NEC 2011 690.47(C)(3) and subpanels

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solarscuba

Member
Location
Winters, CA
Probably an easy one but I'm hung up on a detail.

I'm reading over Mike Holt's. Understanding NEC Requirements for Solar Photovoltaic System. The examples in the
690.47(C) section show a single inverter tie-in to a main panel. What about situations where you have multiple inverters
that tie in to a separate solar subpanel? Does the combined EGC/GEC have to somehow tie directly in to the main panel where the grounding
electrode conductor and grounding electrode exist?

Thanks for any input.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Probably an easy one but I'm hung up on a detail.

I'm reading over Mike Holt's. Understanding NEC Requirements for Solar Photovoltaic System. The examples in the
690.47(C) section show a single inverter tie-in to a main panel. What about situations where you have multiple inverters
that tie in to a separate solar subpanel? Does the combined EGC/GEC have to somehow tie directly in to the main panel where the grounding
electrode conductor and grounding electrode exist?

Thanks for any input.

Yes.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
The code is kind of vague. (What is 'associated AC equipment'?)

I've always been of the opinion that if you're installing a sub, you just continue the combined EGC/GEC along to the service panel, via irreversible splices if necessary.
 

solarscuba

Member
Location
Winters, CA
Thanks. Last question. The combined EGC/GEC is sized to the larger of 250.166 and 250.122.
How would combining the grounds like that affect the sizing according to those articles?
 

Smart $

Esteemed Member
Location
Ohio
Thanks. Last question. The combined EGC/GEC is sized to the larger of 250.166 and 250.122.
How would combining the grounds like that affect the sizing according to those articles?
250.66

With GEC's (250.66), you have to base it on combined cross sectional area of the largest ungrounded circuit conductor of each supply and round up to nearest recognized wire size.

With EGC's (250.122), you just use the largest required base on each circuit's overcurrent protection rating.
 
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solarscuba

Member
Location
Winters, CA
690.47(C)(3)

".....This combined grounding conductor shall be the larger of the sizes specified by 250.122 or 250.166 and shall be installed in accordance with 250.64(E)."

What am I missing?
 

Smart $

Esteemed Member
Location
Ohio
690.47(C)(3)

".....This combined grounding conductor shall be the larger of the sizes specified by 250.122 or 250.166 and shall be installed in accordance with 250.64(E)."

What am I missing?
Nothing. You are correct it is per 250.166. My bad.

I was thinking combined GEC was per 250.66 because it is on the POCO side of the PV System AC disconnecting means.

Now I have to wonder...
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Thanks. Last question. The combined EGC/GEC is sized to the larger of 250.166 and 250.122.
How would combining the grounds like that affect the sizing according to those articles?

In my opinion there is nothing in the code which addresses the situation with respect to 250.166. What you will have is a common grounding electrode conductor with taps, which is addressed very specifically in 250.64 with respect to AC service entrances, but is not addressed in 250.166 whatsoever. 690 simply refers to 250.166. If it were me, I would simply consider the requirement to be the largest ungrounded DC conductor in any of the several systems, minimum #8. Since it would take multiple faults and some bizarre circumstances for too much DC current to flow on this conductor, I reckon that is safe.

With respect to the EGC requirement, you will have a feeder for the solar AC combiner panel. The size requirement here is simply based on the overcurrent protection for the feeder and table 250.122. (But don't forget 250.122(B) which requires you to upsize along with your ungrounded conductors if you did so for voltage drop.)

Probably for the combined GEC/EGC between the service and solar combiner, the AC EGC is the larger requirement (and IMO, absolutely sufficient to provide DC safety).
 
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