the load requires current excess PV array current will flow to the load while the batteries provide any additional current requirements Different types of batteries require different bulk and float voltage settings Table 4 shows some sample settings for 24V systems Always check the manufacturer s instructions to verify the proper voltage set points For 12V systems the settings are half as much and for 48V systems the settings are twice as much Table 4
13. 10 Circuit Name Total Current Wire Size V arop V drop Distance kft Module wiring 0 012 12AWG 0 166 V 0 69 94 J box to Combiner 7 amps 8AWG 0 778 0 436 V 1 8296 Combiner to Charge 0 01 2 amps 6AWG 0 491 0 103 V 0 43 Controller Charge Controller to 0 006 21 amps 6AWG 0 491 0 062 V 0 26 disconnect 0 006 6AWG 0 491 0 062 V 0 26 Disconnect to Service 0 05 6 amps 10AWG 1 2 0 36 V 0 3 Panel Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 22 The example in this table is very typical of a well designed 24V PV system It also illustrates where increasing wire size will initially have the most impact in the J box to combiner circuit By increasing this circuit size from 8AWG to 6AWG the voltage drop will reduce by about 0 7 overall However the larger size wire will require the next size larger conduit to accommodate these circuits An overall voltage drop of less than 5 for a 24V system 1s a good target and getting voltage drop below 3 is extremely difficult for these very low voltage systems A 48V system will drop the dc voltage drop impact in half for the same wire sizes shown in the table yielding an overall system voltage drop below 3 This is one of the main reasons why 48V battery based systems are generally recommended over 24V systems Other unavoidable voltage drops not calculated in this table include voltage drops in fuses circuit breakers and switch
14. 2 and d 60 in equation 2 The result is O kft 0 8143 which means 8 AWG copper wire will work in this case The question is whether maximum power conditions will exist If an MPT controller is used see next section then it will track maximum array power whereas if a non MPT controller is used it will not track maximum power so the array will probably Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 57 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 operate at a lower voltage making the nominal system voltage a better choice for calculation of voltage drop in this circuit The temperature at this location is most representative of the average battery temperature See Table 2 of this Guide If the PV bulk setting on the PV charging source e g inverter is lower than the alternate float setting on the backup charge controller the PV charging source will always be the dominant charging source unless it is deactivated as when a utility interactive battery based inverter goes into standby during a utility outage None of the proposed connections would work The first one a would result in overcharging of the 120 Ah batteries The second b would result in even more serious overcharging of the 120 Ah batteries and the third c would also result in serious overcharging of the 120 Ah batteries T
15. 6 9 17 9 Record Lo deg C 31 7 34 4 23 9 18 9 5 6 1 1 6 1 5 8 3 16 1 22 2 31 7 34 4 Record Hi deg C 22 8 24 4 28 9 31 7 35 6 40 40 38 3 36 1 31 7 26 1 23 9 40 HDD Base 18 3C 608 492 448 280 141 39 0 0 80 238 433 586 3344 CDD Base 18 3C 0 0 0 0 6 71 148 113 35 4 0 0 377 Rel Hum percent 55 56 54 50 52 49 48 49 50 49 56 56 52 Wind Spd m s 3 7 3 8 4 1 44 4 1 3 8 3 6 3 5 34 3 4 3 5 3 6 3 8 Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 61 Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification 1 Version 2 July 2004
16. 9 amps Since the 9 28 ampacity factor for the 14 AWG conductor is greater than the 9 amps of continuous currents this conductor meets initial code requirements for ampacity However the fuse protecting the conductors must also be rated at 1 25 times the continuous currents 1 56 Isc which is 11 23 amps and that fuse must provide overcurrent for the conductor under its conditions of use The fuse rating can be rounded up to the next higher standard value 12 amps but this value of fuse will not protect the cable which has a corrected ampacity of only 9 28 amps The 14 AWG THWN conductor therefore is not acceptable due to the minimum size of the overcurrent protection Ifa 14 AWG THWN copper wire is used the 30 C ampacity of the wire is 25A Furthermore the temperature correction factor for 57 C operation is 0 71 The resulting ampacity of the 14 AWG THWN 2 conductor when corrected for temperature and for conduit fill becomes 25x0 71x0 8 14 2 A which is more than adequate to handle the continuous source circuit current 9 amps It can also be appropriately protected with the 12 amp fuse When using conductors with insulation temperature ratings higher than the terminal temperature rating of the connected devices a check must be made to ensure that the conductor temperature during normal operation does not exceed the maximum temperature rating of the terminals of these devices In this case the module terminals are rated at 90 C and t
17. A PV system is partly electronic and partly electrical This sometimes introduces a mix of color code conventions for negative and grounded conductors In electronics black is generally used for negative or ground but in electrical systems the grounded neutral conductor is white or gray PV systems are considered to be electrical systems are covered under the NEC and are required to follow the conventions for color codes The ungrounded conductors will normally be black red or blue the grounded conductor will be white or gray and equipment grounding conductors will be green or bare VEC 250 119 allows white taping only for conductors larger than 6 AWG Conductors sized 6 AWG or smaller are required to be the correct color However NEC 200 6 A 2 states A single conductor sunlight resistant outdoor rated cable used as a grounded conductor in photovoltaic power systems as permitted by 690 31 shall be identified at the time of installation by distinctive white marking at all terminations A common example of this type of cable is USE 2 which is solid black in color but needs additional white markings at all terminations when used as a grounded circuit conductor Proper wiring technique requires that adequate working spaces be allowed for access to all equipment and junction boxes NEC 110 26 specifies clearances for systems having voltages less than 600 V If it appears that a working space depth of 3 feet is not possible and the system voltage
18. NEC also recognizes the fact that conductors installed in conduit exposed to direct sunlight as is common in PV systems can operate at temperatures that are 17 above the ambient temperature 310 10 FPN No 2 This means that a conduit in an outdoor temperature of 40 should actually be sized based on a 57 operating temperature due to the sunlight exposure Suppose the conductors are exposed to 57 C and that 14 AWG THWN with insulation rated at 75 C 1s being considered According to NEC Table 310 16 when THWN wire is operated at 30 C or less its ampacity is 20 A But the correction factor associated with Table 310 16 requires that the ampacity of the wire be derated to 58 of its 30 C value if it is operated at 57 C This reduces the ampacity of the wire to 20 A x 0 58 11 6 A Although it may appear that this ampacity is just enough to satisfy the source circuit ampacity requirement in our source circuit example the conductors pass through conduit so the conduit fill adjustment factor must be applied The conduit between the junction box JB and the source circuit combiner box contains four current carrying conductors According to NEC Table 310 15 B 2 a a further adjustment of 80 is needed for conduit fill This reduces the ampacity of the 14 AWG THWN conductors to 11 6x0 8 9 28 A The ampacity of the conductor after the application of these conditions of use factors must be equal to or greater than the continuous current of
19. and checking the programming of any applicable equipment For utility interactive systems it is most helpful for the system owner to keep a periodic record of kWh produced by the system Any significant deviation from expected values should be cause for checking the system Monthly records are valuable but monthly weather variations are usually much more significant than annual weather variations Consequently customers might be overly concerned about their output after a particularly rainy month if they do not have a means to estimate their performance based on actual weather conditions Most electricians will be familiar with the instrumentation used for measuring ac voltage and current and for measuring resistance There are a few pieces of equipment that a PV system maintenance person should also know how to use These include a dc clamp on ammeter a pyranometer and a hydrometer The dc clamp on ammeter is different from the familiar ac clamp on unit The ac ammeter will not measure dc because it employs a transformer coupling to the magnetic field surrounding a conductor that carries an ac current The dc clamp on ammeter generally employs a Hall Effect sensor to measure the dc current Because the Hall Effect sensor is also sensitive to ac the dc clamp on ammeter will also measure ac current As dc current travels in only one direction the dc meter will indicate or depending upon whether the current 18 flowing in the direction of the
20. any battery on the side of a battery between two batteries The charge controller connected to the PV output circuit of Figure 7 normally requires adjustment for voltage drop battery type maximum input power maximum input current Suppose an alternate series three stage charge controller is available as a backup to a parallel three stage PV charging source such as a utility interactive battery based inverter Suppose also that the PV source 1s intended to be the dominant charging source To ensure that the PV source is the dominant charging source one should ao Fe set the float voltage of the PV charging source higher than the bulk voltage of the alternate controller set the float voltage of the PV charging source lower than the bulk voltage of the alternate controller set the bulk voltage of the PV charging source lower than the float voltage of the alternate controller set the absorption mode time of the PV charging source lower than the absorption time of the alternate controller Four 6 volt 240 Ah batteries manufactured by manufacturer A and four 6 volt 120 Ah batteries manufactured by manufacturer B are available It is acceptable to incorporate all of these batteries into a 12 volt PV battery storage bank under the following circumstances a b If the 240 Ah batteries are connected in 2 series groups and the 120 Ah batteries are connected in 2 series groups and then the four series groups are connected in par
21. applicant might find on a NABCEP test for PV installer certification The sample questions and answers that are provided here have been chosen to represent an overview of the material presented in this Guide and are not intended to cover every aspect of PV system installation or the requirements of codes or by local utilities Questions referring to the VEC and OSHA can be verified by following the requirements of the VEC or OSHA standards Many questions seek verification of the applicant s knowledge about PV systems and associated hardware Select the best answer for each of the following questions Solutions are given at the end of this Guide 4 1 Working Safely with Photovoltaic Systems 1 A fall protection system must be in place for all work done at heights in excess of a 4 feet b 6 feet c 8 feet d 10 feet 2 OSHA rules for personal protection and life saving equipment are found in OSHA Part 1926 a Subpart A b Subpart E c Subpart M d Subpart Q 3 Ifno specific fall protection system is in place according to OSHA it is acceptable to provide fall protection by using a person competent in recognition of fall hazards who a monitors the operation on closed circuit TV and is in contact with the crew by walkie talkie is a part of the work crew who is capable of warning other workers c is not apart of the work crew but who is stationed at the level of the work crew within sight of and speaking distance of the crew who is capa
22. arrow on the probe It is important to know the conventions used for dc current When dc current leaves a positive terminal power is being delivered Thus if the dc ammeter shows that current is leaving the positive terminal of a battery then the battery is discharging If the current is entering the positive terminal then the battery is charging If current 1s leaving the positive terminal of a PV output circuit the PV array 1s delivering power If current is entering the positive terminal of a PV output circuit the PV array is dissipating power A pyranometer or irradiance meter is used to measure sunlight intensity irradiance Pyranometers range from very expensive units that are calibrated in terms of the spectral content of the sun to more modestly priced instruments that use the photovoltaic effect to produce a current that is proportional to the irradiance Usually these instruments have a scale factor that must be used to convert the output of the device to the equivalent irradiance Normally the pyranometer that is used by the PV troubleshooter will be an instrument that measures total irradiance incident on the array from all directions 1 e direct and scattered The pyranometer must be facing the same direction as the array to properly register the irradiance incident on the array If the PV array has multiple orientations the irradiance must be measured for each orientation A hydrometer is used to check the specific gravity
23. box to the source circuit combiner box has been determined the source circuit fuse sizes need to be determined These fuses or circuit breakers both known as overcurrent protective devices OCPD are installed to protect the PV modules and wiring from excessive reverse current flow that can damage cell interconnects and wiring between the individual PV modules The maximum size fuse is specified by the PV module manufacturer and approved as part of the module listing The fuse size marked on the back of the module must be at least 15696 of the STC rated module short circuit current to meet requirements for overcurrent protection Hence for the modules specified in Table 2 the smallest fuse size that could be marked on the back of the module would be approximately 11 A It can be larger if the module manufacturer has tested and listed the module with a larger value The fuse will generally be a dc rated cartridge type fuse that is installed in a finger safe pullout type fuse holder The finger safe holder is necessary as each end of the fuse holder will typically be at a voltage close to the maximum system voltage These fuses are available in 1 amp increments from A to 15 A with other larger sizes as provided for in NEC 240 6 A The next component sizing consideration is the wire size from the source circuit combiner box to the charge controller This is the PV output circuit as defined by the NEC The size of this wire is based upon 156 of
24. calculated Assuming a distance of 10 feet the V arop be calculated using the equation below to be Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 23 2x10fix14A Q 1000ft kft E nop f A TY 100 14596 This voltage drop is high for such a short wire run and as 8 AWG is being used for the wire runs from the junction box to the source circuit combiner box it is recommend that 8 AWG be used between the combiner box and the charge controller The voltage drop over this circuit will then be reduced to 0 9 Note Check the Q kft for 8 AWG stranded copper given in the NEC to verify this result The wire run from the charge controller to the battery circuit should be sized according to the size of the overcurrent protection in the charge controller circuit 30 A Sizing of wire for the rest of the system is based upon 125 of the continuous current drawn by the load The continuous total current is determined by adding all of the individual continuous load currents 1f they are known The worst case would be if all loads were on simultaneously If this can happen then the LVD and the wiring to and from the must be sized accordingly The correct size of wire is to assume the loads to be continuous and to size all wires to 125 of the expected current that will flow in the wire For example if the combined loads add up to 30 A then the wire disconnects fus
25. consideration for roof mounting PV arrays is to determine whether the roof can support the additional load If the roof appears to be bowed or if it will not readily support the weight of the installer it may not be strong enough to support the array structural engineer should be consulted if the roof structure appears to be inadequate to support the PV array Generally houses built since the early 1970 s have been through more rigorous inspection and tend to have more standard roof structures than those built prior to that period If the attic 1s accessible a quick inspection of the type of roof construction is often worthwhile Also the uplift force during windy conditions of a PV array must also be considered as the total uplift force in a strong wind may reach up to 50 pounds per square foot psf or greater This is particularly important with standoff roof mounts A 10 square foot module could impose an uplift load of 500 pounds when the PV system is attached to the roof A panel of four of these modules may impose a load of 2 000 pounds on the mounting structure If the panel is supported by four roof mounts and if forces are distributed equally there would be a 500 pound force attempting to lift each mount from the roof and the roof mount attachment method must be capable of resisting this maximum uplift force Several manufacturers of roof mounting systems provide engineering analysis for their mounting system hardware Without this docum
26. corrosion resistance for aesthetics or for areas with extreme corrosion rates This is different from steel where once oxidation forms it must all be removed to prevent further oxidation Stainless steel is generally too expensive for structural materials even though it is highly resistant to oxidation The combination of corrosion resistant aluminum structural members and stainless steel fasteners is a practical solution to minimizing the cost while maximizing long term structural reliability The installer should pay careful attention to the location of module junction boxes so the lengths of electrical wiring can be minimized and organized into source circuits as needed once modules are mounted Modules are normally installed in groups that produce the desired source circuit voltage The NEC requires that all junction boxes be accessible but allows for temporarily removing modules to access junction boxes The layout of BOS components should be done in a neat and professional manner that provides for convenient access testing and disconnecting of system components If the array is on the roof it is generally preferable to install combiner boxes containing source circuit fuses or circuit breakers in a more accessible location Because most PV modules carry warranties of 20 years or more any other components installed on the roof should be also be capable of operating for 20 years without significant maintenance The BOS layout should minimize dist
27. for the series connection of the four modules may exceed 100 V At 40 C Voc may reach 110 volts Thus it is important to check the inverter input voltage range for maximum power tracking as well as the absolute limits of the inverter input voltage to be sure that the array and inverter are properly matched It is important to install the correct wire sizes at inverter inputs and outputs The sizing process is straightforward as the wire is calculated for 125 of the inverter input or output current at full rated power and as specified in the inverter instruction manual Full rated ac currents are generally close to the full rated power divided by the output voltage for the output current or by the input voltage for the input current For example a 4000 W inverter that has a 120 V output specifies an output current at full rated power of 33 A Dividing the inverter power by 120 gives 4000 120 33 A However dividing the inverter power by 24 the nominal dc voltage gives 4000724 167 A and the manual states that the maximum current is 200 A The discrepancy between the dc value in the manual and our calculated value at the input is an important one The reason the values differ is because the value in the manual is calculated at the minimum inverter input voltage of 22 V and at full power the inverter operates well below peak efficiency near 80 efficiency Article 690 8 A 4 of the NEC requires that for stand alone PV systems the inverter
28. have an area of about 10 ft and will weigh about 30 pounds Each year however sizes of PV modules have been increasing During the decade of the 1990s most modules produced were in the 35 to 75 watt range while modules are now common in the 100 200 watt range with some producing as much as 300 watts and weighing more than 100 pounds Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 6 Other technologies require multiple cells in series and parallel resulting in different voltage and current characteristics for the modules Temperature coefficients for voltage and current output as well as conversion efficiencies differ for each technology Photovoltaic module performance is characterized by its open circuit voltage o short circuit current ls maximum power voltage V mp and maximum power current Imp Figure 1 shows a typical relationship between module current and module voltage for different levels of sunlight irradiance incident on a PV module Sunlight intensity is called Irradiance which is measured in watts per square meter W m In summer when the sun is nearly directly overhead its irradiance at the surface of the Earth at sea level is approximately 1000 W m This irradiance is defined as full or peak sun and it is the standard irradiance for testing and rating PV modules At peak sun conditions roughly 70 of the sun that is incident at
29. input current shall be the continuous inverter input current rating when the inverter is producing rated power at the lowest input voltage Dividing the full rated power by the lowest input voltage and by the efficiency at full rated power 85 results in 4000W 22V 0 85 214 A slightly higher than the value found in the manual The battery conductors must be calculated for 12596 of the full rated current or 267 A 214 A x 1 25 267 A Regardless of the load connected to the inverter the size of the input and output conductors of the inverter must be based on the inverter input and output currents at rated load if the system has batteries Hence even for the 720 W system of Figure 10 if the inverter is rated at 4000 W then the conductors to the input must be sized for 125 of 214 A or 267 A This will also be the rating of the circuit overcurrent protection and disconnect for the battery Atthe output the wire and disconnect needs to be sized for 125 of the rated inverter output current which in this case would be 125 of 33 3 A or 41 62 A The wiring from the PV array is sized according to 156 of the PV output circuit short circuit current In this case the combined short circuit current of the three source circuits is 7 2 A x 3 21 6 A The PV output circuit conductors then must have an ampacity of at least 21 6 A x 1 56 33 7 A Provided that the voltage drop is not a problem 8 AWG conductors with a 40 A dc circuit breaker
30. is less than 60 V dc NEC 110 26 A 1 b allows for smaller working spaces by special permission from the local code enforcement official The installer should be sure that all components used in the system are listed unless listing is not required of a particular part For example modules charge controllers surge protectors fuse holders fuses circuit breakers terminal strips wiring wire connectors ground lugs and essentially all parts of the electrical system must be listed A few exceptions to listing requirements are the array mounting system and the nuts bolts nails and screws that are used to attach components and materials Batteries generally are not listed separately but could be part of an energy storage system that 1s listed Running conductors from a PV array on a residential rooftop to an inverter near the service entrance can be a challenging task Many retrofit installation use electrical metallic tubing EMT exterior to the building to accomplish this circuit run In some retrofit cases and in most new home construction projects it is most aesthetic and easier to run the conductors through the building to the inverter rather than external from the building The 2002 specifically required that the photovoltaic disconnecting means for the inverter be on the roof or at the first point of entry into the building The 2005 NEC provides an exception to this requirement provided that the conductors are placed in a metallic r
31. of 1 3 Even though 14 AWG THWN wire meets the ampacity requirements of the NEC it falls quite short of meeting the voltage drop requirements for system performance If the target is less than 2 from junction box to combiner box what would be the correct conductor size To find the correct conductor size substitute in the Q kft values for other wire sizes until a size is found that will meet the voltage drop requirements Substituting the value for Q kft for 12 AWG stranded copper gives Va 4 62 which is still too high For 10 AWG stranded copper the result is V a4 2 89 and for 8 AWG stranded copper the result is V drop 1 82 which meets the performance requirement This exercise shows how large the conductors must be in 24V systems to carry small amounts of current To achieve overall system voltage drops that are within 3 5 individual circuits must have much lower voltage drops To illustrate the need to keep these voltage drops at reasonable levels the following table Table 3 shows one way of tracking voltage drop to maintain it within appropriate levels Not all systems will have all these different circuits but it becomes easy to see how voltage drops can add up if care is not taken throughout the wire sizing process Using as an example the system shown in Figure 10 of this manual the following table shows how a typical wire sizing exercise would proceed Table 3 Wire Voltage Drop Example Using Diagram From Figure
32. of generation as backup for the PV system The third is utility interactive in which the unit uses grid power to charge batteries if the PV system fails to do so but does not send power from the PV system to the grid while doing so In the utility interactive mode it is also able to send excess PV power to the grid after meeting the needs of the installation site The installer typically must refer to the instruction manuals for the installation operation and maintenance details for these inverters The installer should be familiar with some of the key characteristics of inverters regardless of the inverter being used Inverters are characterized by input voltage and current limits output voltage and current limits waveform type operational modes and power rating Figure 10 shows a schematic diagram of a common utility interactive PV system Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 28 with one particular type of inverter that uses battery backup and an optional standby ac distribution panel Figure 10 1s used to show the required calculations for wire size system ratings fusing and disconnects Most of the same methods for the calculations that were done for the stand alone system regarding wire sizes fuse sizes and disconnect sizes apply for utility interactive systems For the purpose of characterizing the system assume the modules are the same as the modules used in the
33. operation and will likely void the warranty It is best to install a generator designed for stationary stand by applications Grounding requirements are set forth in NEC 250 112 The ampacity of the conductors from the generator terminals to the PV system is governed by NEC 445 13 which sets the conductor size at a minimum of 115 of the generator nameplate current rating unless the generator design and operation will prevent overloading of the conductor In the example the ampacity of the conductors can be 10096 of the generator nameplate current rating The output of the generator must be protected from overloads either by inherent design circuit breakers fuses or other acceptable overcurrent protective means NEC 445 12 A NEC 445 18 requires a disconnect device unless the driving means for the generator can be readily shut down and the generator is not configured to operate in parallel with another source of voltage For PV systems where the generator is operating as a backup generator a disconnect should be used to isolate the generator from the system for repairs of either 3 6 Installing Subsystems and Components at the Site Tasks 6 1 thru 6 10 of the NABCEP Task Analysis Once the system design has been determined the locations for components wire sizing fuse and disconnect sizing established the installer must determine that the system can be installed safely and correctly The actual installation is primarily a psychomotor endeavor
34. option c represents an implementation of a but is not exact wording 9 c instructions from equipment and battery manufacturers 10 b Seeany instructions from manufacturers of storage batteries or equipment that uses storage batteries 11 b Because only December March September and June are shown on the chart it is necessary to estimate where the curves will appear for other months The only answer that fits the estimate albeit perhaps a bit conservative is b The altitude for months between March and September is high enough to avoid shading 12 d The answer is found by first determining the smallest value of a at 8 a m or 4 p m From the chart it is approximately 10 on December 21 Then note that tana 3 d so that d 3 tana 17 feet If you came up with one of the other answers can you find what you did wrong Many of the questions have choices of answers that would be calculated if common errors were made 13 b Theanswer is read directly from the chart Be sure not to confuse altitude with azimuth When the altitude is 20 the azimuth is close to 45 14 d A rack mount generally is pretty well ventilated so overheating is not a problem Normally all voltages will be protected by junction boxes and enclosed electrical equipment and while earthquake damage is a possibility it is less likely in almost all areas than vandalism or storm damage 15 b DC disconnects are available from PV system integr
35. procedure must be implemented to find the ground fault After these four preliminary checks the next step depends upon the specific system If the system is a stand alone system with a charge controller then the next step usually is closing the battery disconnect to connect the batteries to the charge controller output Then close the PV output circuit disconnect to apply PV power to the charge controller Follow the instructions in the charge controller manual for adjustment of the charge controller When the charge controller has been adjusted for the array and the batteries then the loads can be turned on If there is a charge controller to control the current to the loads then this charge controller will also need to be adjusted for the system batteries and loads Once the charge controllers are adjusted the system should be ready for operation If the system is a hybrid stand alone system without an inverter the next step is to check the generator control system Provided that the generator is controlled automatically the control system should start the generator at the proper time and or battery voltage Systems of this type should have an instruction manual that explains any adjustments that need to be made If the system is a hybrid stand alone system with an inverter the inverter will normally control the generator start sequence The first step in setting up this system is to verify that the correct starting voltage is available to the
36. result in the highest performance gain for tracking mounts Article 314 28 is used when the size of any conductors entering the junction box is larger than 6 AWG The conductor ampacity must be the greater of 15696 of Isc OR 125 of Isc after corrections for conditions of use temperature and conduit fill This time because the conductor ampacity at 30 C 1s requested the derating factors for temperature and conduit fill must be applied to 125 of the module current Be sure to divide by each of the factors 0 87 and 0 8 as the 30 ampacity of the conductors will need to be greater than its derated capacity at 45 C Solving equation 2 for O kft using I 7 0 A V 24 V VD 2 and d 60 gives O kft 0 5714 NEC Chapter 9 Table 8 shows that uncoated 6 AWG copper wire has O kft 0 491 and uncoated 8 AWG copper has O kft 0 778 So uncoated 6 AWG copper wire is the smallest size that has O kft lt 0 5714 The maximum system voltage using Table 690 7 is 2x21 0x1 17 49 14 V Calculating the necessary Q kft using V 24 V I 7 A VD 1 5 gives 3 42 which corresponds to 14 AWG copper which has the next lowest value Note that for short distances sometimes the voltage drop calculation will result in a wire that does not have sufficient ampacity to carry the circuit current That is not the case for this example but the check should always be made Using maximum power conditions means using V 34 1 V 7 A VD
37. s default settings But for the utility interactive system that has battery backup and an emergency panel some programming will be required It will be necessary to follow the instructions in the equipment installation manual It is always advisable to take advantage of specific product training provided by equipment manufacturers and dealers Some equipment can be extremely involved to program properly It is critical to fully understand the meaning and importance of each setting as system performance and safety can be dependent on proper programming of equipment A common cause for poor system performance is improper programming of equipment An improperly programmed inverter could result in the utility supplying all the power to the loads with no contribution from the PV system In fact this is why the remainder of the system must be tested once the inverter is programmed and run through any operating modes just to be sure that every part of the system 1s functioning properly The final step 15 to be sure everything is labeled as required by the NEC or utility source circuits should be identified with permanent markings such as wire tabs or other numbering methods Some systems are quite complex and to facilitate system checkout and maintenance it is good practice to label points of connection to correspond with points on the system schematic The labeling process includes providing a complete set of as built drawings photos and schematics
38. so that at any later date workers will know where every component is located The requires a long list of labels including Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 36 3 7 Ground fault protection device Labels and markings applied at a visible location near the device stating that if a ground fault is indicated the normally grounded conductors may be energized and ungrounded Bipolar source and output circuits The equipment must have a label that reads Warning Bipolar Photovoltaic Array Disconnection of neutral or grounded conductors may result in over voltage on array or inverter Single 120 V supply If single phase 3 wire distribution panel is fed with a 120 V PV inverter output by connecting a jumper wire between the panel busbars the panel must have a label that states WARNING SINGLE 120 VOLT SUPPLY DO NOT CONNECT MULTIWIRE BRANCH CIRCUITS System disconnects Each PV disconnecting means in the system shall be permanently marked to identify it as a PV system disconnect Note that disconnects may not be located in bathrooms Energized terminals in open position In cases where both terminals of a device may be energized even if the device is in the open position a warning sign shall be mounted on or adjacent to the disconnecting means that states the following or equivalent WARNING SHOCK HAZARD DO NOT TOUCH TERMINALS TERMINALS ON BOTH THE LI
39. stand alone examples and the specifications are shown in Table 2 Because each module is rated at 120 watts at STC this system may be referred to as a 720 watt utility interactive system Modules Voc Vinp Imp V mp Temp Dimensions Weight STC STC STC STC STC Coefficient Crystalline 21 0 V 72A 17 1V 70A 120W 0 5 C 66 cm x 142 cm 11 kg Silicon Lay mieni Mcr x ae ees i ee eye hex ACA ey T I I l l 1 1 1 1 1 1 OPTIONAL STANDBY LOAD oco ioo oo joo joo OO Buh 3 1 1 1 INVERTER pes JUNCTION BOX DC NEGATIVE AC 7 CHARGE BUS AQIN CONTROLLER N REA 4 Ed cel 1 MAIN AC LOAD COMBINER DCLOAD F 4 Mes ilo Or CENTER CENTER Q GROUNDING BLOCK E TO UTILITY BATTERY BATTERY DISCONNECT Figure 10 Utility interactive PV System with Battery Backup with a UPS option as it has battery backup to supply emergency ac loads The installer must be aware
40. that enable the system designer to calculate the position of the sun at any time of the day any day of the year at any place on the planet While an installer is not expected to know how to use these formulas the installer should know that devices are available for observing the path of the sun at a location for each month of the year These devices can be used to determine whether any obstructions in the vicinity of the array will shade the array during critical sun times at any time during the year A Solar Pathfinder www solarpathfinder com or similar device can be used effectively to estimate the impact of shading at any location Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 10 The location of the sun in terms of North East West or South is determined by the azimuth angle usually symbolized by the Greek letter Psi y When a conventional compass is used North is 0 or 360 East is 90 South is 180 and West is 270 But for calculating sun paths due South is often used as the zero degree reference as the sun is due South at solar noon Note that solar noon is not necessarily the same as 12 p m clock time due to various offsets including daylight savings time simple method to determine solar noon is to find the sunrise and sunset times in a local paper and calculate the midpoint between the two In summary for solar calculations 0 typically represents true South 9
41. the array will be shaded at any time between 9 a m and 3 p m include a None Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 43 b November December and January c September through March d April through August 12 Suppose a PV array consists of three rows of rack mounted modules facing south as shown in the Figure Suppose also that all rows are on a level surface and that the tops of the modules are spaced three feet higher than the bottoms Suppose also that the array is to be used at latitude 30 N In order to avoid any shading of modules from other modules between 8 a m and 4 p m the spacing between rows d must be no less than a 0 53 feet b 4 feet c 12 feet d 17 feet 13 Using the sun path chart of Figure 5 in this Guide the minimum annual sun altitude between the hours of 9 a m and 3 p m sun time is closest to a 10 b 20 c 30 d 45 14 With the PV array rack mounted at ground level which would normally be of greatest concern Overheating of the modules Electrical hazards from exposed conductors open circuit voltages Earthquake stresses on the modules Physical damage to the array and wiring Boc 15 A concern associated with 12 V PV systems that use large wire sizes to minimize voltage drop is ge the difficulty in obtaining dc rated disconnects with adequate current ratings b the possibility of using jun
42. the attic to ensure that it will also be possible to get out of the attic And adequate lighting will be needed in the confines of the attic Before entering an attic one should be sure to drink water for hydration if the attic is hot and if it is expected that the attic work will take more than 15 minutes It is essential when traversing an attic to support one s weight by stepping only on the ceiling joists or trusses There may be 1x2 furring strips for holding rock lath or plaster board but these will not support the full weight of a worker They may be confused with joists or trusses when they are covered with insulation The ceiling material itself will definitely not support the weight of a worker In many cases planking or boards may be placed above joists temporarily to support the weight of workers in attics making it easier to work Care should be taken not to drop or lay heavy tools onto the ceiling material It may crack the ceiling Also care should be exercized when climbing around wires piping air conditioning ductwork and any other attic protrusions such as recessed lighting fixtures The recessed fixtures may be hot and the other items are subject to damage 1f they are overstressed Furthermore wires pipes etc present a tripping or choking hazard to the worker Special care must be made to avoid crushing air conditioning ductwork or existing electrical wiring Caution is essential where protruding roofing nails may catc
43. the short circuit currents from each source circuit In this case the ampacity of the wire needs to be at least 7 2 x 1 56 x 2 22 46 A Once again temperature conduit fill terminal temperature limitations and voltage drop need to be considered Rather than start with a wire ampacity and then apply correction factors it is also acceptable to follow the reverse process This method finds the correction factors applies them to the required ampacity by dividing rather than by multiplying and then finding the appropriate 30 C wire size For this example assuming only two current carrying wires in the conduit from combiner box to charge controller there is no correction for conduit fill In summer even though the combiner box and charge controller will normally be mounted away from direct sunlight the operating temperature can reach 40 C or more if mounted on an outside wall or in a garage If THWN 2 insulation 15 used then a temperature correction factor of 0 82 1s used if the maximum temperature is 45 C The procedure then 15 to divide 22 46 A by 0 82 to get 27 39 A as the required ampacity of the wire Although Table 310 16 would require a 10 AWG THWN copper wire the circuit breaker fuse at the charge controller input needs to be at least 22 46 A It is appropriate to use 10 AWG wire fused at 30 A for this wire run provided that it meets voltage drop constraints The distance from source circuit combiner box to charge controller also must be
44. x0 97x0 95 2522 W 68 4 The correct result is obtained from 4000 x 0 9 x The correct result is obtained from 4000x0 9x0 95x0 97x0 95 3152 W The MPT charge controller increases the inverter output power by 630 W or 25 from the previous example This is the anti islanding feature of the inverter a requirement of UL 1741 listing The other answers are also good practices The point here is that presumably there is a problem This means that it is possible that a switch is bad so that even if it is in the off position maybe it is not off So after turning off all relevant switches to remove power from points where wires are to be attached or removed the voltages and currents should be measured to be sure there are no surprises Even then gloves and a facemask may be appropriate especially on high voltage systems and battery systems when making the connections Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 59 71 c 72 d The first measurement proposed is not meaningful because unless there is a connection between the module leads and the module case for one of the modules in a source circuit the current in all modules will be the same as the modules are in series The voltage at the inverter input will be determined by whether the inverter tracks array maximum power and will be determined primarily by the properly functioning source circuits Measuring the s
45. 0 represents East 180 represents North and 270 or 90 represents West Azimuth angles west of south are typically represented as negative angles and azimuth angles east of south are conversely represented as positive angles The location of the sun in terms of the angle between the sun and the horizon where horizon is defined at the plane tangent to the Earth s surface at the point of measurement is determined by the altitude angle and symbolized by the Greek letter alpha a At sunrise and at sunset when the sun is on the horizon the altitude is 0 If the sun is directly overhead then the altitude is 90 The sun will be directly overhead only in the tropics located between the Tropic of Cancer and Tropic of Capricorn 23 45 north and south of the equator respectively When the sun is up it is at an altitude angle between 0 and 90 It is interesting to note that everywhere on the Earth on the first E day of spring and on the first day of fall equinoxes the sun rises directly in the east and sets directly in the west and 15 above the horizon for exactly 12 hours SS Knowledge of the azimuth and altitude of the sun at any s particular date and time will enable the determination of whether 7 the array will be shaded but it is very difficult to quantify the i effects of shading due to the variables involved degree of E shading placement of shading series parallel connection of c
46. 0 5 A to the battery The fuses have been confirmed to be good and the charge controller is fully operational and indicating a full battery From this information it can be concluded that the standby loads are interfering with the inverter operation the charge controller is set above the inverter set point the utility disconnect is open the charge controller is set below the inverter set point ao FP Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 55 5 Answer Key to Study Guide Review Questions 1 b as required by OSHA Subpart E 2 b reference OSHA Subpart E 3 c C is the best answer because a is not at the level of the workers b may be distracted and d is not at the level of the workers and not in communication with them while they work 4 b module and wiring can withstand short circuit current of the module even if modules are connected in series or parallel Wiring must sized accordingly 5 c Multiply the open circuit voltage by 1 17 Don t divide and be sure to use 20 C correction factor 6 b The GFP is designed to provide fire protection The GFCI protects people from shock 7 c module maximum power voltage decreases at the rate of 0 5 C for module temperatures above 25 Hence at 60 C the module voltage will have decreased by 0 5x 60 25 17 5 resulting in Vinp 14 1 V 8 a See NEC 480 9 A Note that
47. 2 x 6 Spanners Between shown in Figure 6 The 2x6 pairs provide three inches of wood into which a lag Roof Trusses screw can penetrate as well as a relatively large area to ensure that the roof penetration enters the lumber associated with the pair In order to provide proper support for the array the boards must be nailed or screwed securely onto the rafters or trusses with at least two fasteners on each side of each board The withdrawal load is the force required to remove a screw by pulling in line with the screw The pull strength increases as the diameter of the screw increases and is directly proportional to the length of the screw thread imbedded in the wood When a lag screw must pass through a metal L bracket then roof shingles and roof membrane nearly one inch of the length of the screw does not enter rafter or truss Also note that many lag screws in lengths over one inch are not threaded the entire length of the screw Pilot or lead holes must be drilled for lag screws typically in the range of 60 75 of the lag screw shank diameter Larger pilot holes are required for hard woods than for soft woods Note that actual pull strengths will vary depending upon the wood that is used and this is why using safety factors of four or more is not unusual A safety factor of four simply means that 1f withdrawal strength of X pounds is needed then the design requires withdrawal strength of 4X pounds Table 1 shows allowable with
48. 80 W when the modules are operating at 50 C with an irradiance level of 800 W m If three modules are observed to be shaded which conclusion is most likely The inverter input current is probably too low The system is probably functioning properly The inverter is probably not tracking maximum power The modules probably do not have bypass diodes amp Questions 71 and 72 are based on the system schematic diagram of Figure 10 in this Guide Assume that the following specifications apply to the equipment in the system The system is operating in standard utility interactive mode dime ions 41 cm x 157 cm 25m max one way one way Negligible negligible Wire Sizes 10 AWG Cu 6 AWG Cu 10 AWG Cu 8 AWG Cu Rack ground 39 South 39 N 55 C 15 C 30 psf 71 Suppose the irradiance on the array is measured with a handheld solar meter at 955 W m and the input current to the inverter is measured to be 8 35 A The voltage across the top fuse is 0 01 V and the voltage across the bottom fuse is 10 V From this information it can be concluded that the charge controller is in the float phase the top fuse is blown the bottom fuse is blown the battery disconnect is open Bowe 72 Suppose the irradiance on the array is measured with a handheld solar meter at 955 W m and the dc current from the charge controller is measured to be 0 A and the inverter is supplying
49. 90 64 B 5 states that dedicated circuit breakers back fed by listed utility interactive inverters do not need to be individually fastened to the busbar as NEC 408 36 F suggests when a front panel clamps all breakers in a panelboard This new provision recognizes that a listed utility interactive inverter 1s inherently safe 1f the interconnecting breaker is inadvertently removed from the busbar 3 5 6 Optional Standby System Panels If a PV inverter feeds an optional standby system panel covered by NEC Article 702 the panel must be labeled so any maintenance person will know that an inverter feeds the panel If the optional standby system panel is a single phase 3 wire panel that is wired with a jumper between phases to connect the inverter 120Vac output to both busbars of the panel it must be labeled per 690 10 C There is an interesting technicality here in that 690 10 refers to stand alone systems but often pertains to a utility interactive system with battery backup Backup systems will feed the panel as a stand alone system in the event that the grid voltage is lost If both busbars are connected to the same phase then multi wire branch circuits are not allowed Every circuit connected to a 120Vac emergency panel must have its own neutral otherwise currents on shared neutrals will add rather than subtract overloading the neutral conductor The panel label must read WARNING SINGLE 120 VOLT SUPPLY DO NOT CONNECT MULTIWIRE B
50. Center Albuquerque NM 87185 0753 http www sandia gov pv The NRCA Roofing and Waterproofing Manual 5 Edition National Roofing Contractor s Association http shop nrca net pubstore manual asp ProductID 243 Home Power The Hands on Journal of Home Made Power Home Power Inc PO Box 520 Ashland Oregon www homepower com Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 3 3 Photovoltaic PV Installer Study Guide 3 1 Working Safely with Photovoltaic PV Systems Tasks 1 1 thru 1 9 of the NABCEP Task Analysis Working safely with PV systems requires a fundamental understanding of electrical systems coupled with common sense The common sense aspects can be summed up with the following statements If the workplace is cluttered the possibility of tripping over something is significantly increased If the workplace is a sloped roof with clutter the possibility of falling off the roof is significantly increased If tools are left lying out on a roof the chance of them falling off the roof and injuring someone below is increased If the workplace is a rooftop in bright sunshine the chance of sunburn is increased so a good layer of sunscreen is in order There are the usual subtle hazards as well These include nicks cuts and burns from sharp or hot components Gloves should be used when handling anything that might be sharp hot rough or that might splinter Ther
51. NE AND LOAD SIDE MAY BE ENERGIZED IN THE OPEN POSITION Photovoltaic power source A label must be installed near the PV disconnecting means that states 1 the operating current 2 the operating voltage 3 the maximum system voltage and 4 the short circuit current Identification of power sources stand alone systems A permanently installed plaque or directory to be installed outside the building to indicate the locations of all system disconnecting means and to indicate that the structure contains a stand alone electrical power system Identification of power sources facilities with utility connections A permanently installed plaque or directory providing the location of the service disconnecting means and the PV disconnecting means Busbar or conductor connection equipment Any equipment containing an overcurrent device that supplies power to a busbar or conductor shall be marked to indicate the presence of all sources For example if the point of utility connection for an inverter is at the main disconnect device or meter of a building there may be a fused or circuit breaker disconnect near the main disconnect If so it needs to be labeled Performing a System Checkout and Inspection Tasks 7 1 thru 7 8 of the NABCEP Task Analysis The first step in a system checkout after completion of the installation is to visually check the entire system This means checking to see that the modules and all other system components are all bolted d
52. North American Board o NABCEP Certified Energy Practitioners STUDY GUIDE FOR PHOTOVOLTAIC SYSTEM INSTALLERS AND SAMPLE EXAMINATION QUESTIONS PREPARED FOR U S DEPARTMENT OF ENERGY AND SANDIA NATIONAL LABORATORIES REVISION FOR THE SEPTEMBER 24 2005 PV INSTALLER CERTIFICATION EXAM FOR THE NORTH AMERICAN BOARD OF CERTIFIED ENERGY PRACTITIONERS NABCEP HTTP WWW NABCEP ORG 1 2 TABLE OF CONTENTS INTRODUCTION P 1 REFERENCE RESOURCES AND ADDITIONAL READING MATERIALS eeeeeeee eerte entente tn nenne 3 2 1 REFERENCES tn iR RO e RO Ie e EH REDI PE XE DRE Eo 3 2 2 ADDITIONAL READING MATERIAL ten erret stds a a repr teet nep o eee aere e 3 PHOTOVOLTAIC PV INSTALLER STUDY GUIDE e eeeeeee sense 4 3 1 WORKING SAFELY WITH PHOTOVOLTAIC PV SYSTEMS TASKS 1 1 THRU 1 9 OF THE NABCEP TASK ANALYSIS 4 BLT sOSHA Reg lations ettet caet t eductum etre ar test ere dr fete re dat 4 NE Safe in UI ERE 5 3 1 3 Working Space for Electrical Systems 5 3 1 4 How Photovoltaic Systems Work and Associated Safety and Testing Issues see 6 3 2 CONDUCTING A SITE ASSESSMENT TASKS 2 1 THRU 2 10 OF THE NABCEP TASK 10 I2 Sh ding e ecce ets 10 3 22 Jrrdy Ori ntation
53. Performing a System Checkout and Inspection 61 The first step in system checkout after completing the installation 1s test open circuit voltage visually check the entire system install the source circuit fuses close all disconnects 62 Before applying PV power to either an inverter charge controller batteries a load one should first check the polarity of the PV output install the source circuit fuses call the electrical inspector close all disconnects aoe 63 Assume the STC maximum power voltage of a crystalline silicon PV array is 68 4 V If the irradiance is 800 W m and the module temperature is 50 C assuming the inverter is tracking maximum power with a 1 6 voltage drop between modules and inverter input the inverter input voltage should be closest to a 68 4V b 589V c 547V d 471V 64 A 4 kWsrc crystalline silicon PV array is operated in a utility interactive mode with no battery backup The inverter tracks maximum power and the array is operating at 50 C with 900 W m incident on the array There is a 2 power loss in the wiring and the inverter is 94 efficient On a typical PV system the inverter output power will be closest to a 3316 watts b 2985 watts c 2612 watts d 1492 watts Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 53 65 66 67 4 8 68 69 A typical 4 kW crystalline silicon array is
54. Photovoltaic Systems Assistance Center Albuquerque NM 87185 0753 http www sandia gov pv Also available in PDF format only from the Southwest Technology Development Institute http www nmsu edu 7Etdi Photovoltaics Codes Stds P VnecSugPract html A Guide to Photovoltaic System Design and Installation California Energy Commission Consultant Report 500 01 020 June 2001 http www energy ca gov reports 2001 09 04 500 01 020 PDF Battery Service Manual 11 Edition Battery Council International 401 North Michigan Avenue Chicago IL 60611 http www batterycouncil org publications html Installing Photovoltaic Systems Course Manual 2002 Florida Solar Energy Center 1679 Clearlake Road Cocoa FL 32922 5703 http www fsec ucf edu Working Safely with Photovoltaic Systems January 1999 Sandia National Laboratories Photovoltaic Systems Assistance Center Albuquerque NM 87185 0753 http www sandia gov pv Maintenance and Operation of Stand Alone Photovoltaic Systems December 1991 Sandia National Laboratories Photovoltaic Systems Assistance Center Albuquerque NM 87185 0753 http www sandia gov pv Stand Alone Photovoltaic Systems A Handbook of Recommended Design Practices SAND87 7023 Sandia National Laboratories Photovoltaic Systems Assistance Center Albuquerque NM 87185 0753 http www sandia gov pv Hybrid Power Systems Issues and Answers Sandia National Laboratories Photovoltaic Systems Assistance
55. RANCH CIRCUITS 3 5 7 Grounding Proper grounding is an important safety element of a properly installed PV system Grounding for PV systems is covered in NEC 690 V If the maximum system voltage of a PV system is greater than 50V then one conductor must normally be grounded A new provision Article 690 35 in the 2005 NEC now provides details on how to install a compliant ungrounded PV system of any voltage This new provision is likely to cause some changes in the design of PV systems which are likely also to bring changes in installation methods for these systems If the PV system is a bipolar system with both positive and negative array output voltages then the reference center tap conductor must be grounded grounding system consists of equipment grounding conductors a grounding electrode system and a grounding electrode conductor The purpose of an equipment grounding system is to ensure that there is no voltage between any exposed metal parts of a system and ground If a system is properly grounded then under no circumstances will a barefoot person standing on the ground and touching an exposed metal part of the system experience an electrical shock It is essential that if a current carrying conductor of a PV output circuit is grounded that it be grounded at only one point If the PV system is mounted on the roof of a dwelling the ground fault protection system will necessitate that this single bonding point between the grounded
56. a system without the LCB The exact savings depends on the efficiency of the LCB and on the operational I V curve of the pump motor The principle of operation of the LCB is also the principle of operation of the electronic MPT Maximum power tracking is sometimes used in charge controllers to ensure that a maximum amount of available array power 15 used to charge batteries Both types of systems can allow the PV array to operate at maximum power while supplying that power to the pump inverter or batteries at a lower voltage and higher current A MPT charge controller is particularly useful on cold sunny days in the winter when stand alone systems have low battery voltage and the array voltage is high due to the cold operating temperature 3 5 2 2 Charge Controller Operation Because the terminal voltage of batteries is dependent upon the state of charge the type of battery and the temperature charge controllers often incorporate temperature compensation using sensors located at battery The temperature sensor should be attached to the batteries according to the manufacturer s instructions It may be necessary to adjust the temperature compensation rate in the charge controller for the type of battery so the controller will employ the proper temperature compensated voltage Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 25 Because many charge controllers employ 3 stage battery charging
57. ac system so NEC 690 47 A clearly applies to PV systems comprising ac PV modules For systems with both dc and ac it is advisable to use 690 47 as the guideline for the grounding electrode system and the grounding electrode conductor There will generally be both a dc and an ac grounding electrode conductor If separate ac and dc grounding electrodes are used then they should be bonded together However one grounding electrode may be used for both the ac and dc system NEC 690 48 and 690 49 are new provisions in the 2005 NEC that specifically require that jumpers be installed for the equipment and system output grounding conductors if the inverter is removed from the system for any reason These new provisions are intended to keep system maintenance personnel from inadvertently ungrounding equipment when inverters are removed during service work Ungrounding the system for a few minutes or hours does not necessarily create a hazard these provisions ensure that the system will not be left in an ungrounded condition possibly injuring a future worker unaware that the system is ungrounded 3 5 8 Generators The purpose of the generator is to provide electric generation when the PV system and or the grid are not providing adequate energy to meet the needs of the system loads In regions where the available summer peak sun hours is significantly more than the winter peak sun hours or in other areas where it is desirable to have close to 10096 system
58. aceway from the point of entry to the disconnect 690 31 E It is best if the array faces within 45 of true south and should be tilted so the system will perform as required For latitudes other than tropical and for off grid systems that are designed to perform best in the winter then the array should be tilted at an angle of latitude 15 If the array is designed to perform best in the summer then the array needs to be tilted at an angle of latitude 15 For example if a system 15 to be located in San Antonio TX which is at latitude of approximately 30 and if the system 15 to be designed for optimal performance during the winter then the modules should be tilted at 45 If the system is to be installed for optimal annual performance the best tilt is between latitude and latitude 15 depending on how summer dominated the solar resource is in a particular region Photovoltaic systems are often installed for best aesthetics or for minimal wind loading rather than best performance These systems are often mounted on an unshaded roof that is closest to South at whatever angle the roof happens be To maximize the value to the utility the system may be designed to produce maximum output during utility peaking times which may dictate a southwest facing or occasionally a west facing installation For more information about orientation refer to the tables in the orientation section Study Guide for the Applicant for NABCEP Photovol
59. acid batteries are used they should not be located beneath any electronic components The corrosive vapors from the batteries can degrade the circuitry in the electronic equipment causing premature failures Batteries should always be placed in an enclosure that can safely contain the entire battery electrolyte released in the event of a spill or case breakage When using lead acid batteries it is good practice to have baking soda nearby preferably a supply in the battery container so it is readily available to neutralize the acidic battery electrolyte in the event of a spill If nickel cadmium batteries are used the electrolyte is a strong base so vinegar acetic acid should be kept near the batteries to neutralize any spill of the electrolyte Eye face and hand protection should be used when working near batteries and an apron may be used to protect clothing Because of the danger of working with batteries it is important to allow adequate working clearances for battery maintenance Working clearances shall comply with NEC 110 26 as stated in NEC 480 9 C It is especially important to use insulated tools to prevent battery shorts when working around batteries If battery terminals are shorted currents greater than 10 000 A may flow through the short circuit resulting in damage to the tool explosion of the battery and serious injury to the worker Because such high currents can flow the battery overcurrent protection must have an interruptin
60. aded connected to a motor load on the utility side not working ao Fe 44 If the PV array in Figure 10 of this Guide is operated at a minimum temperature of 20 C then the inverter maximum input voltage rating must be at least 29 2 V b 34 3 V c 42 0 V d 49 1 V 45 Assume a 225 A 42 position single phase three wire main distribution panel fed by a 200 A main breaker is used in a dwelling unit The maximum inverter output current that can be fed to this panel is a 20A b 25A c 56A d 70A 46 For the system of Figure 10 assuming that the grounding electrode is a ground rod the size of the grounding electrode conductor is NOT required to be arger than a 10 AWG copper b 8 AWG copper 6 AWG copper d 4 AWG copper 47 The size of the equipment grounding conductor for each of the PV source circuits in the system shown in Figure 10 should be no smaller than a 14 AWG copper b 12 AWG copper c 10 AWG copper d 8AWZG copper Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 50 48 The size of the equipment grounding conductor type THWN 2 for the PV output circuit should be no smaller than a 14 AWG copper b 12 AWG copper c 10 AWG copper d 8 AWG copper 49 If the output of the inverter is connected to the optional standby system panel through a 30 A circuit breaker the appropriate size of the equipment grounding conductor between the inver
61. age which is the PV system open circuit voltage multiplied by the correction factor of NEC Table 690 7 for crystalline modules or by factors supplied by the manufacturers for other types of modules 3 5 2 Charge Controllers and Linear Current Boosters The purpose of Charge Controller 1 in the system in Figure 7 is to prevent the batteries from becoming overcharged This situation might occur if the loads were turned off while the PV modules continued to produce battery charging current The charge controller must monitor the state of charge of the batteries and disconnect the array from the batteries when the batteries become fully charged Usually the charge controller will simply open the circuit between the PV array and the batteries but it is also possible for the charge controller to short the array or to divert the array output to a different load such as an electric water heater or a water pump The attractive feature of the diversion charge controller is that it uses an alternate load for the array output rather than wasting the array output by shorting or opening the array Charge controllers have maximum input voltage and current ratings specified by the manufacturer and the listing agency It is required that the PV array is not capable of generating voltage or current that will exceed the charge controller input voltage and current limits The charge controller rated continuous current sometimes specified as input current sometimes a
62. aic Installer Certification Version 3 August 2005 4 In cases where no specific fall protection system is used OSHA allows a safety monitoring system in which a person can oversee the operation provided that the person is Competent in recognition of fall hazards Capable of warning other workers Operating at the same level as the other workers where the other workers can be seen by the monitor Close enough to the operations to communicate orally and Having no other distracting duties Even when care is taken to ensure that no person falls it is equally important to ensure that no tool or part is dropped If the object is dropped from overhead anyone below is subject to having the object fall on him or her Furthermore there may be scaffolds lumber or any number of other items on a job site capable of inflicting head or other bodily injury For this reason it is important to review Subpart E where OSHA specifies foot protection head protection eye protection face protection and respiratory protection in addition to fall protection practices 3 1 2 Safety in Attics Some PV installations may involve working in attic spaces Working in an attic generally will require wearing a breathing mask eye protection clothing that will protect skin from insulation and will require knowledge of where it is safe to support the weight of a person without risk of falling through the ceiling It will also involve planning the excursion into
63. allel If the 240 Ah batteries are all connected in parallel the 120 Ah batteries are all connected in parallel and then the two parallel sets of batteries are connected in series If each 240 Ah battery is connected in series with a 120 Ah battery and then the four sets are connected in parallel There is no acceptable connection of the batteries into a single battery bank Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 48 36 A good reason for large wire sizes in battery interconnections even if they are oversized for ampacity is to keep all battery currents as equal as possible provide a lower resistance path for battery short circuit currents allow for increases in load size or array size better secure the batteries in case of high winds 37 The purpose of a linear current booster 1s to a keep its output voltage the same as its input voltage and boost the output current to a value larger than the input current convert a high input voltage and low input current to a lower output voltage and a higher output current convert a low input voltage and high input current to a higher input voltage and a lower input current d keep its output current the same as its input current and boost the output voltage to a value larger than the input voltage oc 38 The purpose of an inverter is to convert dc at one voltage to ac at the same or another voltage convert a
64. alling PV systems This Guide is organized in the following manner Reference Resources and Additional Reading Study Guide Sample Examination Questions Answer Key The Guide is intended to provide an overview of each of the major content areas of the above task analysis A set of practice questions that relate to each of the major content areas is then provided The questions are organized according to each content area The answers to the practice questions along with related explanations are given at the end of this Guide The experienced installer may wish to try the questions first Then if any are missed s he can study the answer key to review the methods for solving the problems used by the technical experts who provided input to the authors of this Guide Individuals that are new to these topics likely will benefit most from starting at the beginning of the material and working through to the end answering questions after completing each section but this Guide alone will neither ensure the applicant will meet the requirements for taking the exam nor provide enough code related information to successfully complete the exam The sample examination questions span fundamental trade knowledge codes and standards and accepted industry practice in the relevant design installation and maintenance of PV systems Many questions are based on system installation scenarios requiring the use of schematics diagrams and equipment specifications Gu
65. an azimuth of about 90 W of S and an altitude of about 60 For the array to point at the sun it must face at the azimuth direction and must be tilted so the perpendicular to the array points toward the sun This means that the tilt must be 30 Make a sketch to prove this to yourself 70 of 10 32 is 7 32 because 7 is 70 of 10 Finally an obvious answer Incorrect polarity can severely damage the electronics in a charge controller or an inverter and if applied to batteries can damage the PV cells because the batteries will be supplying power to the PV array rather than vice versa The power supplied to the PV cells by the batteries will be dissipated as heat This problem involves successive applications of temperature and voltage drop factors The temperature coefficient for power for crystalline silicon PV modules is 0 5 C so the temperature losses are 0 875 0 5 C x 50 C 25 C 12 5 or 0 875 With the combination of temperature and voltage drop the answer is V 68 4x0 875x0 984 58 9 V Note that the array maximum power voltage is not dependent upon irradiance at higher irradiances and is neglected in this problem The correct result is obtained by multiplying 4 000 W x 90 irradiance of 900 w m x 90 for module tolerance array mismatch and dust degradation x 0 875 temperature derate of 5 C x 50 C 25 C 12 5 or 875 x 98 wiring loss x 94 inverter inefficiency 2 612 watts answer c 52
66. ance can be used so the resistance at 48 V can be 16 times as much as the resistance needed for the 12 V system and still result in the same percentage voltage drop The configuration of b has the best aspect ratio next is a then d then c So if cooling is the only consideration then b would be best It is conceivable however that the mount of a may be easier to install and that the effect of a may be more aesthetically pleasing The difference in operating temperatures between a and b will be minimal so it 1s possible that a might be chosen as the best overall choice Note also that for both a and b it is more likely that array mounting feet can more readily be attached to roof rafters of trusses as the mounts will run perpendicular to the them Finally for the configuration of b it may be convenient to mount junction boxes on the mounting rails between the two groups of four thus minimizing roof penetrations Care should be taken to avoid shading of the modules by the junction box es The effective thread length that penetrates wood is 2 3 4 so 2 75x281 773 b The total load is four modules x 10 sf module 75 Ib sf 3000 Ib So each bracket supports 300076 500 Ib Then divide 500 by 300 Ib in to get 1 67 of thread that penetrates the wood Salt spray is the most corrosive atmosphere of those listed thus requiring stainless steel to avoid corrosion Higher latitudes and minimal summer cloud cover
67. ances for dc wiring especially if the system operates at 48V or less Tracking mounts can increase the array output by 2096 to 40 over the year Tracking mounts are most cost effective in areas with minimal cloud cover Tracking can be either single axis or double axis Single axis trackers are generally sufficient at latitudes below 40 Double axis trackers generally are more economical when located at latitudes higher than 40 Higher latitudes need both axes because the sun traverses a path that covers a greater range of azimuth angles from sunrise to sunset For example in northern Canada or in Alaska the sun may rise at an azimuth of 160 and set at an azimuth of 160 resulting in a total movement of 320 The solar irradiation sometimes called insolation data for the installation site should be checked carefully to determine whether single axis double axis or no tracking is best for a particular location Another consideration for evaluating tracking as an option is wind loading An array on either a tracking mount or on a rack mount will be subject to a larger wind load than a standoff mounted array making them less attractive for very windy regions Finally initial cost and maintenance costs are key to evaluating the value of a tracking system relative to a fixed system Tracking systems require periodic maintenance if they are to deliver more energy than a fixed system Seasonal variations are another important consideration when
68. ators and manufacturers insulation is rated for dc or ac and if animals are going to chew on wire they most likely will not be fussy about the wire size But if a circuit is listed as having a current of 30 A and if a 60 A disconnect is used it is very likely that the housing of the disconnect and possibly the lugs will be too small to accommodate 1 0 copper wire 16 d Hereone either needs to convert square feet to square meters or square meters to square feet Either conversion yields the same answer Because 1 m equals 10 76 ft 1 square meter of PV will produce 10 76x6 64 6 W The available roof area is 30 m so 30 m of PV will produce 30x64 6 W 1937 W Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 56 17 18 20 21 22 23 24 25 26 27 28 29 30 31 690 5 See 690 7 The correction factor for 25 C is 1 25 so 1 25x504 630 V See NEC 690 7 Note that the maximum system voltage at 25 C is 504 V At 20 C this voltage must be multiplied by 1 17 which results in 589 7 V which is less than 600 At lower temperatures the correction factor is 1 25 which results in a maximum system voltage of 630 V which exceeds the 600 V NEC limit for residential systems Because the ratio of the voltages is 48 12 4 the ratio of the resistances is 47 16 At higher voltages wire with greater resist
69. ble of warning other workers d provides safety instructions to the crew before the crew begins work 4 Temporarily shorting the output terminals of a PV module will destroy the module if the short is not immediately cleared have no effect on the module destroy the insulation on the module wiring if the short is not immediately cleared cause damage only if the module is connected in series with other modules 5 Ifthe open circuit voltage of a crystalline silicon PV array is 315 V at 25 C then according to NEC if the array is operated at 20 C maximum system voltage must be corrected to a 269V b 315V c 369V d 394V Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 42 6 The purpose of the ground fault protection device in a PV system 15 to aoe reduce the probability of electrical shock to service personnel reduce the possibility of fire from an arcing fault to ground reduce losses of the PV output energy to ground reduce degradation of structural supports from rapid electrolysis 7 Ifthe maximum power voltage of a crystalline silicon PV module is 17 1 V at STC then at 60 C module temperature and 1000 W m incident the module the maximum power voltage of the module will be closest to 20 1 V 17 1 V 14 1 V 12 0 V 8 According to the National Electrical Code if flooded lead acid batteries are chosen for energy storage
70. c at one voltage to dc at the same or another voltage convert dc at one voltage to dc at another voltage convert ac at one voltage to dc at another voltage ao Fe 39 Sine wave inverters are required for connection to utility lines because they are more efficient than other types of inverters they are the only inverters that have low enough harmonic distortion only sine wave inverters can be designed to disconnect from the utility when utility power is lost non sine wave inverters cannot develop adequate power for utility interconnection Ross 40 A 2500 W inverter is used to supply a 120 V ac load of 1500 watts This means that the ampacity of the wire at the inverter output must be at least a 125A b 15 6A c 208A d 260A 41 A 2500 W inverter with an input voltage range of 22 V to 32 V has an efficiency of 88 at full output This means the maximum inverter input current at full rating will be c osest to a 129A b 100A c 89A d 69A Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 49 42 If the maximum ac output rating of an inverter with 120 V ac output is 1500 W the rating of the circuit breaker at the point of utility connection should be a 15 b 20A c 25A d 30A 43 If the inverter in a utility interactive PV system begins to hum quietly about five 5 minutes after closing the connection to the utility then it is most likely working overlo
71. circuit conductor and the grounding electrode conductor be in the equipment providing the ground fault protection If a current carrying conductor either positive or negative is connected to the grounding system at more than one point then part of the current that would normally be carried by the grounded conductor Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 32 will be carried by the equipment grounding conductor Equipment grounding conductors are intended to carry current only if a ground fault occurs Under normal operating conditions equipment grounding conductors do not carry current Figure 10 which has no ground fault protection device shows the bonding block at the input of the inverter If the PV array of a utility interactive system is installed on the rooftop of a dwelling unit the ground fault protection GFP device will typically be designed and built into the inverter where it will interrupt the ungrounded conductor and also disconnect the grounding connection between the grounded conductor and the system ground This action typically will occur if approximately one amp is detected to be flowing in the wire that connects the grounded conductor to the grounding block Note There is no NEC requirement for a particular fault current level that must be detected If the PV installation has a battery backup then it will probably have one or more charge controllers When
72. ction boxes terminal blocks or switch boxes that are too small to house the large wire the difficulty in obtaining wire with dc rated insulation d the possibility of animals chewing on the wires 16 If a proposed PV installation site has an unobstructed south facing roof area of 60 m and if thin film modules with six watts per square foot power output at STC are to be installed on 5096 of the roof then the maximum available PV array output power based on the sum of module ratings at STC will be closest to a 4500 watts b 3600 watts c 2250 watts d 1900 watts Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 44 4 3 Selecting a System Design 17 Which of the following devices does the require to be a part of PV systems mounted on residential dwellings A stand off mount for the PV modules A utility interconnection A ground fault protection device An accessible source circuit combiner box aoe 18 A crystalline silicon PV array that has bipolar outputs of 252 V and 252 V with a common grounded conductor under Standard Test Conditions is selected for a large single family residence The lowest expected temperature at the installation site is 25 C For this system the maximum system voltage is closest to a 252V b 315 V c 504 V d 630 V 19 A crystalline silicon PV array that has bipolar outputs of 252 V and 252 V with a commo
73. d inverters in PV systems are subject to UL standards however this only provides a measure of safety of the device and does not indicate field performance or reliability There are relatively few resources to find comprehensive and unbiased analyses on field performance of these products And even so a given product may perform quite well in one set of conditions but under perform in other conditions e g at given temperatures voltages etc Also manufacturer specifications are listed in a laboratory and it is important to recognize that field performance 15 far more dynamic than what is seen on a specification sheet For modules the tolerance of the manufacturing processes i e modules rated as a given number of Watts X is one key parameter Likewise the performance in the temperature conditions expected at your site will be relevant Modules also achieve a lower performance due to voltage degradation over time so understanding the stability of the products is valuable in determining the long term system output Meanwhile system designers should also consider other issues like how well modules endure various environmental effects Inverters are also complex components Inverter performance and reliability is important to good system design since it enhances or limits the effectiveness of a given system Inverters have many more variables than modules Although DC AC conversion efficiency is often the primary factor looked at to determine
74. d these may be interpreted by some maximum power trackers as the maximum power point This typically occurs when one or more modules in an array are shaded or not operating properly causing bypass diodes to conduct The current and voltage available from a PV module are temperature dependent The voltage is much more temperature dependent than is the current for crystalline silicon based modules For crystalline silicon modules as the module temperature increases the module current only increases slightly while the module maximum power voltage V mp decreases by approximately 0 5 per resulting in a maximum power decrease of approximately 0 5 per as shown in Figure 2 Conversely as the temperature of the module decreases the voltage and power increase by approximately 0 5 per Thus crystalline silicon based PV modules achieve their highest voltages at the lowest temperatures Table 690 7 of the 2005 National Electrical Code gives voltage correction factors for crystalline silicon PV modules that are operated at low temperatures Note that the PV array open circuit voltage at the lowest temperature is the maximum system voltage as defined by the NEC Note also that PV modules using other than crystalline silicon may exhibit significantly different temperature characteristics The manufacturer for thin film PV modules provides the module relationships to irradiance and temperature Because the electrical characteristics of PV modules
75. derstand This information is intended primarily as a Study Guide to help individuals better prepare for the NABCEP PV installer examination but does not provide all of the materials needed for completing the certification examination Knowledge of the information presented knowledge of pertinent sections of the National Electrical Code and appropriate experience and qualifications are generally required of those applying for and completing the NABCEP certification process Applicants and certificants are also reminded that local installation codes can differ from national codes and the information presented in this Guide This Guide is based on a task analysis for the PV system installer which includes the following eight major job task areas 1 Working Safely with Photovoltaic Systems 15 2 Conducting a Site Assessment 5 3 Selecting a System Design 5 4 Adapting the Mechanical Design 15 5 Adapting the Electrical Design 20 6 Installing Subsystems and Components at the Site 20 7 Performing a System Checkout and Inspection 10 8 Maintaining and Troubleshooting a System 10 The percentage following each task area represents the approximate emphasis that each topic is given in this Guide and is similar to what would be expected for a typical distribution of emphasis in training programs and on formal testing for certification and or licensure in the trade How this Guide is used may depend upon how much the reader already knows about inst
76. drawal loads for various lag screw sizes driven into the side grain of four common types of kiln dried wood The allowable load accounts for a safety factor of four Table 1 Allowable withdrawal loads for lag screws in seasoned wood pounds per inch of penetration of threaded part Lumber Type Southern Yellow Pine Douglas Fir White Spruce Specific Gravity G 0 58 0 41 0 45 Screw Diameter D in Allowable withdrawal load P Ib in 1 4 281 232 192 5 16 332 274 227 3 8 381 314 260 Adapted from Marks Standard Handbook for Mechanical Engineers atest edition McGraw Hill Withdrawal loads are calculated using the formula P 1800 G D where P withdrawal resistance in lb in D screw shank nominal diameter in inches and specific gravity The minimal wind loading of a PV array occurs when the array is mounted parallel to the roof surface at height of 6 inches or less and at least three feet away from the edges of the roof In regions where wind load may be significant it is desirable to keep the modules away from the edges of the roof as much as possible Most roof structures above cathedral ceilings may require the mounting screws to penetrate a sandwich of foam insulation between two layers of roof decking before the screw will enter a support beam Other cathedral roof structures are built over scissors trusses with the insulation above the ceiling rather than u
77. e 1s always the possibility of dropping tools or materials on either oneself someone else or on sensitive equipment or materials Dropping tools across battery terminals 1s an especially dangerous hazard When a PV system is being assembled it presents the possibility of shock to personnel Improperly installed systems may result in fire should an electrical short circuit occur The surprise elements on the job site cannot be overlooked There is nothing more disconcerting when trying to move to a new position on a roof only to bump into someone who has quietly moved into the space that you had hoped to occupy It can cause a loss of balance and result in a serious fall When working in a potentially dangerous location it is wise to keep a conversation going so workers know exactly where everyone is located 3 1 1 OSHA Regulations All PV installers should be familiar with construction standards established by The Occupational Safety and Health Administration OSHA which are covered in Chapter 29 of the U S Code of Federal Regulations Part 1926 Safety and Health Regulations for Construction Part 1926 consists of 26 subparts labeled from A to Z All parts are important but some parts such as underwater construction refer to situations that will not be encountered with typical PV installations The OSHA rules most relevant to PV installations include Subpart D Occupational Health and Environmental Controls Subpart E Personal Protecti
78. e PV system may be connected using a 70 A breaker so the sum of the main plus the PV breakers is 270 A But because the PV breaker is rated at 125 of the inverter output current the inverter output current can not exceed 56 A See NEC 250 166 C See NEC 690 45 This is a case where there is no ground fault protection so the size must be 125 of the source circuit short circuit current calculated in 690 8 or 1 25 x 7 2 x 1 25 2 11 2 A Even though smaller wire will carry 11 25 A 14 AWG is the best available answer The array short circuit current is 7 2 Ax 3x 1 25 27 A so 125 of 27 A is 33 75 A and 10 AWG THWN 2 is rated at 40 A See NEC Table 250 122 per NEC 690 45 A 30 A breaker requires a 10 AWG equipment grounding conductor Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 58 50 2 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 115 of 42 A is 47 92 A 8 AWG THWN has an ampacity of 50 A All the other ampacities are larger than this The batteries will discharge 40 per day so the generator will need to run four hours per day See NEC 250 122 See NEC 250 119 Note that the conductor may also be green with yellow stripes or bare See NEC 250 119 See NEC 110 26 A 2 See NEC 110 26 A 1 b See text in this Guide See Figure 5 in the Guide On June 21 at 2 30 p m the sun is at
79. e the PV 1000 Wm array produces sufficient current to start the pump A linear current booster LCB 15 a device that converts the high input voltage and low input current into a lower voltage and a higher current so the pump can start at a sunlight intensity level of 75 W m rather than 375 W m for examples and can continue operating until the intensity level drops again to 75 W m Furthermore at intermediate intensity levels some LCBs may continue to track array maximum power A or pump current w So for example at 375 W m rather than operate at point B in 0 Figure 8 as it would without the LCB it will operate at point D which corresponds to the maximum available power at this intensity level As the sunlight intensity increases toward maximum the point on the pump operating curve moves closer to the array maximum power point thus when sunlight is near maximum intensity the LCB improvement in pump performance is minimal The greatest improvement in pump performance occurs at low and intermediate sunlight intensity levels 0 20 40 60 80 or pump voltage V Figure 8 Dependence of array and pump current and voltage on incident sunlight levels Manufacturers of LCBs claim that a pump with a LCB controller will pump at least 20 more water in a day than a pump without the LCB controller What this means is that a LCB will deliver at least 2096 more energy to the load as opposed to
80. e to these connection points 3 5 Adapting the Electrical Design Tasks 5 1 thru 5 8 of the NABCEP Task Analysis Photovoltaic systems may be either stand alone utility interactive or a combination of both types of systems Stand alone DC LOAD CENTER O O O eo 1 1 1 1 LOW VOLTAGE JUNCTION BOX DCNEGATIVE DISCONNECT T CHARGE BUS CONTROLLER 1 1 GROUNDING BLOCK CIRCUIT COMBINER BATTERY DISCONNECT cj cj f e BATTERY Figure 7 A 24 Volt DC Photovoltaic System systems must be a complete independent power system including a PV source usually a storage medium and other electronic components A hybrid stand alone system uses an alternate source of electrical generation such as a motor generator or wind turbine to back up the PV source in the event of any energy shortfalls from the PV source during times of below average irradiation or during excessive load usage A utility interactive PV system uses an inverter to convert the dc power from the PV array to ac power that is compatible with the utility supply The inverter must have all the necessary controls to safely interact with the utili
81. ells and modules etc Devices can be purchased that allow the level user to view potential shading problems and quantify the impact 827 of the shading and provide a reasonable energy loss estimate Sde view Although understanding the sun s position using azimuth and altitude is important calculating the impact of shading is even more important Figure 3 Tree Shading Example For example suppose a tree is located as shown in Figure 3 Suppose also that one wishes to determine whether the tree will shade the array during critical times at any time of the year Because the south row of PV modules is closer to the tree and it is also lower in height it is most susceptible to shading problems Therefore if the bottom row of modules is not shaded then the top row will not be shaded from the same obstruction The critical points to observe are the two bottom corners of the array where the sun s lower elevation combined with the azimuth angle can result in a shadow Remember that some trees may grow a foot or more per year both in height and width so understanding growth rates and the ultimate size of a tree is also important when estimating the shading impact from trees Notice next that the westernmost corner the one closest to the tree will be shaded first and most likely in the afternoon as the position of the sun moves east to west Before solar noon as t
82. en the dimension parallel to the eaves 15 longer than the dimension perpendicular to the eaves In the landscape layout air spends less time under the module before escaping and provides more uniform cooling Modules operate cooler when they are mounted at least 3 inches above the roof A number of pre engineered standoff mounts are available commercially When installed properly engineers or test laboratories certify these mounts to be capable of withstanding specified wind loads If commercial mounts are used verification is necessary to ensure that the instructions indicate wind load compliance which may be needed as documentation for local inspectors Most standoff array mounts are designed so the support rails can be attached to the roof by brackets that can be lag screwed into the rafters or trusses from the roof surface This is the most common method since many roofs do not have access to the underside for other mounting methods It may be difficult to locate the exact center of the rafters or trusses but compared with techniques that require attic access these problems are more minor and can be overcome For roofs with adequate attic access and when it is extremely difficult to mount standoffs from the roof surface it may be necessary to install blocking boards spanners in the attic between the roof trusses Typically a solid anchor between trusses can use pairs of 2x6 boards that are attached between rafters or trusses as Figure 6
83. en the module currents add just as in the case of batteries If higher currents are needed then modules are connected in parallel until the desired current is reached Most PV arrays consist of series parallel arrangements of modules to achieve the desired operating voltage and current Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 7 1000 60S eS 6 0 I lt 750 Wm 4 5 4 5 2 3 0 500 Wm 2 2 2 250 Wim gt 1 5 m 1 5 0 0 l l LS 0 4 8 12 16 20 24 0 4 8 12 16 20 24 Module voltage V Module voltage V Figure 1 Dependence of current and voltage Figure 2 Dependence of current and voltage on incident sunlight levels on temperature for sunlight level of 1000 W m The maximum power voltage and maximum power current are the module voltage and current levels for which the module delivers the maximum possible power for a given irradiance level For each irradiance level there is only one voltage and one current at which maximum power output occurs as illustrated by point A in Figure 1 Many inverters and some charge controllers that are used with PV arrays are designed to electronically track the maximum power point MPP of the array and thus deliver maximum power to the batteries or loads A PV module or PV array may exhibit more than one point where the power curve may have extra peaks an
84. entation local inspectors may require that a custom mounting system have a structural analysis for approval The need for engineering documentation easily justifies the additional costs of purchasing mounting hardware from a mounting system manufacturer The final consideration for roof mounting is the age and condition of the roof If the roof is due for replacement within the next 10 years it typically makes sense to re roof the building before installing the PV system as the array would need to be removed when the roof is replaced Some roof mounting systems developed for the commercial PV market use ballast instead of lag screws into the structure to hold the array in place These systems are engineered for specific wind speeds and for specific roof structures and have very specific stipulations on how to install the array Before recommending any roof mounting system provide detailed information to the mounting system manufacturer to confirm whether or not your specific application is acceptable for the mounting system design Far too often installers make assumptions about the applicability of mounting systems without consulting the manufacturer Also many installers often copy manufactured mounting systems and make the assumption that the self designed system is equivalent to the manufactured product Unfortunately many installers lack the background and experience to properly design and fabricate a mounting system and these structures often det
85. er separate systems as mentioned above The generator either starts automatically when the utility voltage fails or is started manually by the system owner in the event of a utility outage The charging rate for batteries is generally defined by the number of hours that it takes to fully charge the batteries at a constant charging rate The C rate is the charging rate expressed as a fraction of battery capacity For example if C is 100 amp hours and it takes 5 hours to charge the batteries the charging rate is 5 or 20 amps Normally the PV system designer will specify a generator that will fully charge the system batteries in about 10 hours or C 10 This means that 1f the batteries are 80 discharged and the generator is programmed to charge the batteries until they are only 30 discharged that it would take 5 hours to do so at the C 10 rate If this sizing formula is used the maximum daily run time for the generator can be determined if the number of days of battery storage for a specified load 15 known For example suppose the batteries provide two days of storage with no more than an 80 depth of discharge In battery terminology this means that if there is no other electric source the batteries will discharge 80 over the two day period with possibly unequal discharge rates each day If the generator 15 sized to restore 1096 of the battery charge for each hour of operation the generator will need to run for eight hours to make up thi
86. ere multiple micro inverters can be wired in parallel before connecting to the dedicated branch circuit In no case can loads can be connected on the circuit The last requirement is that if more than one source feeds any one busbar and then all sources must be marked This requirement is common sense for safety especially for maintenance The requirement that the point of interconnection be on the line side of a GFCI is necessary because the function of the GFCI can be destroyed by feeding power to the load side once it trips When the point of interconnection is on the line side then the GFCI will continue to provide protection to all circuitry on its load side The second requirement is often the most involved provision and is intended to prevent overloading a busbar If for example a 200A distribution panel is fed by a 200A main breaker then 1f an additional 30A of current from a PV system is fed into the panel the 200A bus of the panel might be subjected to a current of 230A There is no NEC special allowance for the PV source to be connected at the opposite end of the busbar from the main breaker due to the concern that the PV Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 31 breaker could be moved next to the main breaker at a future date creating a potential hazard In commercial and industrial services the busbar may be nearly fully loaded The NEC requires that the sum of a
87. eriorate or fail prematurely These self designed examples are why local jurisdictions often require some engineering documentation to qualify that the mounting system as designed by the manufacturer will keep the PV array on the roof and not pose a hazard to the system owner or those in the immediate vicinity of the array Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 14 3 2 4 Array Mounting Methods Common PV array mounting methods include integral mounting standoff mounting rack mounting and pole mounting Integral mounting is where the modules are integrated into the roofing or exterior of the building itself Standoff mounting is where modules are mounted above and parallel to the roof surface For flat roofs standoff mounting may be used for small arrays Large scale flat roof commercial projects are often accomplished with fully engineered and certified systems and some have no roof penetrations The most common mounting method on a sloped roof is the standoff mounting method as this method provides for air circulation behind the modules to reduce the PV module operating temperature For standoff mounting a spacing of between three and five inches between the modules and the roof is recommended to provide adequate ventilation for the modules Often it is desirable that the array be pole mounted or rack mounted at ground level Once again the usable area available for the ar
88. ers and switches are open and that all fuse holders are empty Then measure the voltage across the circuit where the shorting jumper is to be installed It must be zero If it is not zero then stop Do not install the shorting jumper If it is zero then install a shorting jumper across the load side of the PV output circuit disconnect while the disconnect is open Do not close any disconnects breakers or insert any fuse toward the inverter or the battery while performing these steps Then insert a source circuit fuse in the first source circuit fuse holder and close the associated disconnect Measure the short circuit current then open the disconnect Repeat the procedure by moving the fuse to each source circuit fuse holder measuring the current each time Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 37 3 The disconnect on the PV output circuit should be open and the shorting jumper should be removed All the source circuit fuses should be installed The next step is to measure the open circuit voltage of the PV output circuit It should be close to the lowest individual source circuit voltage measured 4 If the array is on a residential rooftop and the ground fault protection device trips when any of the tests are performed the next step is to make sure there is only one grounding point If there is only one and the ground fault protection still trips then a troubleshooting
89. es and circuit breakers should be sized for 37 5 A If the power ratings of the loads are known the continuous load currents can be estimated by dividing the power by the system voltage Proper wire size from the distribution panel to the loads is also important Load current temperature conduit fill and voltage drop are the factors that need to be considered when sizing branch circuit wiring to the various loads Normally the temperature and conduit fill correction factors will not be needed for branch circuits in residential wiring as NM cable is most common However if the system voltage is 48V or less it is highly likely that branch circuit wiring size will need to be increased to reduce voltage drop The battery disconnect and wire needs to be sized to carry 125 of the maximum of either the load current or the charging current whichever is greatest The battery disconnect must also be a full load rated switch or circuit breaker The battery disconnect must also be a full load rated switch or circuit breaker Any device used in this circuit for overcurrent protection must be able to interrupt any battery short circuit current It 1s important to remember that dc circuits require dc rated components It is NOT acceptable to substitute ac fuses disconnects or circuit breakers for dc applications NEC 690 9 D Some components are rated for both ac and dc The dc voltage ratings of all components should be based upon the maximum system volt
90. es which can add up to 0 5 for a 24V system Additionally charge controllers can cause another 1 4 voltage drop depending on the product If the wiring from the modules to the junction box is exposed the requires the wire must be listed as or marked sunlight resistant A suitable insulation type for this application is USE 2 Even if exposed wiring is used the ampacities of Table 310 16 must still be used if the conductors terminate at equipment PV modules As a final note on voltage drop it is common practice to use smaller wiring between modules and junction boxes and then increase the wire size between the junction box and the string combiner box As the wire size is increased to meet voltage drop requirements then it is important to be sure that lugs or terminals in each of the boxes can accommodate the larger wire size It is required that the box itself be large enough for the wire If wire sizes in junction boxes are 6 AWG and smaller the minimum box size is found from either Table 314 16 A or Table 314 16 B If conductors larger than 6 AWG are in the box then the installation must comply with 300 4 F and the box size should be determined in accordance with NEC 314 28 A Listed PV combiner boxes will have terminals and wire bending space consistent with the current ratings of the device Some will accommodate the larger wires necessary to address voltage drop requirements Once the wire size from the junction
91. et then the smallest wire size between junction box and combiner box that will limit the voltage drop to less than 2 when I is flowing is a 10 AWG copper b 8 AWG copper c 6 AWG copper d 4 AWG copper 29 If the lowest temperature of the PV modules is expected to be 10 F then the maximum system voltage for the PV system of Figure 7 will be closest to a 24 6 V b 34 2 V c 40 0 V d 49 1 V 30 Referring to Figure 7 if the length from the junction box to the circuit combiner is five feet the smallest wire size needed to keep the voltage drop in this circuit less than 1 when the current in the circuit is the maximum power current is a 14 AWG copper b 12 AWG copper c 10 AWG copper d 8 AWG copper Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 47 31 If the distance from the junction box to the combiner box 15 60 feet to keep the voltage drop between the module junction box and the source circuit combiner box less than 2 under maximum power conditions at STC the smallest wire size that can be used for each source circuit in the system in Figure 7 is 32 33 34 35 10 AWG copper 8 6 4 AWG copper The charge controller battery temperature sensor should be connected to ao Fe the side of the battery compartment the top of a battery that is at the end of the row of batteries the bottom of
92. etermining the mounting method for the array Determining where the Balance of system BOS components will be located and Determining how the PV system will interface with the existing electrical system 3 21 Shading A site assessment involves determining whether the location of the PV array will be shaded especially between the hours of 9 a m and 3 p m solar time This is important as the output of PV modules may be significantly impaired by even a small amount of shading on the array Crystalline silicon module outputs are generally more susceptible to shading than thin film module outputs because the thin film cell structure traverses the full length of the module requiring more shading for the same effect For maximizing benefits to utility companies in some utility interactive PV installations it may be desirable for the array to face either southwest or even west provided that the array tilt is below 45 Westerly orientations tend to maximize the array output in the afternoon during utility peak usage hours but do not necessarily maximize the benefit to the customer Some net metering programs offer time differentiate rate structures to encourage the production of energy during utility peak hours A careful assessment using an hourly computer simulation program is necessary to determine the benefits of westerly orientations A minimum of six hours of unshaded operation is still important for best system performance There are formulas
93. for a PV system the battery enclosure must a have provisions for sufficient diffusion and ventilation of the gases from the battery to prevent the accumulation of an explosive mixture and if a tray is used it shall be resistant to deteriorating action by the electrolyte have adequate ventilation at the top only with vent holes screened to keep out bugs and small animals as well as a plastic tray under the batteries to contain spills of electrolyte have adequate ventilation at bottom and top with screened vents and a plastic tray under the batteries to contain spills of electrolyte be sealed and insulated so the battery temperature will not drop below 40 F and have a plastic tray under the batteries to contain spills of electrolyte 9 Ifelectronic equipment is to be housed in a container above the battery container with battery cables passing between the two containers then the batteries should be of what type Flooded lead acid Nickel iron acid Valve regulated lead acid Any type of nickel cadmium 10 If the electrolyte freezes ao Fe the battery should be slowly charged the battery should not be charged the battery should be slowly discharged the battery should be heated with a hair dryer 4 0 Conducting a Site Assessment 11 Using the sun path chart of Figure 5 in the this Guide assuming that if the altitude 15 less than 30 when the azimuth is 45 that the array will be shaded the months of the year when
94. g rating sufficient to interrupt the battery short circuit current Battery circuits should nearly always be equipped with charge controllers for overcharge protection If a battery is overcharged its life is generally reduced Furthermore severe overcharging may cause a dangerous pressure build up in sealed units and will cause vented units to release gases at a much higher rate The gases come from the electrolysis of the electrolyte which also causes the electrolyte level to decrease If the level drops below the top of the lead plates the tops Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 9 of the plates will be degraded If a lead acid battery is over discharged its lifetime is also significantly decreased Some systems include overdischarge protection to prevent battery damage resulting from prolonged discharge The NEC requires that 1f the total nominal voltage of a battery bank exceeds 48 V then the batteries shall not be installed in conductive cases The term nominal is used in the NEC and assumes 2 volts per cell for lead acid batteries Note that 48 V nominal battery banks exceed the 50 V limit for allowed ungrounded PV systems in NEC 690 41 This mix of terminology is often confusing and requires a thorough understanding for interpreting the NEC If the batteries are mounted on metal racks the metal of the rack must not come closer than six 6 inches to the battery term
95. generator The next step for this system will be to adjust the inverter generator starting and stopping sequence for the desired operation of the generator The inverter will also need to be programmed for battery type and desired battery charging and discharge limits All programming is normally done after the inverter is connected to the batteries so the inverter control electronics will be powered up If the system is a utility interactive system with no batteries the next step is usually to connect the inverter to the PV array and the utility source Connection sequence is not critical for most inverters since either or both of these power sources may be turned off and on in a random sequence during normal operation Once this is done the inverter should remain off for at least five minutes with the input voltage close to its open circuit value After the inverter has determined that the utility voltage is stable it should turn on and begin delivering power to the utility grid This can be determined by watching the revenue meter turn backwards or by observing the inverter input current and voltage Normally the input current and voltage will quickly converge to their maximum power values and the inverter output current will reach a maximum value If the system is a utility interactive system with battery backup it will probably incorporate a sophisticated inverter that requires a long list of software checks After the software checks have been com
96. h such as the one shown in Figure 5 is to be used to determine whether the array will be shaded then the worst case values for the altitude and azimuth must be measured for the site T Because the tree will likely be higher in the center as 3ft 38 shown in the side view of Figure 3 the altitude angle to the highest part of the tree needs to be measured from each of the lower corners of where the proposed array will K d 3 d 3 be installed Even if a sun position measuring device is used the sun position will need to be measured from these two points to check for shading from the tree Figure 4 Example showing inter row shading of modules When the worst case altitude and azimuth angles corresponding to a shading problem have been measured they are then compared with the position of the sun on a sun position chart for the latitude of the installation The chart in Figure 5 is for latitude 30 N The chart indicates that on the first day of winter December 21 the sun rises at about 7 a m sun time and sets at about 5 p m Solar noon is when the sun is directly south and highest in the sky for that day Notice that on December 21 the highest sun altitude is about 37 at noon On March 21 and September 21 the first days of spring and fall the sun rises at 6 a m at an azimuth of 90 and the highest sun altitude is 60 at noon On June 21 the first day of summer the sun rises at about 5 a m reaches a maximum a
97. h on clothing or cause lacerations and punctures to the worker 3 1 3 Working Space for Electrical Systems The National Electrical Code is very specific on the working spaces that must be accommodated for maintenance personnel to operate on equipment safely Article 110 26 covers the requirements relative to working spaces Rather than restate all the requirements in 110 26 this guide 1s highlighting the fact that knowing the requirements for proper working spaces 15 essential to preserve the safety and accessibility of an electrical working space Proper working clearances are the first priority when locating balance of system hardware for a PV system Generally clearances are 3 feet but several qualifiers determine the appropriate clearance to use Voltages from 150 600V require greater clearances if live parts are on one side and grounded parts on the other or if live parts are on both sides of the working space The width of working spaces must be the width of the equipment or 30 inches whichever is wider For dc voltages less than 60 V smaller working spaces may be permitted by special permission of the Although this is allowed in the code permission must be secured prior to mounting equipment should smaller clearances be sought Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 5 314 How Photovoltaic Systems Work and Associated Safety and Testing Issues Both the modules an
98. hat are often incompatible with this waveform include light dimmers laser printers chargers for cordless tools smoke detectors and the electric 1gniter circuits on some gas ranges Modified square wave inverters are not suitable for connection to the utility grid even though the inverter may be used as a replacement for the utility when the inverter is used in a UPS application The ideal ac waveform is the sine wave shown in Figure 9c A sine wave inverter is suitable for either stand alone or utility interactive applications Stand alone inverters that have a sine wave output are suitable for all loads within their power and current ratings They have low harmonic current and voltage distortion An inverter is considered utility interactive provided that it meets the requirements of IEEE Standard 929 2000 and is listed to UL 1741 These standards ensure that the inverter output waveform has less than 596 total harmonic distortion and that the inverter will disconnect from the grid if grid power 1s lost Once disconnected the inverter will continue to sample the grid voltage After the grid voltage has again stabilized and after a required five minute delay the inverter will reconnect to the grid and deliver power from the PV system Some sine wave inverters are capable of multiple modes of operation The first of these modes is stand alone in which the inverter operates independent from the utility grid The second is hybrid which uses another form
99. he fuse terminals are rated at 60 or 75 The ampacity of the 14 AWG conductor taken from the 75 or 60 insulation column in NEC Table 310 16 is 20 amps The continuous current in this circuit is only 9 amps so it is assured that the 14 AWG conductor will operate at temperatures well below 60 C at the fuse terminals if the terminals are in an ambient Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 20 temperature of 30 C If the fuse terminals are in a 40 environment similar to conduit not exposed to sunlight the maximum allowable current must be corrected by the 40 C correction factor of 0 82 or 16 4 amps 0 82 x 20 A 16 4 A Fortunately the maximum continuous current is only 9 amps which is well below the maximum of 16 4 amps If the terminals are in a box on the roof exposed to direct sunlight they would have to be rated for 75 minimum in order to not overheat on a hot sunny day since the temperatures could reach 57 similar to the conduit 3 5 1 2 Voltage Drop for Circuits In the above example before purchasing a roll of 14 AWG THWN 2 it is also necessary to check for voltage drop Because each installed watt of PV costs approximately 5 00 it is wasteful to dissipate energy to heat wires when the cost of larger wires is usually minimal compared with the cost of PV modules Voltage drop is often the determining factor in wire sizing particularly for s
100. he Mechanical Design Tasks 4 1 and 4 2 of the NABCEP Task Analysis Once the PV system components have been selected the installer must decide how best to install all the parts so the system will be safe perform as advertised and look pleasing aesthetically If the chosen design calls for installation on a sloped roof the mounts need to be fastened solidly to the roof trusses or rafters not to the roof decking Depending upon the type of roof the mounts need to be attached in a manner that will ensure that the roof will not leak at the roof penetrations Other methods may be allowed with engineered systems that have been certified by an accredited organization Manufacturers of commercially available roof mounting systems provide instructions for attachment to many types of roofs Handling and mounting the modules must ensure that the module edges are not chipped or impacted Small chips or nicks in the glass result in high stress points where cracks can begin with the expansion contraction associated with temperature Torque values given for compression types of PV mounts must be followed PV Module layout is important for aesthetics and to assist in cooling the modules A landscape horizontal layout may have a slight benefit over a portrait vertical layout when considering the passive cooling of the modules Landscape is Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 16 wh
101. he sun moves toward the west the altitude increases until the sun appears due south As the sun moves farther to the west its altitude decreases The altitude and the azimuth must both be considered In this example the array may be shaded for a short time in the morning then be fully illuminated around solar noon and then be shaded later in the day as the sun s altitude decreases Inter row shading refers to the situation where one row of modules shades an adjacent row of modules A six inch shadow from an adjacent module is capable of shutting down a whole section of modules and can even shut down the entire PV system down A simple rule for minimum spacing between rows is to allow a space equal to three times the height of the top of the adjacent module The example in Figure 4 suggests that a separation distance d should be nine feet since the height of the adjacent row is three feet above the front of the next row In the southern half of the United States a closer spacing may be possible However even in the lowest latitudes the spacing should not be less than two times the height of Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 11 the top of the adjacent module In most cases it is better to take the orientation penalty of using a lesser tilt angle in order to prevent inter row shading than it 15 to take the penalty of the shading loss If a chart of sun altitude vs sun azimut
102. his answer is consistent with the preceding discussion The others are not This is the normal operating mode of a LCB when used with a pump A MPT will also convert a low input voltage and a high input current into a higher output voltage and a lower output current if required to supply maximum power to the load See text in this Guide Harmonic distortion relates to power quality The lower the distortion the higher the power quality UL Standard 1741 establishes an upper limit of 5 for harmonic distortion The wire must be sized at 12596 of the inverter rated output current The inverter output current can be determined by dividing the inverter rated power by the rated output voltage which in this case is 2500 120 20 8 A and 125 of this value is 26 0 A The maximum input current is the rated power divided by the lowest input voltage divided by the efficiency which gives 2500 22 0 88 129 A The rated output current is 1500 120 12 5 A So 125 of 12 5 is 15 6 A Hence the next higher circuit breaker i e 20 A should be used When a utility interactive system in the sell mode is connected to the utility there is usually about a five minute time delay during which the inverter establishes that the utility voltage 15 stable The STC open circuit voltage 15 42 0 V so this must be multiplied by 1 17 per 690 7 to get 49 1 V The bus rating is 225 A but the main breaker is only 200 120 of 225 A is 270 A Thus th
103. hoosing the appropriate components for a system it is best to get input from experienced users on the products you may specify prior to installing them 3 1 4 1 Photovoltaic Modules Photovoltaic modules generate electricity as a result of sunlight shining on the PV cells and these cells may be made of any of several technologies Some of these technologies used in flat plate modules as opposed to concentrator modules include crystalline silicon cells multi crystalline silicon cells thin film amorphous silicon cells deposited on a substrate thin film cadmium telluride CdTe cells deposited on glass or other suitable substrate thin film copper indium diselenide sometimes referred to as CIS deposited on glass or other suitable substrate or other technologies now being developed Using crystalline technology for example where individual cells produce dc voltages of approximately 0 5 volt and dc currents in the range of one to eight amps with the amperage depending on the surface area of the cell takes a large number of cells to produce appreciable amounts of voltage and power Usually PV cells are grouped into series strings of 36 or 72 cells to produce open circuit voltages of approximately 20 or 40 V though these configurations change with manufacturing trends 120 watt PV module made of crystalline silicon PV cells encapsulated between layers of glass antireflective coating and an encapsulant material in an aluminum frame will typically
104. hort circuit current does not single out any one module Hence the last test is the only one that may yield useful information It is likely that the voltage across one of the modules will be significantly different from the voltages across the other modules If this is the case it indicates a problem with the module such as a high resistance in a portion of the module causing the bypass diode to conduct or a shorted bypass diode When this happens the string has a lower maximum power voltage than the other strings causing the string to operate well above the maximum power point of the individual string indicated by the low operating current The problem does not say how much the three modules are shaded but for crystalline silicon modules a little shading goes a long way toward reducing module power output When this happens the rest of the modules try to generate full rated current so the bypass diodes of the shaded modules begin to conduct thus reducing the voltage across the modules and providing an alternate path for the current from the other modules So in the worst case the STC power rating of the unshaded modules will be 1200 W But then at 50 there 15 a 12 5 degradation along with the module mismatch and dust degradation of 10 and then the compensation factor of 0 8 to account for the sun irradiance of 800 W m instead of 1000 W m Then there is probably 3 wiring loss and 5 inverter loss So the inverter output power should be app
105. htly above 0 5 1 0V higher the inverter set voltage Also both inverter and charge controller must have temperature compensation probes that are measuring battery temperature to properly set the charge voltage It also may be possible that the PV array disconnect is open or the temperature compensation probe on the inverter and or charge controller has failed but these are not supplied answers Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 60 6 Appendix 6 Irradiation Formerly called Insolation Data for Denver CO Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Average Minimum Maximum Element Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year Temp deg C 1 3 0 8 3 9 9 14 19 4 23 1 21 9 16 8 10 8 3 9 0 6 10 2 Daily Min deg C 8 8 6 6 3 4 14 6 4 11 3 14 8 13 8 8 7 24 3 7 8 1 2 3 Daily Max deg C 6 2 8 1 11 2 16 6 21 6 27 4 31 2 29 9 24 9 19 1 11 4
106. idelines for determining solutions to the questions may be found in the text of this Guide or from the references listed below The suggested reading materials are also listed to broaden resources for the applicant but have not been validated by legal entities or checked for accuracy by NABCEP In addition to the specific content areas listed above knowledge and skills in the following content areas are required for successful completion of the examination Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 1 e Reading and interpreting plans and specifications e Reading and interpreting codes and standards e Using basic mathematics and some trigonometry addition subtraction multiplication division calculations of area and volume fractions decimals percentages calculating the sides of triangles square roots powers of numbers and solving simple algebraic equations for unknown variables One does not need to understand algebra or calculus to be able to engage in safe work practices around PV systems but it is useful to better understand how PV systems work to understand how to calculate expected system performance and to evaluate the results of the system operation Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 2 2 Reference Resources and Additional Reading Ma
107. inals The exception to this rule is for Valve Regulated Lead Acid VLRA batteries or sealed units that use steel cases NEC 690 71 D G requirements state that if batteries are connected in series to produce more than 48 V nominal then the batteries must be connected in a manner that allows the series strings of batteries to be separated into strings of 48 V or less for maintenance purposes The means of disconnect may be non load break bolted or plug in disconnects For strings in excess of 48 V there must also be a means of disconnecting the grounded circuit conductors of all strings under maintenance without disconnecting the grounded conductors of operational strings If batteries are used outdoors in cold climates the electrolyte freezing temperature increases as the batteries discharge If the electrolyte freezes DO NOT CHARGE THE BATTERY Let it thaw slowly in a place where if it should rupture the electrolyte will be contained Do not attempt to accelerate the thawing process with anything that might be capable of igniting any gases that may be liberated 3 2 Conducting a Site Assessment Tasks 2 1 thru 2 10 of the NABCEP Task Analysis A PV installer needs to know how to determine whether a proposed site for a PV installation will be adequate for proper operation of the system A site assessment involves Determining whether the array can operate without being shaded during critical times Determining the location of the array D
108. ked to ensure that the batteries have not been connected with different lengths of cable that will result in uneven charge and discharge of batteries The installer also needs to be able to verify and demonstrate the complete functionality and performance of a system including start up shutdown normal operation and emergency bypass operation As there are many different system configurations each will have its own functionality and performance parameters Because mechanical problems are generally evident by something being loose or bent or broken or corroded they can generally be found with a visual check Note that for further information on troubleshooting essentially every equipment installation manual has a section on troubleshooting with a table that lists problems probable causes and test procedures The reader is encouraged to review all equipment manuals to gain a familiarity with the sorts of problems that might occur with the system and the procedures that need to be followed to confirm the problem and implement the solution Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 41 4 Study Guide Review Questions The following questions were developed from the reference materials listed in this Guide and from other supporting material used by professionals in electrical mechanical and PV installations These sample questions provide insights into the types of questions an
109. ld be better Figure 11 Determination of tilt angle for rack mount on a sloped roof Some equipment such as inverters can be VERY HEAVY and it may be necessary to reinforce a wall with an extra piece of plywood before mounting the equipment Pay careful attention to the manufacturer s instructions If anchors are used in concrete or drywall they must be installed according to instructions The voltage current and polarity from the PV modules should be methodically checked as each disconnect is closed in turn These measurements must be checked prior to any programming that may be performed After all the equipment is installed the next step often is to program the charge controllers and inverters for the desired operating modes Inverters and charge controllers nearly always come from the factory with programs in a default mode However the default mode is rarely the best operating mode as there are so many different operating modes for some systems For the simple dc stand alone system if the system has a charge controller it will need to be adjusted for the type of battery used in the system and the setting calibrated with a reliable digital multimeter If the system has a discharge controller it likely will also need to be adjusted for the battery type For the simple utility interactive system in the sell mode with no battery back up or emergency panels the inverter will often operate as a plug and play unit with the manufacturer
110. le of thumb in conjunction with the irradiance factor 0 7x0 7x2000W 980W A good estimate for summer operation in most areas is to first take 70 of the STC rated array power and then multiply by a factor that expresses the actual sunlight intensity on the array as a fraction of 1000 W m Array temperature although important does not vary significantly enough on a daily basis to adjust the array output in light of all the other losses involved in the system performance For a utility interactive system with battery backup the calculation of expected voltages currents and powers is somewhat more complicated The difference between a battery backup system and a system without batteries is that the PV array does not operate at its maximum power voltage unless a MPT charger is used and the battery requires constant charging to remain fully charged The output expectations of this system can be reasonably estimated if the system is programmed in the sell mode if the batteries are fully charged and if charge controller and inverter settings allow for full PV current to enter the inverter when the batteries are fully charged Typical battery based systems will lose 6 0 94 for maximum power tracking errors and about 4 0 96 for additional inverter losses when considering instantaneous power These additional losses result in a system factor of 0 63 for a battery based system instead of the 0 7 used for non battery systems Battery systems with ma
111. les only consider annual energy production and do not take into account rate structures that vary based on time of day 3 23 3 Array Location Arrays can be mounted on roofs racks and poles The installer needs to determine or verify which method is best for the location of the installation Roofs are popular locations for PV array installations Roof mounted arrays provide protection for the modules from many forms of physical damage Additionally rooftops usually provide better sun exposure and installations do not occupy space on the ground that might be needed for other purposes Several disadvantages of roof installations are that they require lifting all of the modules mounting materials and wiring materials to the roof they Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 13 present a falling hazard they are susceptible to leakage at the attachment points and they need to be removed and replaced occasionally when the roof needs to be repaired or replaced If a roof is selected for the array location then it is necessary to determine whether the roof is large enough for the proposed number of PV modules Sometimes when a roof has non rectangular shapes it is a challenge to determine the amount of useful roof area When laying out a plan for mounting modules on a roof access to the modules must be provided in case system maintenance 15 needed For easiest maintenance acces
112. ll ampere ratings of overcurrent devices that supply current to the busbar should not exceed the busbar rating In dwellings where full loading of the busbar is very rare the NEC allows the sum of overcurrent devices connecting multiple sources to a single busbar to equal 120 of the busbar rating The 120 allowance is somewhat subtle and should be looked at carefully It states that the sum is of the overcurrent devices NOT the sum of currents This means that if a 200A busbar is fed by a 200A main circuit breaker then a circuit breaker connecting a PV source may not exceed 40A which is 2096 of the busbar rating But this also means that the output current of the inverter may not exceed 32 as the circuit breaker at the point of utility connection is sized at 125 of the inverter rated output current and 32A x 1 25 40A Note however that this current may be at 120V or at 240V as at 240V 32A is added to each busbar while at 120V 32A is added to only one busbar If the current is at 240V the power rating of the inverter can be twice as large The other possibility is two inverters one connected to each busbar through a double pole breaker to ensure that both busbars are used An issue that that is clarified in the 2005 NEC addresses the additional fastening means requirement in NEC 408 36 F for back fed devices which is intended to protect against inadvertent removal of back fed energized breakers from a busbar The new provision in NEC 6
113. ltitude of about 83 and sets at about 7 p m sun time At 9 a m on June 21 the azimuth is approximately 95 slightly north of east and the altitude is approximately 49 about half way between the horizon and directly overhead 90 Altitude degrees 6 120 90 60 30 0 30 60 90 120 4 Bs Azimuth degrees West Figure 5 Seasonal sun position at a latitude of 30 N If the installation happens to be at 30 N and if the worst case altitude is 45 and worst case azimuth is 45 then according to the chart the array will have some shading from the tree in winter but will not be shaded between about March 1 and October 1 It is suggested the reader verify the observations and thoroughly understand how to read the chart If an inclinometer or transit or other measuring device is not available then a ruler a straight stick a level and trigonometry can be used to determine the angles Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 12 3 2 2 Array Orientation Next to shading orientation of the PV array is one of the more important aspects of the site assessment Fully understanding what the orientation will be at construction must be understood very early in the project Often the roof tilt is used as the orientation of a residential rooftop system due to the improved aesthetics of a parallel standoff roof moun
114. n grounded conductor under Standard Test Conditions STC is selected for a large single family dwelling The lowest allowable ambient temperature in which the system can be installed at a single family dwelling is a 0 C b 10 C c 20 C d 40 C 20 A PV system is to be selected for operating a PV water pumping system The pump will require 300 W of PV modules for proper operation A 12 Vas model and a 48 V4 model are available If both pumps operate at the same power level the resistance of the wire to the 48 V pump compared to the resistance of the wire to the 12 V pump assuming the same percentage voltage drop in the wiring may be 1 16 as much 1 4 as much 4 times as much 16 times as much ao Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 45 44 21 22 23 24 25 Adapting the Mechanical Design Assume a roof mounted PV array is to consist of two source circuits of four modules each Assume the drawings are to scale and that the roof is large enough d for any of the configurations shown Which of the following configurations can be expected to result in the coolest operation of the modules A x3 lag screw that has a 3 thread is used to attach an L bracke
115. nals connected to the optional standby ac distribution panel the inverter should not supply power to either set of ac terminals the inverter should no longer take dc power from the batteries aoe Maintaining and Troubleshooting a System When connecting and disconnecting wires while troubleshooting a PV system the best way to avoid electrical shock is to inspect all questionable terminals wear rubber gloves and turn off all switches keep one hand behind your back with all switches turned off and only touch grounded surfaces turn off switches measure voltages and currents and wear protective equipment wear shoes with soft rubber soles turn off all switches and don t touch metal surfaces If the current one source circuit is significantly lower than the currents in the remaining source circuits of a PV array and all modules are in full sun then without disconnecting any conductors an appropriate follow up test is measure the individual module currents in this source circuit measure the voltage at the inverter input measure the short circuit current of this source circuit measure the individual module voltages in this source circuit Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 54 70 A utility interactive PV system with no batteries consists of 15 100 W modules in series that feed a 1500 W inverter The inverter output power is found to be 7
116. nder the roof decking If there is any uncertainty over the roof composition roof loads uplift loads or roof materials the installer should consult with a structural engineer professional roofer or building contractor Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 17 Materials used for mounting hardware must be suitable for the ambient environment and compatible with other materials that may be in contact In dry areas such as Southwestern United States a plated steel screw or bolt may not degrade appreciably with time In other areas such as Florida it is essential that most mounting hardware be corrosion resistant stainless steel Manufacturers of commercial array mounts and racks generally supply the mounts with stainless steel hardware to be sure it will be adequate for specified installation locations and site conditions This also precludes the aesthetic problem of discoloration of hardware that occurs in any climate Materials for array mounts can also vary widely especially depending upon geographic and site requirements In some areas painted wooden mounts may be acceptable Other mounts might be made of galvanized steel A common structural material used for commercial array mounts is corrosion resistant aluminum such as 6061 or 6063 alloy aluminum Aluminum develops a thin oxide coating very quickly and this coating prevents further oxidation Anodizing can increase the
117. nformation presented in this publication is intended to inform candidates as they prepare to apply for and complete the certification process for the Certified Solar Photovoltaic Installer Certification Program While NABCEP has made a reasonable effort to provide current and accurate information neither NABCEP nor its employees volunteers or representatives warrants or guarantees the accuracy completeness timeliness merchantability or fitness of the information contained herein for a particular purpose Reference herein to any specific commercial product process training program or service by trade name trademark manufacturer or otherwise does not constitute or imply its endorsement affiliation or support by NABCEP or other contributors to this document While individuals certified as a Certified Solar Photovoltaic Installer are required to meet established certification requirements NABCEP maintains no control over these certificants or their related organizations and disclaims all liability to any party for any action or decision made in reliance on the information contained herein or otherwise provided by NABCEP or for any actions or inactions of candidates or certificants to any party or for any loss or injury resulting from the use or non use of such information 1 Introduction This Study Guide presents some of the basic cognitive material that individuals who install and maintain photovoltaic PV power systems should know and un
118. ns the system it will often be connected on the load side of the service disconnecting means If the output of an inverter is connected to the line side of the main breaker it is important that an acceptable termination procedure be used If two wires are to be attached to a single point the lug s used must be approved for this application When possible it is generally most convenient to establish the point of utility connection at a main distribution panel This can be done by connecting the inverter output to the load side of a dedicated circuit breaker in the main distribution panel When connected in this fashion the requirements of NEC 690 64 B apply NEC 690 64 B requires that e theconnection to the panel must be made via a dedicated circuit breaker or fusible disconnecting means e the sum of the ampere ratings of the overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed the rating of the busbar or conductor e the interconnect point shall be on the line side of all ground fault protection equipment e equipment containing overcurrent devices in circuits supplying power to a busbar or conductor shall be marked to indicate the presence of all sources The requirements should come as no surprise to an experienced electrician The first requirement is that each inverter must be connected by means of its own dedicated circuit breaker or fuse There is an exception for this in VEC 690 6 for ac PV modules wh
119. nt temperature in the neighborhood of 25 30 C But if ambient temperatures drop to freezing then the array will tend to operate at close to STC and the overall remainder becomes 2000x0 9x0 97x0 94 1641 watts which is 8296 of the sum of the STC rated module powers As irradiance decreases the array current will decrease nearly in direct proportion but the array power may not decrease in the same proportion as the array temperature will tend to decrease as the irradiance decreases For example suppose the irradiance on the array has decreased to 700 W m This means the solar power delivered to the PV array is only 7096 of the power delivered at 1000 W m which means another loss factor needs to be included in the calculation If this is the case the cell temperature increase should only be about 70 of its value at 1000 W m Because the increase at 1000 W m was 30 C the cell temperature increase at 700 W m should be about 21 C with a resulting cell temperature of 46 C This would result in a loss of 11 5 due to temperature degradation The expected inverter output power can be calculated by Inverter Output Power 2000 X 0 7 X 0 9 X 0 850 X 0 97 X 0 94 976 W STC array power Irradiance factor Mismatch and dust factor Array temperature factor Wiring efficiency factor Inverter efficiency factor Note that the value for the 700W n is 976 W and it still can be approximated using the 70 system loss factor ru
120. nverters can be set to inhibit the charging of the batteries at night from the grid to allow maximum energy to be extracted from the PV array and not the grid For a system programmed to sell to the utility it is still important that the sell voltage setting on the inverter be below the bulk setting of the PV charge controller If it is not the PV charge controller may limit the PV output and consequently limit the output from the inverter to the utility Recall that at the beginning of the bulk charge mode the charge controller is supplying as much current to the batteries as the PV array can generate It is important to note that the system of Figure 10 has the array configured for effective battery charging It should also be noted that the use of different types of inverters having different multimode capabilities would be connected in significantly different ways In larger utility interactive systems with 48V nominal inverter input voltage and no battery backup the inverter may track maximum array power over array voltages ranging from approximately 44 to 88Vdc For this range of input voltages it 1s possible to use four 12 volt modules in series to meet the input voltage requirements For example with four modules in series V for each will drop to 14 1 V at 60 C resulting in Vmp for the series combination of 56 4 V Note however that if four modules are used and the module temperature falls very much below 25 C it is possible that Vinax
121. of the electrolyte in battery cells The caps of the cells are removed and the hydrometer is used to withdraw electrolyte into the hydrometer The hydrometer incorporates a float that floats higher if the specific gravity is high and floats lower if the specific gravity is low If the specific gravity is significantly lower in one cell than in the other cells of a battery it is an indication of a bad cell If after a normal charging period filling the cell with distilled water and then applying an equalizing charge to the battery does not increase the specific gravity then the cell or battery will need to be replaced Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 40 When checking batteries it is important to wear eye and face protection Rubber gloves are also advised as is protection for your clothing and it is also important to have water and baking soda for lead acid batteries nearby in case any acid spills need to be neutralized Along with the specific gravity check of battery cells battery voltages and currents should also be checked The voltages of all series connected batteries should be the same within a few tenths of a volt and the currents of all batteries in parallel should be the same within about 5 under charging and discharging conditions If these are not the results then all battery connections need to be checked The lengths of battery cables also need to be chec
122. oltage to ground It is the maximum voltage that may appear between the two ungrounded conductors The maximum system voltage requirement combined with its relationship to temperature make it 1s possible that a system meet code requirements in one location but not in another location even if both locations subscribe to the same code An experienced PV installer will likely also be experienced in PV system maintenance Experience with the reliability of systems or with the ease or difficulty of installation of specific systems enable an installer to inform a potential PV system owner of good and bad points associated with different systems allowing the consumer to make a more intelligent choice of systems for the intended application After a PV system has been installed it is essential that the system do what it has been advertised to do and do it safely and reliably The experienced PV installer should be able to identify the advantages and disadvantages of systems that operate at different dc voltages ranging from 12V systems to systems greater than 400Vdc The major disadvantage of 12V systems is that currents in 12V systems are twice as high as 24V systems and four times as high as for 48V systems These higher currents require proportionately larger wire sizes In fact to maintain a voltage drop within certain limits say 396 for the same load at 24V as opposed to at 12V the allowable wire resistance can be 4 times as high as for the 12V loads
123. operating at STC in a utility interactive system with battery backup The STC maximum power voltage rating of the PV array is 68 4 volts The system uses a conventional charge controller that does not track maximum power Wiring losses are 3 and inverter losses are 5 If the batteries are at full charge at a voltage of 52 V and if all PV output is delivered to the grid assume that no power is being used to hold the batteries at 52 volts the inverter output power will be closest to a 1261 watts b 2207 watts c 2522 watts d 3152 watts A typical 4 kW crystalline silicon array is operating at STC in a utility interactive system with battery backup The STC maximum power voltage rating of the PV array is 68 4 volts The system uses a MPT charge controller that has 5 losses Wiring losses are 3 and inverter losses are 5 If the batteries are at full charge at a voltage of 52 V and if all PV output is delivered to the grid assume that no power is being used to hold the batteries at 52 volts the inverter output power will be closest to a 1261 watts b 2207 watts c 2522 watts d 3152 watts In the event that the utility voltage is lost in a utility interactive system that has battery backup and an optional standby system ac distribution panel and if the inverter is programmed in the sell mode the inverter should not supply power to the terminals connected to utility point of connection the inverter should not supply power to the termi
124. own securely that all wiring connections have been made properly according to the system schematic diagram and manufacturers instructions that all weather sealing is properly applied that wires are bundled neatly and that the system has been installed in a workmanlike manner NEC 110 12 Before performing any other checks on the system the grounding system should be checked to be sure that all required parts are properly grounded Presumably all disconnects and breakers were turned off and fuses were left out of the holders It should be verified that this is still the case system check out includes the following 1 Polarity of all source circuits should be checked WARNING Improper polarity can cause severe damage to the array and system electronics and has been known to cause fires in some systems At the same time the polarity check 1s made the open circuit voltages should also be checked Open circuit voltages of source circuits should normally be within 296 of each other if the irradiance remains constant during the test If not then it becomes a troubleshooting procedure to be covered in the next section of this Guide Short circuit currents should be within 5 of each other provided that the irradiance remains constant during the test The short circuit currents will only equal STC values if the modules are exposed to 1000 W m of irradiance To measure short circuit currents it should be verified again that all break
125. pace in front of an inverter that is 24 inches wide must be at least a 24 inches b 30 inches c 36 inches d 42 inches 56 The minimum depth of the working space in front of a charge controller for which the input voltage never exceeds 60 V dc is a 30 inches b 36 inches c 42 inches d negotiable 57 Which of the following items does NOT require UL or equivalent listing the concrete anchors the surge protectors the battery cables the charge controllers aoe 58 In order for a PV array to directly face the sun at 2 30 p m sun time on June 21 at 30 N latitude see Figure 5 of this Guide which array orientation is correct 60 W of S with a tilt of 40 with respect to the horizontal directly west with a tilt of 60 with respect to the horizontal directly west with a tilt of 30 with respect to the horizontal 45 W of S with a tilt of 60 with respect to the horizontal ao Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 52 59 If 5 16 inch lag screws are used to fasten a charge controller to wooden studs an appropriate pilot hole size would be closest to a 1 8 inch b 19 64 inch c 7 32 inch d 1 4 inch 60 When mounting a heavy inverter or other piece of equipment to a dry wall type of wall to provide a solid attachment to the wall it may be necessary to use a thumb tacks b plastic anchors c plywood d moly bolts 4 7
126. pleted inverter input and output should be tested for proper operation Because the system has batteries it may need a separate charge controller Unless the system incorporates a MPT charge controller the array most likely will be operating slightly below the maximum power Some multimode inverters incorporate MPT circuits for the PV array However unless the charge controller is operating in a current limiting mode the full array current for the given irradiance should be flowing The installer should have a good 1dea of what to expect from the measurements For example assume that a 2 kW STC crystalline silicon array is in place that feeds a utility interactive inverter with no battery storage Assume also that the installer has a solar irradiance meter or pyranometer available to measure the irradiance incident on the array and that the array is receiving full sun 1000 W m and is at a temperature of 55 C Finally assume the inverter has a rated efficiency of 94 and the inverter efficiency remains nearly constant at and around the operating point The problem is to determine how much power to expect from the inverter Even under STC the array will probably not supply the full 2 kW to the PV output circuit Manufacturers tolerances allow for a 5 lower output or more These tolerances along with slight mismatches among the modules and dust on the modules will likely reduce the output to 90 of the rated value With a temperature coefficient of
127. r the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 27 120 Volts a Square Wave b Modified Sine Wave c Sine wave Figure 9 Three types of inverter output waveforms The simplest waveform to generate is the square wave of Figure 9a which can be used in simple stand alone systems where insensitive loads can tolerate the low quality power Square waves have high harmonic distortion thus they are not advisable for use with some electronic equipment and they can cause motors and other magnetic components to overheat If a square wave inverter 1s used it is important to check the instruction book to see what loads are not suitable for use with the unit The output of square wave inverters 1s not suitable for connecting to the utility grid Square wave inverters are now generally considered obsolete and are seldom found in stand alone systems The next waveform for stand alone inverters is the modified sine wave of Figure 9b This waveform is commonly used in uninterruptible power supplies UPS for electronic equipment and is also used by many moderate cost stand alone PV inverters The harmonic distortion is less than the harmonic distortion of a square wave and the waveform is suitable for use with more types of loads Inverters with modified sine wave outputs more accurately known as modified square wave inverters are in common use in stand alone PV and recreational vehicle applications Common loads t
128. ray must be determined Ground mounted arrays are more susceptible to vandalism than pole or roof mounted systems If an array is mounted at ground level it may need to be protected by a fence to avoid or reduce the possibility of breaking modules in the array and contact with the array by other than qualified service personnel The addition of a fence requires a greater array area so that the fence does not shade the array One way to reduce access to the modules 15 to ground mount the array at least eight feet above the ground This provides protection and can provide some much needed shade on hot sunny days 3 2 5 BOS Locations Balance of system components include support and security structures charge controllers source circuit combiner boxes batteries inverters disconnects overcurrent devices electrical wiring and junction boxes Some of the electrical components may need to be installed in weather resistant or rain tight enclosures if they are not installed indoors Other components such as an inverter may already be rated for wet and outdoor exposure If hardware is enclosed in weather resistant or rain tight boxes the installer must be sure that heat can still be dissipated or removed It is important to know the dimensions and the required spacing of all components so that adequate space for the hardware and access can be assigned in the system layout In low voltage systems wire sizes are often larger than 6 AWG in order to minimi
129. roximately 1200 0 9 0 875 0 8 0 97 0 95 697 W Because the measured power is actually more than this the shaded modules are apparently making some contribution to the total output power So the system appears to be working properly When the charge controller is in the float phase the current to the batteries is reduced but the voltage across each fuse should be very close to zero When one fuse is good the voltage across it is zero so the voltage on the charge controller side of the fuse should be at approximately the battery voltage provided that the charge controller disconnect is closed When a fuse is open the voltage on one side will be the battery voltage and the voltage on the other side will be the open circuit source circuit voltage Hence the problem is that the bottom fuse is blown If no current is measured from the array and the inverter is charging the battery during a sunny day it is likely that the charge controller has either failed or is set improperly The problem states that the charge controller is good so this rules out failures and turns the attention to charge controller settings In a grid connected PV system with battery backup and no MPT charge controller the inverter provides the primary charge control under normal conditions The backup charge controller is only to be used in the event of a utility outage To keep the backup charge controller from regulating during normal operation it must be set slig
130. s a walkway should be provided between each row of modules However this consumes valuable roof area so a balance needs to be made between the area for the array and access to the array Often only 50 to 80 of the roof area that has a suitable orientation can be used for mounting modules when room for maintenance wiring paths and aesthetic considerations are taken into account To determine the size of the PV array ultimately the power rating of the system that can be installed the usable roof area must be determined The physical size of the modules to be used is important depending upon the shape of the roof area to be used When a module is selected it is necessary to check the total array dimensions against the roof dimensions to be sure the array will fit the roof As a rule of thumb crystalline silicon modules with 1096 efficiency will generate about 10 watts per square foot 100 watts per square meter of illuminated module area typically today s thin film modules require more area for the same rated output Hence by multiplying the usable and available roof area in square feet by 10 the size of the PV array in Watts that can be installed can be estimated For example a roof with dimensions of 14 by 25 350 ft has a usable area of 250 ft 71 of total This roof area would be sufficient for a 2 5 kW 250 ft x 10 W ft 2500 W crystalline silicon array or an 8 efficient thin film array of 2 kW An extremely important
131. s are mounted so they always face the sun they will generate the maximum possible current for that level of irradiance In areas that have minimal cloud cover and long daylight hours array mounts that track the sun can increase the daily energy from a PV module by as much as 40 in summer months In cloudy areas the enhancement from a tracking array is less Figure also shows that the current produced by a module is limited even when it is shorted V 0 Shorting a PV module generally does not damage it however there are exceptions In fact measuring the short circuit current of a module when it is disconnected from the rest of the system is one way to test for good or bad modules Some PV charge controllers regulate battery charging by short circuiting the module or array Note that short circuits for extended periods of time greater than several minutes under high irradiance may damage some thin film modules Manufacturers data sheets provide applicable cautions The open circuit voltage of a module occurs when the module is not supplying any current when the module is disconnected from all loads Note from Figure 1 that the open circuit voltage of a module is minimally dependent on irradiance If modules are connected in series then the module voltages add just as in the case of batteries So if higher voltages are needed then modules are connected in series until the desired voltage is reached If modules are connected in parallel th
132. s charge Knowing the maximum run time is important when estimating the generator fuel consumption and for estimating generator maintenance intervals The generator manufacturer will specify the hourly fuel consumption at rated load on the generator and will specify maintenance intervals for oil changes tune ups and rebuilds When the generator is running inverters usually switch the optional standby system load to the generator and then use any excess generator output for battery charging This does not affect the average daily generator run time because if the generator is supplying power to the loads then the batteries are not supplying that power and thus less charge is required from the batteries to meet the daily load requirements Generators are covered in NEC 445 Because generators used in PV systems are permanently connected they must be grounded Normally the case of the generator should be bonded to the equipment grounding conductor The generator neutral assuming an ac generator is not connected to the equipment grounding conductor except at the single point of grounding for the system Note that many portable and RV style generators make the bond between the neutral and ground internally and this connection should be removed if used in a PV system This ensures that no current will flow on the equipment grounding conductor However removing the internal connection may compromise the listing of the generator may result in improper
133. s consult the battery manufacturer before attempting to equalize batteries In some systems the PV output current may exceed the input current rating of a single charge controller The PV output circuit must then be split into two or more circuits each of which has an output current that is within the input current limits of a charge controller Two or more charge controllers can then be connected with their outputs in parallel to the batteries If multiple charge controllers are used then multiple combiners will be needed and it is advisable to use a separate surge suppressor at the output of each combiner box Also each charge controller will need to have separate disconnects at its input and output Diversion charge controllers are often used in stand alone systems to take advantage of excess generation that would normally be wasted once the batteries are fully charged Diversion charge controllers typically use a resistive load such as a water heating coil in a water heater to consume the excess power from the array Diversion charge controllers that are used as the sole means of battery charge regulation are required to have an independent controller to prevent overcharge of the battery should the load circuit fail The 2005 NEC includes a new requirement that the power rating of the diversion load be 15096 of the power rating of the array This provision ensures that the load is sufficiently oversized to handle the entire output of the PV array
134. s output current must be at least 12596 of the PV array short circuit output current and the charge controller maximum input voltage must be higher than the maximum system voltage as determined by NEC 690 7 Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 24 NEC 690 15 requires disconnects from all ungrounded conductors at inputs and outputs of all system components It also requires a provision for disconnecting the PV system from any other electrical system to which it may be connected NEC 690 17 specifies that any disconnects that can be opened under load must have interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment Normally the output current of the charge controller will be less than or equal to lt the input current The exception to this rule is a maximum power tracking MPT charge controller for which the output current may exceed the input current If a MPT charge controller is used it is important to consult the manufacturer s specifications for the device to determine the maximum charge controller output current 3 5 2 1 Linear Current Boosters and Maximum Power Tracking Charge Controllers A PV array produces nearly full operating voltage at very low irradiance levels 75 W m In a water pumping system without 6 batteries it may be several hours after sunrise befor
135. selecting the tilt angle for a PV array Either the designer or installer must determine whether summer or winter performance of the array is most important Lower angles of tilt will optimize the summer performance and higher angles will optimize the winter performance When a stand alone system with energy storage is installed there may be trade offs that depend upon the load requirements at specific times of the year e g decorative Christmas lights in December If a system incorporates a heavy inverter and some inverters weigh up to 150 pounds or more it is very important to follow the manufacturer s instructions for mounting the inverter It is often practical when mounting a heavy inverter over plaster or drywall to firmly attach a sheet of plywood to the wall studs The inverter can then be safely secured to the plywood It often requires two people or some other means of carrying the weight of the inverter when lifting the inverter to its mounting location Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 18 The locations of all dc and ac connections to electronic system components should be carefully planned to accommodate a neat and professional installation For example if the ac connections are on the left side of an inverter and the dc connections are on the right side locations of batteries charge controllers and disconnects should be arranged as conveniently as possibl
136. serving as a disconnect device and overcurrent protection would be adequate If the wiring to the inverter 1s located in a hot location such as a garage or on the wall of a building the temperature corrections of the wiring must also be considered If voltage drop is a problem which is nearly always the case in 24 V systems then larger wire will need to be used Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 30 The inverter bypass switch is shown in Figure 10 The inverter bypass switch allows for inverter maintenance while supplying the emergency loads directly from the utility line This switch in this circuit consists of a double pole circuit breaker ganged together with a single pole circuit breaker in a manner such that both breakers cannot be simultaneously ON but both can be simultaneously OFF When the double pole breaker is on the hot lead of the ac input from the utility passes through one pole of the breaker and enters the ac in terminal of the inverter The ac out of the inverter passes through the other pole of the breaker from which it 1s connected to the optional standby load When the double pole breaker is in bypass mode the utility line in 1s blocked from the inverter but 1s connected through the single pole breaker to the emergency load This is the inverter bypass position When both breakers are off power is disconnected from both the inverter and the emergency panel
137. st be limited to less than 2 each in order to stay within 5 overall Determining Voltage Drop If the one way distance between two points is expressed as length d in feet recognizing that the total wire length of a circuit between these two points will be 2xd Ohm s Law V4 I x R provides the basic equation to find voltage drop in conductors where V is the amount of voltage drop in the conductor at the highest expected current level The O kft term is the resistance of the conductor in ohms 1000 feet and is presented in Chapter 9 Table 8 V IxR m 1000 ft Aft V REN x Eqn 1 1000 ft kft kft 2xdxI s LO rop Where I is the circuit current in Amperes which for source circuits is usually taken as the maximum power current Imp Vnom 15 the nominal system voltage which in this case is 24V and Q kft is found from NEC Chapter 9 Table 8 Conductor Properties Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 21 In this case using 14 AWG stranded copper uncoated wire O kft is found to be 3 14 Assuming the distance from junction box to source circuit combiner box to be 40 ft the arop is found after substituting all the numbers into the formula to be 2x40fix7A 1000 fi kft a WV trop Ji 100 7 3 Clearly a value of 7 3 is high and is well above the recommended target
138. summer cloud cover In areas of moderate wind latitude greater than 30 and moderate year around cloud cover In areas of moderate wind latitude less than 30 and minimal year around cloud cover Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 46 4 5 Adapting the Electrical Design Questions 26 31 are based on the system schematic diagram of Figure 7 in this guide and the equipment specifications that follow the Figure 7 as shown in Table 2 26 Referring to Figure 7 if 4 AWG THWN 2 copper wire in a single run of 1 5 inch conduit is used between the array junction box and the source circuit combiner box and the wiring from the modules to the junction box is exposed 10 AWG copper wire that enters through a one inch cord connector at right angles to the 4 AWG conductors and if the volume of the terminal strip in the junction box is six cubic inches the junction box must be sized from a NEC Article 314 16 b NEC Article 314 17 c NEC Article 314 27 d NEC Article 314 28 27 For the PV modules used in Figure 7 if the maximum wire temperature ambient temperature that the wire sees is estimated to be 45 C between the array junction box and the source circuit combiner box then the ampacity of the conductors at 30 assuming THWN 2 insulation must be at least a 161A b 129A c IL2A d 78A 28 If the distance from the junction box to the combiner box of Figure 7 is 60 fe
139. sure that the installed system will meet code requirements It is not unusual for something to be left out of a design often because different jurisdictions have somewhat different requirements for system installation or different interpretations of pertinent sections of the NEC For example a homeowner may have selected and perhaps even purchased a PV system If the PV array is to be installed on the roof of a dwelling the PV installer needs to know that a ground fault protection device is required for the system by the National Electrical Code The installer also needs to know that a residential PV system 1s prohibited from having a Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 15 maximum system voltage in excess of 600 V Thus it is important for the installer to know the correction factors of Table 690 7 for crystalline PV modules so the maximum system voltage at Standard Test Conditions can be corrected for the lowest anticipated system temperature It is also important for the installer to know that the maximum system dc voltage 15 the maximum dc voltage that may appear between any two ungrounded conductors in the system For a two wire system this is simply the maximum voltage across the two wires but for a 3 wire bipolar system that has both a positive supply and a negative supply with respect to a common grounded conductor the maximum system voltage is NOT the maximum v
140. system performance this alone is incomplete and can be misleading since this efficiency changes with DC input voltage AC output voltage percentage of load and several other factors The output of a system is also dictated by its maximum power point tracking effectiveness its ability to output high power at the site s expected temperature and product reliability The most critical objective of any inverter is to output kWh throughout the life of the system Choices should typically be based around this principle and then the other features of the products Warranties manufacturer reputability and ease of installation use are also worth considering for both modules and inverters As with any other type of products there are a wide array of manufacturers offering a range of performance reliability and features Discussions with other experienced installers 15 often the most effective way of determining a product s true value in the field For example some modules may be rated within 5 but often perform at the lower end of this tolerance while others with the same tolerance may regularly achieve their output target or even above Some inverters offer sophisticated self diagnostic systems to assist installers in servicing systems or data monitoring systems that allow installers or end users to view performance more easily over a computer Though looking at specification sheets and product literature is often a good place to begin your search for c
141. t to an asphalt shingle roof If the combined thickness of the L bracket shingles and roof membrane 15 34 inch and if the screw penetrates directly into a roof truss made of Southern Yellow Pine into a properly sized pilot hole then the withdrawal resistance will be closest to 632 pounds 773 pounds 843 pounds 984 pounds Four PV modules each with an area of 10 ft are to be mounted with a stand off mount that is secured to a metal seam roof with six L Brackets If the modules can withstand a load of 75 pounds per square foot and if it is desired to support the full load with one lag screw in each bracket and each screw has a withdrawal resistance of 300 pounds per inch including a safety factor of four the minimum screw thread length that will need to penetrate wood will be closest to ao ee 1 1 1 67 2 25 6 67 Stainless steel hardware is most important in which of the following areas Coastal areas where the air contains salt spray Desert areas where the air contains a mixture of dust Mountainous areas where the solar spectrum contains more ultraviolet rays Inland regions that are subject to freezing temperatures For the situations described which would result in the most cost effective use of a two axis tracking mount ao In areas of low wind latitude less than 30 and moderate daytime summer cloud cover In areas of low wind latitude greater than 30 and minimal daytime
142. taic Installer Certification Version 3 August 2005 35 What 1f the array is to be rack mounted on a roof that is not flat An inclinometer sometimes called a roof protractor determines the roof angle and the roof angle is subtracted from the desired array angle Trigonometry can also be used to perform the same calculations using a tape measure and an inexpensive scientific calculator A 3 12 pitch roof represents a 14 angle from horizontal If the desired collector tilt angle is 30 then the rack must have a 16 tilt angle The height of the tilt leg can be determined experimentally using an inclinometer or by using trigonometry to solve for the tilt leg height If multiple rows of modules are necessary remember that adequate room must be given to prevent inter row shading in the winter months Next the mounting holes need to be drilled For pilot holes into roof truss timbers the pilot hole for screw threads should normally be 65 to 75 of the size of the screw For a 4 in diameter lag screw the pilot hole should be drilled with a bit with a diameter between 0 65x0 25 in 0 16 in and 0 75x0 25 in 0 187 in But what are these sizes in 64ths To find out multiply each by 64 as there are sixty four 64ths in an inch The result is 0 15 9 6 64 and 0 187 12 64 3 16 in So anything between a 5 32 inch and 3 16 inch bit would be appropriate For softer woods the smaller bit would be best and for harder wood the larger wou
143. ted array rather than an array that is tilted to an angle greater than the roof tilt Most roof orientations are not the most ideal for the array orientation so the impact of a less than optimal orientation must be understood prior to solidifying the system orientation Much erroneous information has been circulated with respect to the impact of systems at non optimal orientations and it is difficult to find sources with accurate information on the subject Correlations between optimal orientations and proposed orientations are extremely region specific In other words an evaluation that was done for California will not apply to New York or Florida Orientation tables similar to ones that have been developed for California and New York can be produced fairly quickly using a variety of available computer simulation programs Caution should be exercised when applying any basic table to prevent misapplication for a location or site that is atypical of the conditions used to create one of these orientation tables The following tables show the multiplication factor used to adjust annual energy production for various tilt angles relative to the orientation that achieves the maximum annual energy production These tables are provided as examples to provide a better understanding of the impact of orientation DPen 1 SESW 039 095 09 093 080 050 oo oo vencal Pen __ Soun 096
144. ter and the emergency panel is a 14 AWG copper b 12 AWG copper c 10 AWG copper d 8 copper 50 A 5 kVA 120 V generator has a rated output current of 42 A It does not have a mechanism to limit its output current to the rated value Assuming they are run in conduit the output conductors should have an ampacity of no less than a 8AWG THWN b 8AWG THHN c 6 AWG THWN d 6AWG THHN 51 A 5 kVA 120 V generator is used as a backup generator for a system designed with two days of battery storage to 80 depth of discharge If the generator is sized for charging rate of C 10 and if the generator burns 1 gallon of fuel per hour of run time the average daily fuel consumption when the generator is the only power source will be closest to a 4 gallons b 5 gallons c 8 gallons d 10 gallons 52 If the 5 kVA 120 V generator is protected with a 50 A circuit breaker then the equipment grounding conductor must be no smaller than a 12 AWG copper b 10 AWG copper c 8 AWG copper d 6 AWG copper Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 51 4 6 Installing Systems and Subsystems at the Site 53 In a PV system the equipment grounding conductors should be a white b black c red d green 54 The 2005 NEC allows marking conductors with colored tape provided that they are larger than a 10 AWG b 8 AWG c 6 AWG d 4 AWG 55 The width of the working s
145. terials 2 1 References The following reference list identifies training materials reports and codes and standards associated with the design and installation of PV systems and are complementary to this Guide Many of the sample questions come from these references as does other content in this Guide Perhaps the most important reference on this list is the 2005 National Electrical Code NEC or its expanded 2005 National Electrical Code Handbook and as so much of the material in this Guide is based on the NEC it is strongly encouraged that a copy be kept close at hand while reviewing this document l 2 2 10 2001 Code of Federal Regulations Chapter 29 Part 1926 Safety and Health Regulations for Construction U S Department of Labor OSHA OSHA Publications P O Box 37535 Washington D C 20013 7535 http www osha gov 2005 National Electrical Code NFPA 70 National Fire Protection Association 1 Batterymarch Park Quincy 02269 or 2005 National Electrical Code Handbook http www nfpa org nec Additional Reading Material Objectives and Task Analysis for the Solar Photovoltaic System Installer North American Board of Certified Energy Practitioners Technical Committee Document Approved 6 4 2002 http www nabcep org pdf resources SolarP V TaskAnalysis pdf Photovoltaic Power Systems and the 2005 National Electrical Code Suggested Practices SAND2005 0342 February 2005 Sandia National Laboratories
146. teries NEC Section 690 74 permits but does not require flexible cables Article 400 to be used as conductors for connecting batteries Welding cable listed or not listed automotive battery cables diesel locomotive cables marked DLO only and the like often do not meet requirements for connection to batteries There are several methods of connecting batteries in common practice that can be found in battery literature and in literature from charge controller manufacturers or inverter manufacturers In general no more than four batteries or series strings of batteries should be connected in parallel It is better to use larger batteries with higher ampere hour ratings than to connect batteries in parallel 3 5 4 Inverters Inverters are used to convert the dc power to ac power Inverters are designed to be used as stand alone system inverters or as utility interactive inverters Some utility interactive inverters are capable of operating in a stand alone mode Any inverter used in a PV system shall be identified for such use 690 4 D Inverters that are suitable for use in utility interactive PV systems must be listed and identified for that use 690 60 Modern inverters are capable of operating with peak efficiencies over 90 with very good power quality Stand alone inverter output waveforms vary in shape and or power quality as shown in Figure 9 Note that all three waveforms of Figure 9 have Vins values of 120 V Study Guide fo
147. that while it may be referred to as a 720 watt system the maximum output of the PV array will fall short of 720 watts The reasons for the shortfall will be discussed in detail later in this Guide The nominal dc voltage is 24 V with each battery being a six volt deep discharge unit The system is essentially the same as the stand alone system discussed earlier up to the inverter This system does not have a peak power tracking inverter and the charge controller is used to maintain the batteries at the correct state of charge Other multimode inverters may have PV maximum power tracking circuits and exclude the charge controller The charge controller is generally set to charge the batteries to the bulk charge level and the inverter is generally set so that when the batteries reach the bulk level current from the charge controller is diverted to the inverter where it is converted to ac and used either to supply the emergency loads or to sell back to the utility Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 29 If the inverter has a battery charging option from either the utility or from alternate generation means the charge settings in the inverter should be set lower than the settings of the PV charge controller otherwise the PV system sensors will find the batteries to be fully charged and the PV charge controller may limit the current from array to charge controller output Some i
148. the system has charge controllers then the ground fault protection device normally is connected at the input to the charge controllers as the charge controller s will be the termination point of the PV output circuit NEC 690 43 requires that all exposed non current carrying metal parts of components be grounded in accordance with NEC 250 134 or 250 136 A regardless of the system voltage This means that even if the current carrying conductors of the PV array do not need to be grounded all non current carrying metal equipment parts and cases must still be grounded with equipment grounding conductors NEC 690 45 provides the requirements for sizing of equipment grounding conductors If the system incorporates a ground fault protection device then the equipment grounding conductor is sized according to VEC 250 122 But if the system does not have a GFDI then the equipment grounding conductor must be sized at 12596 of the PV short circuit current The size of the grounding electrode conductor is determined in accordance with NEC 690 47 Note that for ac systems NEC 250 50 through 250 60 governs the grounding electrode system NEC 250 64 gives the installation requirements for the grounding electrode conductor In a PV system with an inverter there is both dc and Fortunately the dc and ac requirements are similar In fact in both cases the grounding electrode conductor 15 sized in accordance with 250 66 An ac PV module is considered an
149. the top of the atmosphere penetrates to the surface of the Earth If PV modules are mounted perpendicular to the sun s rays it is possible to receive close to peak sun nearly every sunny day at noon for much of the continental United States However at different times of the year and at higher elevations sunlight intensity can be as much as 1250 W m more for hours at a time As the sun moves to a lower position in the sky solar radiation passes through more of the Earth s atmosphere resulting in more of the irradiance being absorbed Note also that the output of the PV module is also affected by the spectrum of the incident light reaching the module The spectrum of light from the sun changes based on how much atmosphere the light travels through Photovoltaic module testing uses the spectrum of light produced when the sun shines through the equivalent air mass of 1 5 times the atmosphere thickness at the equator as a standard so that all modules are evaluated under the same conditions This is the amount of atmosphere sunlight travels through to get to the Earth on March 21 at solar noon at Cape Canaveral Florida This is referred to as Air Mass 1 5 in testing specifications As shown in Figure 1 the current generated by a module is directly proportional to the irradiance incident on the module If the sunlight strikes a module at an angle the effective irradiance on the module is reduced and the module current decreases Thus if module
150. ting creating a fall hazard Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 8 The National Electrical Code in Article 690 5 requires that any PV array installed on the roof of a dwelling must incorporate a ground fault protection device to reduce fire hazards The device shall be capable of detecting a ground fault interrupting the flow of fault current and providing an indication of the fault Note that the purpose of this dc ground fault protection device differs from that of the ac ground fault circuit interrupter in that the GFCI is intended for prevention of serious shock while the ground fault protection is intended to detect and interrupt higher currents that can create a fire hazard 3 1 4 2 Battery Safety Some PV systems are designed to provide power to loads even when the sun is not shining and these typically use battery storage Batteries used in PV systems are normally lead acid batteries of the deep discharge type Occasionally nickel cadmium nickel iron or nickel metal hydride batteries are used Deep discharge type batteries differ from automobile starting batteries deep discharge units are designed with heavier thicker plates for better deep discharge performance Automobile starting batteries have a larger number of thin plates to provide a greater plate surface area so it can provide high starting currents for short times If an automobile batter
151. tteries In fact 1t is possible to group loads in order of importance and then have different disconnect settings on the charge controllers depending upon the priority of the loads connected to the output of the charge controller 3 5 3 Batteries and Battery Wiring The goal of battery wiring is to charge and discharge all batteries equally To do so requires an effort to equalize the currents flowing through each battery at all times If batteries are connected in series this is automatic but if batteries are connected in parallel the currents may be unequal due to different and very subtle cable resistance First of all all batteries must be the same type same manufacturer the same age and must be maintained at equal temperatures The batteries should all have the same charge and discharge properties under these circumstances It is important to minimize voltage drop in wiring and to attempt to keep the voltage across every battery the same Minimizing voltage drop in wiring means using large conductors such as 2 0 or 4 0 battery cable The battery connections in Figure 7 show one way to keep battery currents nearly equal when the batteries are connected in a series parallel configuration The method is to connect the positive and negative leads to the battery bank at opposite corners of the battery bank This tends to equalize the voltage drop across each pair of batteries which in turn results in equal current flow either to or from the bat
152. ty grid Some utility interactive systems have the PV array directly connected to the inverter while others incorporate battery storage and a multimode inverter that can operate in either a utility interactive or stand alone mode This configuration allows the inverter to continue supplying dedicated loads from battery power when there is a loss of utility service Regardless of the intended use of the PV system it is the installer s responsibility to install and configure the system correctly so it will perform as intended Figure 7 shows a typical 24V PV system that supplies 24V 4c loads The system consists of PV modules a module wiring junction box JB a source circuit combiner box a charge controller to prevent overcharging of the batteries and a low voltage disconnect LVD to prevent overdischarge of the batteries The system also includes overcurrent protection disconnects grounding wiring between components batteries and a dc load center Note that the PV modules are connected as two source circuits Each source circuit consists of two modules connected in series and the two source circuits are ultimately connected in parallel at the source circuit combiner box Table 2 shows the PV module specifications for examples to follow Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 19 Table 2 Module Specification for Discussion and Examples in 3 5 1 Modules Vac M Vos
153. vary with irradiance and temperature the nameplate on the back of a module lists the electrical properties of the module under Standard Test Conditions STC Standard Test Conditions are a cell module temperature of 25 C an Air Mass of 1 5 and under an irradiance level of 1000 W m These conditions are conducive to testing in a manufacturing environment but tend to overestimate actual performance as the cell temperature is rarely at a temperature of 25 C and an irradiance of 1000 W m at the same time An operating cell temperature of 50 is much more common when the module is at mild ambient temperatures For crystalline modules that lose 0 596 per C this corresponds to a typical performance that is 1296 below the value tested under STC 50 C 25 C x 0 596 12 5 Working safely with PV modules involves taking precautions to avoid electrical shock from potentially high dc voltages especially when several modules are connected in series It also involves care in handling of the modules so they are not dropped and care in protecting open wire ends from shorting to each other Because a short circuit may not blow any fuses opening the short circuit produces a very intense dc arc between the wires Once initiated this arc can reach temperatures of 10 000 C over a distance of half an inch or more This arc is a potential fire hazard burn hazard ultraviolet eye hazard or might surprise the electrical worker causing him her to lose foo
154. ve and Life Saving Equipment Subpart I Tools Hand and Power Subpart K Electrical Subpart X Stairways and Ladders Several especially pertinent issues relating to PV installations include the following e No construction worker will be required to perform work under conditions that are unsanitary hazardous or dangerous to his safety or health 1926 10 a e The employer shall initiate and maintain a job site safety and health program 1926 20 b 1 e The use of any machinery tool material or equipment that is not in compliance with safety standards is prohibited 1926 20 b 3 e Every employee shall be instructed in the recognition and avoidance of unsafe conditions 1926 21 b 2 e Every employee shall be instructed in the safety and health regulations applicable to his her work 1926 21 b 2 Falls are the leading cause of workplace fatalities with 150 200 workers killed each year and nearly 10 000 injured Because nearly every PV system involves climbing on a ladder onto a roof or both it is essential that PV installers be familiar with OSHA fall protection regulations OSHA requires that fall protection must be provided where needed Any work done at more than six 6 feet above ground level must be done with fall protection considerations Fall protection systems may include safety net systems warning line systems covers toe boards safety harnesses or lanyards Study Guide for the Applicant for NABCEP Photovolt
155. ximum power tracking charge Study Guide for the Applicant for NABCEP Photovoltaic Installer Certification Version 3 August 2005 39 controllers can reduce the power tracking losses to only 2 0 98 so the resulting battery system loss for maximum power tracking battery systems is about 0 66 0 7x0 98x0 96 0 66 This is about 94 of a non battery system performance 3 8 X Maintaining and Troubleshooting a System Tasks 8 1 thru 8 7 of the NABCEP Task Analysis Photovoltaic system maintenance includes maintaining all parts of the system Photovoltaic systems are becoming more reliable and once a system is running properly it should require very little maintenance except for batteries when they are included The amount of maintenance required by the batteries will depend upon the battery types and the charge discharge cycles The manufacturer s recommendations for maintenance should be followed Flooded lead acid batteries will require the most maintenance as the electrolyte level will need checking on a regular basis System maintenance should be done at least on an annual basis but the installer should also teach the system owner to monitor the system performance If something seems to be wrong the installer may become the troubleshooter Annual maintenance should involve checking all mechanical and electrical connections checking all source circuit currents and voltages checking battery electrolyte if the batteries are not sealed
156. y 1s used in a PV system it will generally degrade much faster because it is not designed for deep cycling Electric golf cart batteries are one type of relatively inexpensive deep cycle battery that is often used for energy storage in smaller PV systems Lead acid storage batteries come in sealed and vented designs and each has different charging and discharging characteristics The charge controllers must compensate for these differences through the charge controller settings Sealed units are more expensive but require less maintenance Vented batteries are less expensive but require more maintenance The level of electrolyte needs to be checked periodically and distilled water needs to be added when it is low Furthermore vented batteries release more hydrogen and oxygen along with other corrosive gases even under normal operating conditions Because hydrogen is lighter than air it tends to accumulate at the top of the battery container and if the hydrogen concentration gets too high the mixture becomes explosive Therefore battery containers should be vented at the top to allow the hydrogen to escape NEC 480 9 A states that all battery locations must provide ventilation but it does not provide any guidance for that ventilation Normally the containers will not be vented at the bottom because acid containment is another requirement NOTE Hydrogen venting is quite different than providing combustion air for appliances When vented lead
157. ystems operating below 100 Volts Voltage drop is not a safety issue therefore it is not covered in great detail in the NEC However PV systems with excessive voltage drop are inefficient and can perform poorly Once you have determined NEC requirements for ampacity and related conditions of use as described above verify that voltage drop is within acceptable limits for efficiency and quality performance For any given wire size voltage drop increases with increasing currents and or increasing wire lengths Therefore circuits with high current and or long lengths deserve close scrutiny with respect to voltage drop This is particularly true of systems operating at 12V 24V or 48V but even higher voltage systems can have significant voltage drop issues as a result of long circuits There is no specified code compliance limit for voltage drop in any given circuit Generally accepted practices within the industry limit overall system voltage drop within a range of 2 to 5 percent of the circuit operating voltage The PV system designer must use their best judgment considering performance and economics Five percent is generally considered a maximum overall acceptable voltage drop from source to load In order to achieve this 5 limit you will have to limit intermediate runs within a circuit to a lesser percentage voltage drop For instance intermediate circuit runs such as PV array to PV combiner box and PV Combiner box to PV charge controller mu
158. ze voltage drop When large wire is used it is important to be sure that all boxes and terminals are large enough to accommodate the size of wire that is used There must be adequate room to install an enclosure that will be large enough for bending and installing the large wire If a 12 volt system is proposed it is important to keep the electrical paths as short as possible to avoid the expense and inconvenience of large wire If the distance is too great it usually is better to consider a system that operates at a higher voltage This also reduces the current levels for the same power level and often requires smaller and less expensive hardware Battery containers were discussed in the section on safety They must be placed in locations that are acceptable under the requirements of the Inverters are usually located near the batteries and close to the panel board and electrical services to which they are connected Adequate clearances and appropriate accessibility must be allowed for all electrical equipment 3 3 Selecting a System Design Tasks 3 1 thru 3 4 of the NABCEP Task Analysis The PV installer is often required to make judgments and recommendations concerning the system design based on site considerations customer needs and other factors Inevitably the installer is asked for advice on system designs so every installer should have some knowledge of PV system design considerations For that matter it is up to the installer to en

NABCEP Study Guide - Brooks Engineering

Contents

Download Pdf Manuals

Related Search

Related Contents