Ground Rod and Gauge requirements in a ground Mount Array

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pgrovetom1

Member
Location
Sonoma California
Occupation
Telecom System Engineer
I'm preparing a detailed spec for a friends system I intend to be quoted by local installers in Northern California. I've been studying the various options for inverters and batteries to spec and now I'm exploring the ground mount array itself. I want the spec to be as precise and code compliant to the extent possible so I'm comparing apples and apples. I'm looking at the grounding requirements for a ground mount array about 300ft from the home. 300ft is substantial and the cost of the wires will not be cheap given the dramatic cost increases of copper wire in recent years.

In 2008 I built a system in Sonoma County and it had 4 separate arrays with each a separate string. The 4 strings returned to 2 SMA 6000 Inverters along with a #10 green ground wire. That green ground wire tied into the equipment ground and into the homes main 8 foot ground rod located near the inverters and main panel. I also drove 8 foot ground rods at each array of 4 and strung bare #6 to every panel and metal component ( 2 1/2 galvanized pipe and struts) with appropriate ground lugs. The 4 arrays were electrically ( except by the ground rods earth path) not connected even though they were about 4 feet spaced. In 2008 the local AHJ approved this arrangement. I'm now 72 and am unable to build this myself so I'm trying to create a package spec so my quotes are for the same thing as close as possible.

My friends home has a large field where I was intending to run one linear rack to carry 4 sets of panels with their 4 strings with 2 sets combined on red/black #10 in conduit carried about 300 ft to the home inverter. I was planning on including one #10 green THWN-2 ground from the combiner along with the string wires back to the main inverter at the home main panel and tied to the ground rods for the home. The linear rack ( not in spaced sections) will have a #6 bare copper wire tying all the panels together and metal parts ( as not provided by panels or racks) and return to the combiner box. There it will connect to the #10 green wire being carried back to the home and its main ground rod.

Is this done correctly and a sufficient description for a bid spec?

Since 300 ft is a long distance and #10 wire is not cheap, I want to be sure I'm using the right number of wires and gauge to meet requirements. Once I have the spec and have chosen all my components spec sheets and think its right, I will sit down with a local AHJ inspector and make sure its ok or make any adjustments needed. Then I plan on giving it to some local installers for quotes and feedback. Any feedback if this is a good strategy and if the ground mounting array grounding scheme is appropriate for a California installation is appreciated.

thanks.
 
The 10awg equipment ground between array and house is fine. You do not mention an electrode at the array; that is required, although it could possibly be one or more of the steel supports.

Other things that have changed since 2008 are:
- rapid shutdown, but that shouldn't apply if the inverter is outside the home or if you can put a disconnect where the DC conductors reach the home
- some AHJs may be more likely to require a fence around the array or other guards to comply with 690.31(A).
 
Yes I was planning on specifying 2 eight foot ground rods at the array separated by feet and tied together by the same #6 bare copper wire that ties all the panels and metal parts together in one continuous length. This would also tie to the combiner box and at that point transition to a #10 green THWN-2 stranded conductor that travels back through the underground conduit to the inverter where its tied to the home ground rod near the main panel.

My reading of the NEC Rapid Shutdown requirements for a ground mount system suggests that if the string conductors pass through part of the home for the purpose of only going to the inverter, the requirements for Rapid Shutdown are waived.

"Exception: Ground-mounted PV system circuits that enter buildings, of which the sole purpose is to house PV system equipment, shall not be required to comply with 690.12."

This doesn't make sense to me as any high voltage wires that transition from underground conduit to under the house (my case), even if in conduit, any distance from the entry could pose a risk to fire fighters. Is my reading correct? Thinking out loud: I would think there should be a device located at the physical string entry as it exits the underground conduit to the home that would act as a DC disconnect for string wiring marked with a big sign for fire fighter use. A DC disconnect at the inverter located in a garage ( my case) might be inaccessible or actually be the fire location. Its purpose would be similar to the home's main electrical cutoff switch or gas shutoff valve so any fire fighter arriving at the house would automatically disconnect all dangerous electrical or gas entering the home before attacking the fire.
 
pgrovetom1 said:
...

My reading of the NEC Rapid Shutdown requirements for a ground mount system suggests that if the string conductors pass through part of the home for the purpose of only going to the inverter, the requirements for Rapid Shutdown are waived.

"Exception: Ground-mounted PV system circuits that enter buildings, of which the sole purpose is to house PV system equipment, shall not be required to comply with 690.12."
Click to expand...

No, it's the building (not the conductors or raceway) whose 'sole purpose' must be 'to house PV system equipment.' A home is not such a building.

I would think there should be a device located at the physical string entry as it exits the underground conduit to the home that would act as a DC disconnect for string wiring marked with a big sign for fire fighter use. ...
Click to expand...
Such a switch would deal with the rapid shutdown requirement as long as it is on the *exterior* of the home and labeled "RAPID SHUTDOWN SWITCH FOR PV SOLAR SYSTEM". Bringing an underground DC conduit directly up into the interior of the home is problematic and I don't see how it could be compliant.
 
jaggedben said:
- rapid shutdown, but that shouldn't apply if the inverter is outside the home or if you can put a disconnect where the DC conductors reach the home
Click to expand...
I don't see how a DC disconnect excludes the conductors on the non-PV side of that disconnect from needing to comply with 690.12. Those conductors would still be "PV system DC circuits".

Cheers, Wayne
 
wwhitney said:
I don't see how a DC disconnect excludes the conductors on the non-PV side of that disconnect from needing to comply with 690.12. Those conductors would still be "PV system DC circuits".

Cheers, Wayne
Click to expand...
It doesn't exclude them, it provides the switch to de-energize them.
It's true there may be functional issues with a strict compliance depending on the inverter.
 
jaggedben said:
It doesn't exclude them, it provides the switch to de-energize them.
Click to expand...
Ah, right, that makes sense, and deenergizing the conductors inside the house seems like it satisfies the intent of rapid shutdown with respect to a ground-mount array.

But doesn't the wording of 690.12(A)(1) mean that all the PV system DC conductors all the way back to the array are controlled conductors? Or are you saying that the first sentence of 690.12 effectively modifies 690.12(A)(1) to be "PV system circuits installed on or in buildings"?

Thanks,
Wayne
 
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