Question on grounding rod for solar PV over two structures on the same property

[brycenesbitt]

In the case of a single PV system spread over two buildings, what's the
right thing for code or best practice, regarding grounding or bonding the upstream building?
Attached is a diagram.
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The upstream building has nine panels on micro-inverters, with an AC disconnect on the side of the building, then plastic conduit to a combiner. The two systems join at the combiner. The interconnecting conduit is specified as plastic.
This system will never connect to the existing MSP (main service panel), instead get it's own backfeed meter.
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My question is on code or best practice for grounding/bonding.
Should there be a ground rod at the AC disconnect on each building, or just a single ground rod at the combiner or MSP?
And would that AC disconnect ground rod if installed be better for lightning or for clearing electrical faults?

 

[jaggedben]

One grounding electrode system per building. The PV array EGC is connected at each building to a GEC going to the grounding electrode system at each building. The grounding electrode system is shared with any other power sources at each building.

For example, if the PV system at House 1 has its own service disconnect then see 250.64 (D) and choose a method for grounding all service disconnects. If the PV is a separate feeder to House 2, it needs a disconnecting means there; connect a GEC to the groundbar in the disconnect and run it to the same electrodes as might be there already.

In case it needs to be said, an EGC is also run with all solar circuit conductors, including between buildings. Grounding electrodes do NOT clear ground faults here.

This is what code requires. I can't say it will help with lightning issues or even won't make those issues worse, actually, (but if you want true lightning protection that's a whole different ball of wax and you will talking to a lightning protection company).

 

[brycenesbitt]

One grounding electrode system per building. The PV array EGC is connected at each building to a GEC going to the grounding electrode system at each building. The grounding electrode system is shared with any other power sources at each building. see 250.64 (D) and choose a method

That sounded right at first, but it's not a direct match:

250.64(D)Building or Structure with Multiple Disconnecting Means in Separate Enclosures. If a building or structure is supplied by a service or feeder with two or more disconnecting means ...
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House #1 is not fed by the PV at all, so even with solar it is "supplied" with just one disconnecting means.
If "supplied" has the meaning of electrical flow.
House #2 is not feed by the PV either: all electrcal flow goes to a generation meter and off to the POCO, any energy feeding the property is metered again on the way back in.
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There are TWO separate POCO drops:
POCO Drop->Cluster Meter for four homes-> feed in conduit to home #1.
POCO Drop->Cluster Meter for two homes, located at home #2.
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The AHJ approved the plans showing the above treating home #1 as a passive support structure.
The field inspector had nothing to add. Thus I'm asking here to try and work out to:

• Say nothing, let it be installed isolated.
• Ask for a separate ground rod at home #1, presumably to reduce resistance to ground in a lighning strike or whatever.
• Ask to run a ground connection under home #1, through it's conduit back to the cluster meter on yet another building.

 

[brycenesbitt]

The generated electricity connects ahead of every meter on the property,
directly to the drop (a utility side tap). Physically the two homes simply support the equipment, having no other electrical connection.

The question is what code section if any regulates the ground. The Green Book is curiously silent on this topic: such as if neutral and ground should connected at the same point as L1 and L2, or....

 

[jaggedben]

Meters don't matter at all whatsoever. The NEC doesn't give a f--- about how things are metered.

'Supplied' means energized by the utility, as it normally does in chapter 2. Forget which direction energy may flow from the PV system. Pretend it's a load for the purposes of interpreting 'supplied' or 'fed'.

I'm unclear at which house the PV system connects the utility, but whichever it is, you have multiple service disconnects as shown in your last drawing. 250.64(D) applies there.

At the other house, whichever it is, I presume you have loads, so you have either
- a separate service for the loads plus the PV system feeder, or
- a load feeder from somewhere else, plus the PV system feeder
Either way, you still have a "Building or Structure with Multiple Disconnecting Means in Separate Enclosures". So 250.64(D) applies there, too. But you can’t use the 'Common Location' option there because there's no grounded conductor common to both supplies. Anyway, 690.47 requires that the EGC from the PV array be connected to the GES at that building and 250.64(D) gives you some guidance on how to do that.

It's good if the Greenbook stays out if the electrical code's business. About the only good thing I can say about PG&E is they don't have stupid grounding requirements of their own and they let the AHJ enforce the electrical code as they see fit, so far as I'm aware.

 

[brycenesbitt]

I'm unclear at which house the PV system connects the utility, but whichever it is, you have multiple service disconnects as shown in your last drawing. 250.64(D) applies there.

Here's a better scribble (um "single line diagram"), per solar contractor's plans, as approved by AHJ. Six units over three buildings numbered here 1 2 and 3:

 

[jaggedben]

Okay.
So at building 3 follow 250.64(D), (or maybe the local AHJ's preference for grounding and bonding a solar supply side connection).

At building 2 a grounding electrode conductor should be run from the Ac disconnect to the grounding electrodes for MSP 2.

(They are really letting you do VNEM for units fed by multiple services?)

 

[wwhitney]

At building 2 a grounding electrode conductor should be run from the Ac disconnect to the grounding electrodes for MSP 2.

Can you provide a code reference for that?

Doing that will connect the neutrals from the lefthand POCO service and the right hand POCO service. My first reaction is that wouldn't be good, but then I guess there are large buildings with multiple services where that happens, so I guess it's probably just another example of not-so-objectionable current.

Thanks,
Wayne

 

[jaggedben]

Can you provide a code reference for that?

690.47

Doing that will connect the neutrals from the lefthand POCO service and the right hand POCO service. My first reaction is that wouldn't be good, but then I guess there are large buildings with multiple services where that happens, so I guess it's probably just another example of not-so-objectionable current.

Interesting point. It also depends whether the lefthand and righthand services are powered from the same transformer, which the installer might not know.

 

[brycenesbitt]

Can you provide a code reference for that?
Doing that will connect the neutrals from the lefthand POCO service and the right hand POCO service. My first reaction is that wouldn't be good, but then I guess there are large buildings with multiple services where that happens, so I guess it's probably just another example of not-so-objectionable current.

Thanks,
Wayne

That's my concern as well. What weird cross current could result?
Had the cross conduit between building #2 and building #3 been metallic, that would have formed an additional ground connection for lighting, which seems Ok. But that's why I'm asking here: what are the code implications, the risks, the tradeoffs. What could go wrong with each configuration. Here I'd rather focus on function not code.

 

[brycenesbitt]

Interesting point. It also depends whether the lefthand and righthand services are powered from the same transformer, which the installer might not know.

I'm pretty sure they are different transformers. But that said I don't care to find out, because it could change over time and it's the POCO's business not mine....

 

[brycenesbitt]

(They are really letting you do VNEM for units fed by multiple services?)

The CPUC tariff is very specific. The POCO's don't want to do this at all, so they are excluding any borderline cases.

The tariff applies to meters on adjacent properties under the same ownership. No telling if a corner parcel touch is enough . But a cluster of parcels all sharing a common boundary would count, no matter how they are fed electrically. This particular instance is a single parcel owned by an HOA, with individual unit ownership. Some decades ago power was insufficient to the parcel so new POCO service was brought in from a different direction.

 

[jaggedben]

I had recalled that PG&E insisted all meters served under a VNEM agreement had to be on the same service. But, that could be out of date, and also could have been PG&E 'going rogue' with their own interpretation that isn't quite in line with actual CPUC regulation. (Wouldn't be the only time I've seen that.)

 

[wwhitney]

690.47

Not quite seeing it: that section requires connection of the PV array EGC to "a" GES, without actually specifying that it connect to the same GES as non-PV wiring on the structure or building supporting the array. The array on building 2 is already connected to building 3's GES.

It also depends whether the lefthand and righthand services are powered from the same transformer

I'm under the impression that when the utility primary is a wye system, the utility connects the primary neutral to the secondary neutral at its transformers. In which case if you have two such utility transformers on the same primary wye system, their secondary neutrals will be interconnected via the primary neutral.

Cheers, Wayne

 

[jaggedben]

Not quite seeing it: that section requires connection of the PV array EGC to "a" GES, without actually specifying that it connect to the same GES as non-PV wiring on the structure or building supporting the array. The array on building 2 is already connected to building 3's GES.

That's an interesting take. What would be the point of requiring a GES if the PV array isn't required to be connected to it? By your reasoning, in a scenario where a structure (say a carport) has a PV array but no other electrical power, I must install electrodes at the carport ... but I can leave them unconnected to anything if the PV array is ultimately connected to a GES at a service at another structure?

Also if 690.47 doesn't do it for you, maybe explain why 250.32(B) should not apply.

That said, I agree that the code is not well thought out here, and that the PV system EGC probably actually shouldn't be grounded at building 2? Ordinarily (if everything were loads, that is) something like this situation would most often be prohibited by 230.2 and such. But the allowance in 230.2(A)(5) clearly allows the separate service connection for a PV solar source like this. As you say, the code allows multiple services to a building to be tied to the same GES in some situations. However in most of those situations each service would be directly grounded to the GES, and the requirements for GECs and bonding jumpers would require larger minimum conductors, and/or likely be made through building steel. Here, the connection between the two GES could theoretically be as small as a 14awg EGC and that doesn't seem good.

 

[jaggedben]

That's my concern as well. What weird cross current could result?
Had the cross conduit between building #2 and building #3 been metallic, that would have formed an additional ground connection for lightning, which seems Ok. But that's why I'm asking here: what are the code implications, the risks, the tradeoffs. What could go wrong with each configuration. Here I'd rather focus on function not code.

I don't think it makes a very meaningful difference if the conduit between buildings is metallic or not. Either way you have an EGC with your circuit conductors that is a path between exposed equipment on the two buildings. A metal conduit will be bonded to that.

Regarding lightning, my understanding is that you don't really want a conductor (EGC or neutral) to be grounded at two different structures because a lightning induced voltage gradient in the soil could induce a current over that conductor that damages it and maybe stuff around it. However, the code requires that grounding (see 250.32, again), apparently to avoid shock hazard from the human engineered electrical power source. So it's a tradeoff. Higher stakes in areas with a lot of lightning, I guess, which is not where I am.

 

[wwhitney]

That's an interesting take. What would be the point of requiring a GES if the PV array isn't required to be connected to it? By your reasoning, in a scenario where a structure (say a carport) has a PV array but no other electrical power, I must install electrodes at the carport ... but I can leave them unconnected to anything if the PV array is ultimately connected to a GES at a service at another structure?

I guess the first sentence of 690.47 would require you to use the carport GES to earth the PV array's EGC, as it calls for the GES to be "utilized." It just seems like 690.47 goes to a little bit of trouble to avoid explicitly saying that you have to utilize that GES for the PV array earthing. But in your example, there's no other way to utilize it, unlike the OP's drawing.

Also if 690.47 doesn't do it for you, maybe explain why 250.32(B) should not apply.

I'll take 250.32(B) as a good answer to my original question as to why you indicated that Building #2's GES should be connected to the PV array's EGC.

On the other hand, if I strongly felt like that connection was a bad idea, then I'd start thinking about whether the PV conductors between Building 2 and Building 3 constitute a "supply" to Building 2 that would trigger 250.32.

I'm inclined to say they don't, but then I'd have to think about all the other ramifications of that position. And I'm not that convinced that interconnecting the neutrals of the Building 1 service and the Building 3 service is that bad, seeing as how I understand there are plenty of other installs where something similar will occur.

Cheers, Wayne

 

[ggunn]

I had recalled that PG&E insisted all meters served under a VNEM agreement had to be on the same service. But, that could be out of date, and also could have been PG&E 'going rogue' with their own interpretation that isn't quite in line with actual CPUC regulation. (Wouldn't be the only time I've seen that.)

Maybe I am missing something, but aren't separate revenue meters by definition on separate services?

 

[jaggedben]

Maybe I am missing something, but aren't separate revenue meters by definition on separate services?

No, they are most certainly not. A service is a single circuit connection to a utility. That connection may be branched to multiple service disconnects that each have a meter.
See for example: https://www.gordonelectricsupply.co...er-Mp-Device-Ringless-125A-6-Position/5711484

Additionally, the NEC has a definition of a service and the NEC doesn't give a f--- if it's metered or not. The NEC prohibits multiple services to a building in most cases, without regard to how many occupancies one might wish to be separately metered. Every building with multiple meters that I've looked at closely had one service.

 

[brycenesbitt]

My takeaway from the above is not seeing a compelling electrical safety reason to get involved questioning using building #2 as a passive mounting rack for the solar. Here I have the choice of saying nothing....
I'd feel better if building #2 had a lighting focused ground rod.