Code Corner is presented as a courtesy of Jerry Flaherty, Chief Electrical Inspector, Electrical Inspection Service, Inc. Jerry is a certified electrical inspector, a NABCEP certified solar PV installer, licensed NYS teacher and a former licensed master electrician. Jerry teaches photovoltaic and National Electrical Code topics at several colleges, universities and professional training centers. Jerry is heavily involved in LISEIA, is a board member of the LISEIA steering committee.

General

What electrical code applies to my area?

How much clearance do I need for PV equipment?

Are surge-protective devices required in PV systems?

Can I connect my PV system to the stand-by generator?

Calculations

How do I calculate the maximum voltage for a string inverter and single or multi-crystalline modules?

How do I calculate the system voltage for a string inverter using a coefficient?

How do I calculate the current for two strings in parellel?

How do I calculate a string wire size?

How do I calculate the current rating of a fuse or circuit breaker?

How do I calculate the AC circuit breaker for the inverter output?

Grounding

I have a commercial PV system with six 10kW inverters. What size does the grounding electrode conductor (GEC) have to be? M. A.

I used a #8 AWG for my grounding electrode conductor (GEC) and the inspector gave me a violation for it not being protected. What gives? M. A.

I ran my grounding electrode conductor in EMT and the *&%@ inspector said it was no good. He suggested that I change it to PVC conduit. Now what's the problem? M. A.

Can I connect the PV system GEC to the service neutral/ground bar?

Can I make a tap on the AC service GEC for the PV DC GEC?

Are grounding clips, WEEB and other type,permitted for bonding PV systems? If so, how many are required on a panel?

How do I size the equipment grounding conductor (EGC)?

Can I use a crimp connector on the module equipment grounding conductor (EGC)?

Disconnects

Service

What electrical code applies to my area?


As of December 28, 2010 New York State will be going to the 2010 Unified Fire and Building Code requiring all one and two family dwellings and town houses to use the 2008 National Electrical Code. All other types of buildings will also be using the 2008 National Electrical Code. It is permitted to used the 2008 NEC before this date.

New York City- has its own building code that is also based on the 2008 National Electrical Code with several amendments including the requirement that all PV systems must be listed by a NRTL as a system.



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How much clearance do I need for PV equipment?


The requirements for PV electrical equipment are the same as for all electrical equipment.

110.26(A) requires that working space 30 inches wide or the width of the equipment by 6 ½ feet high and 3 to 4 feet from the front of the equipment depending on the voltage of the equipment.
110.3(B) requires that the manufactures instruction be followed; if the manufactures instructions require additional clearance then that additional clearance must be provided.
110.26(D)requires adequate lighting to service the equipment.
110.26(F) requires dedicated space above and below equipment for use exclusively for the electrical equipment associated with the PV system.



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Are surge-protective devices required in PV systems?


No. Surge-protective devices are primarily used to mitigate line surges usually associated with power quality and lightning surges. Long Island has a low rate of lightning.

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Can I connect my PV system to the stand-by generator?


Yes, your PV system can be connected to a stand-by generator but the generator must be listed for use with PV systems. A better method is to connect the stand-by generator to only the service and forgo the small amount to electricity that the PV system will deliver.

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How do I calculate the maximum voltage for a string inverter and single or multi-crystalline modules?


690.7(A) NEC requires the maximum voltage be calculated by multiplying the open circuit voltage (Voc) of the modules by the voltage correction factor (table 690.7) (vcf) by the number of modules in series (Nmod).



Example #1 Twelve modules (Nmod) with an open circuit voltage (Voc) of 36.5 volts DC and the lowest expected temperature (tlow) of -7° F.
NEC table 690.7 = tlow -7° F = 1.20 vcf



Voc	x	vcf	x	Nmod	=	VNECmax
36.5 v DC	x	1.20	x	12	=	VNECmax
		526.6 volts DC			=	VNECmax



Note the 2008 NEC requires using the temperature coefficient if available. See coefficient?
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How do I calculate the system voltage for a string inverter using a coefficient?



690.7 require using the open circuit voltage temperature coefficient (Vockt) when available.
Manufactures list the coefficent Vockt in mili-volts per °C rise as in example #2 or percentage per °C rise as in example #3.



Example #2 Twelve modules (Nmod) with an open circuit voltage (Voc) of 36.5 volts DC, coefficient (Vockt) of -131mv/°C and the lowest expected ambient temperatur (tlow) of -7° F.



Convert °F to °C		°C = (°F - 32°) x 5/9 = (-7 - 32°) x 5/9 = -22°C tlow
Convert mV to volts		Vockt of -131mV / °C = -0.131volts / °C





(	Voc	+	((	tlow	-	STCC°	)	x	Vockt	))	x	Nmod	=	VNECmax
(	36.5 VDC	+	((	-22°C	-	25°C	)	x	-0.131 v/°C	))	x	12	=	VNECmax
(	36.5 VDC	+	(		-47°C			x	-0.131 v/°C	))	x	12
(	36.5 VDC	+				6.156 VDC				)	x	12
				42.656 VDC							x	12
						511.9 volts DC							=	VNECmax



Example #3 Twelve modules (Nmod) with an open voltage (Voc) of 36.5 volts DC, coefficient (Vockt) of -0.346% / °C and the lowest expected ambient temperatur (tlow)of -22° C



(	Voc	+	(((	tlow	-	STCC°	)	x	Vockt	)	x	Voc	))	x	Nmod	=	VNECmax
(	36.5 VDC	+	(((	-22°C	-	25°C	)	x	-0.346%/°C	)	x	36.5 VDC	))	x	12	=	VNECmax
(	36.5 VDC	+	((		-47°C			x	-0.346%/°C	)	x	36.5 VDC	))	x	12
(	36.5 VDC	+	(			16.309%					x	36.5 VDC	))	x	12
(	36.5 VDC	+							5.95 VDC				)	x	12
									42.45 VDC					x	12
									509.4 VDC							=	VNECmax

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How do I calculate the current for two strings in parellel?


The maximum current (INECmax) of one string is 125% of the short circuit of the string, so the maximum current of two strings is two times the one string.
To calculate the conductor current click.

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How do I calculate a string wire size?


To calculate the conductor size of one string of modules multiply the short circuit current (Isc) by 156% then divide by the temperature correction factor (tcf) at the bottom of table 310.16.(90 °C column)



Example #4 The short circuit of the string is 8 amps and the ambient temperature is 110 °F.



Isc	x	156%	/	tcfa	=	Icond
8 amps	x	1.56	/	0.87	=	14.34 amps	(less the .5 can be dropped)
		14 amps			=	Icond





To calculate the conductor size for two or more strings (Nstg), multiply the short circuit current (Isc)of one string by 156% plus the short circuit current (Isc)of the remainding strings multiplied by 125% then divide by the temperature correction factor (tcf) at the bottom of table 310.16.(90 °C column)



Example #5 Five strings, the short circuit of each string is 8 amps and the ambient temperature is 110 °F.



((	Isc	x	156%	)	+	((	Isc	x	125%	)	x	Nstg	))	/	tcfa	=	Icond
((	8 amp	x	156%	)	+	((	8 amps	x	125%	)	x	4	))	/	0.87	=	Icond
(			12.48 amps		+	(			10 amps		x	4	))	/	0.87	=
(			12.48 amps		+				40 amps				)	/	0.87	=
									52.48 amps					/	0.87	=	60.32 amps
									60 amps							=	Icond




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How do I calculate the current rating of a fuse or circuit breaker?


690.7(B)(1) requires that the overcurrent device rating (Iocpd) be at least 125% of the maximum current (INECmax) calculated in 609.7(A). So the rating can be calculate two ways. First, multiply the maximum string current (INECmax) by 125% or multiply the string short circuit current (Isc) by 156%.

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How do I calculate the AC circuit breaker for the inverter output?


The minimum rating for the AC circuit breaker is the inverter output wattage divided by the nominal voltage the multiplied by 125%. The maximum rating of the circuit breaker in listed on the name plate of the inverter.

Example #8 What is the circuit breaker and conductor rating of a 4,000 watt inverter (Pac)? The line voltage is 240 volts (Vac).

(	Pac	/	Vac	)	x	125%	=	Iolpd
(	4,000 watts	/	240 VAC	)	x	125%	=	20.8 amps

240.4(A) permits the next higher standard fuse or circuit breaker. The minimum size circuit breaker is 25 amps.
The maximum size circuit breaker would be in the manufacture manual or not to exceed 175% of the current. Using the formular above, the maximum circuit breaker would be have to be smaller the 28.3 amps or also 25 amps.


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I have a commercial PV system with six 10kW inverters. What size does the grounding electrode conductor (GEC) have to be? M. A.


According to 250.166 the DC GED has to be a minimum of #8 AWG or the same size as the largest DC conductor in the system. If all the DC conductors are #8 AWG or smaller then you can use a #8 AWG for your grounding electrode conductor.

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I used a #8 AWG for my grounding electrode conductor (GEC) and the inspector gave me a violation for it not being protected. What gives? M. A.


250.64(B) states that a GEC smaller then a #6 AWG shall be protected by RMC, IMC, RNC (PVC), EMT or cable armor.

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I ran my grounding electrode conductor in EMT and the *&%@ inspector said it was no good. He suggested that I change it to PVC conduit. Now what's the problem? M. A.


The inspector should have explained the code requirement of 250.64(E) to you. 250.64(E) states that "When ferrous metal enclosures are not physically continuous from cabinet or equipment to the GEC shall be made electrical continuous by bonding each end of the raceway to the GEC". You could have just put a jumper (same size as the GEC) from bonding bushings on the EMT to the GEC using a split bolt tap. Other options would be running the #8 AWG GEC in PVC or using a #6 AWG GEC.

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Can I connect the PV system GEC to the service neutral/ground bar?


No. 690.47(B) of the 2002 NEC requires that the GEC be installed as per 250.64. 250.64(C)requires that the GEC be "continuous without a splice or joint, unless spliced only by irreversible compression-type connector… or exothermic weld" The 2005 NEC added 690.47(C) for PV system with both AC and DC grounding requirements. This allowed for two grounding methods: (1) a separate grounding electrode (ground rod) for the PV DC GEC and then bonded to the AC grounding electrode. (2) The DC GEC and the AC GEC connected to a common grounding electrode.

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Can I make a tap on the AC service GEC for the PV DC GEC?


No. 690.47(B) NEC requires that the GEC be installed as per 250.64. 250.64(D) allows only AC service to have the GEC taped.

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Are grounding clips, WEEB and other type, permitted for bonding PV systems? If so, how many are required on a panel?


WEEB's and other grounding clips are listed for bonding several rail systems and modules, but not for all rail systems and not for all modules. Refer to the manufactures for rail system and modules that are listed.
How many clips are required also depends on the clip, the rail system and the module. Again refer to the manufactures literature.

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How do I size the equipment grounding conductor (EGC)?

250.122(A) requires that the EGC be sized using the circuit overcurrent rating as applied to table 250.122. 250.122(B) requires that the EGC be proportionally increased in size if the circuit conductors are increased in size. Usually PV systems increase the circuit conductor for the elevated ambient temperature.
Rule of thumb is to use the same size EGC as the string conductors.
If the circuit conductors are increased in size for voltage drop then the EGC does not have to be increased in size. This might apply to a low voltage PV system but usually not to grid-tied systems since the conductors are increased in size for temperature correction factor per 310.16 not for voltage drop.
#14 AWG is the minimum size equipment grounding conductor allowed.

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Can I use a crimp connector on the module equipment grounding conductor (EGC)?

Yes, but the crimp connectors must be listed for the voltage and for a wet location. Automotive type crimp connecteos are not listed for the voltage or for a wet location.

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LISEIA and Jerry Flaherty do not make code recommendations. The contents of this page are the code interpretation of Jerry Flaherty. LISEIA and Jerry Flaherty assume no responsibility for the use of this information. These code interpretations may differ from the local AHJ (electrical inspector) and the local AHJ has the final say as to what is code compliant.