Rooftop Community Solar and 250.58

[pvgreeze]

Hello everyone, I'm curious to know if anyone has any thoughts/experience with a rooftop community solar installation and interpreting the needs for a common grounding electrode system per NEC 250.58. I've come across a number of high level designs where a community solar project is proposed on an existing building with its own service, but the community solar is interconnected either on the low voltage bushings of the existing utility pad mounted transformer or via a completely separate service and new pad mounted transformer. Therefore, while the building itself is shared, the community solar and the building distribution exist as completely separate services.

I personally don't think there is much ambiguity in 250.58; the PV system inverters and combiner panel ("ac system") is located "in or on that building or structure;" therefore, a connection must be made between the PV equipment grounding electrode system and the existing grounding electrode system. 250.58 specifically calls out "Where separate services...are required to be connected to a grounding electrode(s), the same grounding electrode(s) shall be used." 250.58 contains the language both "in or at a building or structure" and "in or on that building or structure" which is confusing, especially "at" versus "on," but, beyond just the legalese of the NEC, I think the intention is quite clear regardless.

Lastly, assuming NEC 250.58 requires that a community solar PV system be connected to the existing building grounding electrode system, I'm curious thoughts on the installation requirements mandated by 250.64 for the GEC. This scenario would presumably follow under 250.64(D), and the installation would need to fall under 250.64(D)(1/2/3). I would interpret 250.64(D)(2)(2) as allowing the GEC to connect the existing grounding electrode system to the PV grounding electrode system via the use of the EGC/EG busbar at any point in the PV system circuit, and not specifically at the service disconnect (covered by 250.64(D)(2)(1)). I am interested in this use due to the fact that a project I am reviewing has the service disconnect located across a driveway from the building, and tying the PV grounding electrode to the AC combiner panel would be far more practical, as that panel is located on the building.

Any thoughts or feedback is appreciate. Thanks!

 

[jaggedben]

First of all, if you haven't looked at 690.47, look at it.

I think perhaps you are overthinking this. You don't need a PV grounding electrode system, you use the same GES as for the structure. Both 250.58 and 690.47 (effectively) say this. If your PV connection is on the load side of a (properly grounded) service then it only needs an EGC. If it is using a separate service disconnect enclosure (either an additional disconnect on the same service, or an additional service) then 250.64(D) comes in as you've noted; treat it the same way as if the disconnect was for a load.

The one thing that's 'interesting' about your post is the 'service disconnect located across a driveway from the building'. In that case it sounds like you need a GES at two locations. But I'm unclear if that's the existing service disco, or the new PV disco, and whether they are both across the driveway or one is on the building.

 

[pvgreeze]

I think perhaps you are overthinking this. You don't need a PV grounding electrode system, you use the same GES as for the structure. Both 250.58 and 690.47 (effectively) say this. If your PV connection is on the load side of a (properly grounded) service then it only needs an EGC. If it is using a separate service disconnect enclosure (either an additional disconnect on the same service, or an additional service) then 250.64(D) comes in as you've noted; treat it the same way as if the disconnect was for a load.

Thank you for the thoughtful reply. I was more focused on 250.58 & 250.64 because 690.47 redirects to Part III of Article 250. Between 250.58 & the opening line of 690.47, as you mentioned, answers my first question. I was especially confused because the developer sent me a reference set of drawings that did not have utilizing the same GES called out, and I wanted to make sure I wasn't missing some 'Exception' or something. But yes, this isn't a load side or even a line side BTM system; it is a completely separate service and even a separate utility transformer (from the same MV line as the existing service); otherwise, I agree it would be straightforward as you mentioned.

Regarding the second point, that is where I'm still confused. I figured bond the neutral and the ground at the new PV disco (utility is doing/owning all service conductors between the new XFMR and the new PV disco), and run the EGC to the building mounted AC combiner panel. I figured the best course of action would be a ground rod at the new PV disco and (as required by 690.47 and 250.58) tie the EGC busbar in the AC combiner panel to the GES for the building that serves the existing (and completely isolated as far as the PV system is concerned) service. This method would satisfy 250.58/690.47(A), would utilize 250.64(D)(2)(2) for GEC installation to the GES, and would satisfy the requirements of 250.24 regarding grounding the new service DS.

Thanks again for any insights!

 

[jaggedben]

...
Regarding the second point, that is where I'm still confused. I figured bond the neutral and the ground at the new PV disco (utility is doing/owning all service conductors between the new XFMR and the new PV disco), and run the EGC to the building mounted AC combiner panel. I figured the best course of action would be a ground rod at the new PV disco and (as required by 690.47 and 250.58) tie the EGC busbar in the AC combiner panel to the GES for the building that serves the existing (and completely isolated as far as the PV system is concerned) service. This method would satisfy 250.58/690.47(A), would utilize 250.64(D)(2)(2) for GEC installation to the GES, and would satisfy the requirements of 250.24 regarding grounding the new service DS.
...

Yes, this sounds right to me. I'm reading you as saying the new PV disco will be across the driveway but the existing service disco for the building is on the building.

690.47 doesn't require the rod at the disco, it requires the GEC connection from the combiner (or other appropriate spot on the PV circuit EGC) to the building GES. So does 250.32.
The rod(s) at the service disconnect are required by 250.24(A).

 

[pvgreeze]

Yes, this sounds right to me. I'm reading you as saying the new PV disco will be across the driveway but the existing service disco for the building is on the building.

Thanks! Yeah, sorry if my descrption is confusing. I've never been to the site, but I know the existing service disco is inside of the building and the community solar disco will be outside of the building across this driveway that goes around the perimeter. I appreciate all the feedback!