Bus rating for PV systems

earshavewalls

Senior Member
I have an ongoing discussion with a PV designer concerning bus ratings on the AC side of the inverters. (2010 California Elect. Code, based on 2008 NEC).

Setting aside 690.54(A)&(B), 705.12(D)(2) contains similar verbiage concerning not exceeding 120% of the rating of the busbar based on the ratings of the overcurrent devices in circuits supplying power to the busbar. The argument from the designer is that you can exceed the rating of the busbar as long as no branch circuits (loads) are expressed in the panel. In other words, they feel that they may use load centers as AC combiners for the output of the inverter(s) and do not need to take into account the power from the utility at the busbar(s).
I have trouble going along with this, but at the same time see what they are trying to say.....they are not adding loads to these panels, they are only passing power through them, for the most part in one direction (except for the inverter power from the utility), and they suggested signage on each of the AC load centers stating that no loads are to be installed on these panels (that option is not in the code, either).

I am leaning toward interpreting that they must consider the feeds from BOTH directions (Utility and the PV output) for each AC busbar that are connected to each power supply, even if they add signage dissallowing any loads on these panels.

Any input out there? Give me something that helps their case, if you can......otherwise, I've been right all this time.....and we can't have that!
 
This is actually something that folks have been trying to add to the NEC without success for several cycles. The thinking on the CMP is that someone can come by and add a load to the panel so you have to size it assuming that someone will. I have seen many installers try to justify using smaller ratings by saying that this panel is a "PV Combiner Only" panel and therefore only has to be rated for the PV output. That would be great but it does not comply with the NEC.
 

earshavewalls

Senior Member
I agree with both posts, thank you very much! The argument is coming from a designer with a company that is trying to get into the 'thin film' module market, so their viewpoint is quite biased.
I am not permitting what they are suggesting. I didn't get much resistance once I told them the results of my further research, which solidified my stance.

Thanks, as always, for the educated input!

Wayne
 
Another Bus Rating configuration question

Another Bus Rating configuration question

What if the solar inverter interconnects inside a wireway that is spliced upstream of a distribution panel (but still on the load side of the service disconnect)? Would the bus rating requirement still apply?

The solar inverter would terminate at a fused disconnect prior to the interconnection inside the wireway. given current would be flowing in two directions, it seems this should work even if the sum of the solar inverter fuses and the circuit breaker upstream of the wireway fed from the utility is greater than 120% of the "bus" (in my case it would be the distribution panel feeder).

If the distribution panel has a main circuit breaker, it would be impossible for the distribution panel bus to be overloaded, since the inverter ties in upstream of the distribution panel.
 

earshavewalls

Senior Member
I think that Article 705.12(D) covers all of the requirements fairly clearly:

(D) Utility-Interactive Inverters. The output of a utilityinteractive inverter shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any distribution equipment on the premises. Where distribution equipment including switchboards and panelboards is fed simultaneously by a primary source(s) of electricity and one or more utility-interactive inverters, and where this distribution equipment is capable of supplying multiple branch circuits or feeders or both, the interconnecting provisions for the utility-interactive inverter(s) shall comply with (D)(1) through (D)(7).

(2) Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.

It appears from (D) that disconnects and wireways may be exempt from the 120% requirement since they are not "capable of supplying multiple branch circuit or feeders". Any equipment (load centers) that you COULD add branch circuit breakers to would have to comply with the 120% rule if used between the inverter output and the service main.
 
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cowboyjwc

Moderator
Staff member
If you have any questions about PV systems, I would suggest that you contact this gentleman.

John Wiles
Southwest Technology Development Institute
New Mexico State University

jwiles@nmsu.edu

I have a phone number to if you would like to PM me.
 

earshavewalls

Senior Member
Hey John......I was at the latest seminar, I think it was in Alhambra (or could have been Irvine), that John Wiles taught. I was checking to see if someone knew something that I had not heard, but as it turns out, I had the right interpretation all along.

Hope to see you at another electrical or energy seminar soon. Didn't you instruct in a seminar in Simi on the 2010 CEC several months ago?

Wayne Webb
MEP Plan Check Assistant Engineer
City of Santa Clarita, CA
 

cowboyjwc

Moderator
Staff member
Hey John......I was at the latest seminar, I think it was in Alhambra (or could have been Irvine), that John Wiles taught. I was checking to see if someone knew something that I had not heard, but as it turns out, I had the right interpretation all along.

Hope to see you at another electrical or energy seminar soon. Didn't you instruct in a seminar in Simi on the 2010 CEC several months ago?

Wayne Webb
MEP Plan Check Assistant Engineer
City of Santa Clarita, CA
I was also at the one in Alhambra. I don't necessarily care for his style, but the guy does know what he's talking about.

No, I was there, but that was Vince and Steve that put on the training. I would much rather listen to those two than to me.:roll:
 
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