Hmmm. That's not the way I understood it. I could be wrong.
690(B)(2): 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. In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors. The way I read it, both the inverters and the feeder circuit are supplying power to the conductor between the main and subpanels. The sum of the OC protection is 100A + 80A = 180A per leg. Therefore, the conductor must be rated for 180A / 1.2 = 150A. Maybe this changes if there are no loads in the subpanel, but I don't think so. In many cases we have a circuit for an energy monitoring device in the subpanel, so we almost always have loads anyway. Another way to read it would be that ONLY the inverter circuits shall be used for calculations, in which case the conductors would need to be 100A / 1.2 = 83.3A. However, I don't think that is the intent. While on the subject, I always wondered about that Lennox "Solar A/C" system that uses Enphase backfed through the A/C compressor circuit. It seemed like the conductor would need to be upgraded. Jason From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Mark Frye Sent: Wednesday, March 30, 2011 11:36 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] sizing a sub-panel used tocombinemultipleinverteroutputs Jason, I think, going back to the diagram in the article, the feeder breaker is rated at 80A, the feeder conductors are rated at 80A, and the subpanel is rated at 80A, assuming that there are no loads connected in the subpanel. Even if there were up to 100A of loads in the subpanel, you would not exceed the allowance for the feeder or the sub-panel. Still, if you do have loads in the subpanel, you have to up sizes the sub-panel, but not the feeder conductor. Mark Frye Berkeley Solar Electric Systems 303 Redbud Way Nevada City, CA 95959 (530) 401-8024 www.berkeleysolar.com <http://www.berkeleysolar.com/> ________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Jason Szumlanski Sent: Wednesday, March 30, 2011 5:18 AM To: RE-wrenches Subject: Re: [RE-wrenches] sizing a sub-panel used tocombinemultipleinverter outputs Another key is to remember that this discussion also applies to the conductor between the main panel and subpanel. In a large PV system, this could result in a pretty large wire between the two panels, and a significant cost that is often overlooked. In some cases it makes sense to locate the subpanel close to the main panel and run multiple sets of smaller wires from the inverters to the subpanel. And because the calculation is based on the first OC protection connected to the inverters, adding a main breaker (theoretically 80A in this example) in the subpanel doesn't change things. Even though this wire would be theoretically protected by an 80A breaker at each end, you can't size the wire for 160A / 1.2 = 133.3A. You have to size for 180A/1.2 = 150A. (not that it makes much of a difference in this example, but it still must be considered) At least that's how I understand it... Jason Szumlanski Fafco Solar From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Mark Frye Sent: Wednesday, March 30, 2011 1:33 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] sizing a sub-panel used to combinemultipleinverter outputs Opps! My bad, I was thinking of a single phase system, not the three phase system shown in the article. For the three phase system Kent is correct in counting 180A of supply per bar. Mark Frye Berkeley Solar Electric Systems 303 Redbud Way Nevada City, CA 95959 (530) 401-8024 www.berkeleysolar.com <http://www.berkeleysolar.com/> ________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Mark Frye Sent: Tuesday, March 29, 2011 10:17 PM To: 'RE-wrenches' Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multipleinverter outputs I think Kent and I agree. For the case where the subpanel is not dedicated a PV sub-panel he is calculating for 2 - 50A breakers and I calculated for 3 - 50A breakers. Mark Frye Berkeley Solar Electric Systems 303 Redbud Way Nevada City, CA 95959 (530) 401-8024 www.berkeleysolar.com <http://www.berkeleysolar.com/> ________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Kent Osterberg Sent: Tuesday, March 29, 2011 9:26 PM To: RE-wrenches Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multiple inverter outputs Per 705.12(D) the sub-panel could be any distribution equipment on the premises. So the question becomes: is the sub-panel capable of supplying branch circuits or feeder loads? If yes, then the sum of the breakers (potentially) feeding the bus is 180 amps so a 150-amp rating is required and the inverters would have to feed the opposite end of the bus bars. If no, the code is not clear on the requirement, but obviously the 80-amp breaker in the main panel limits the maximum current flowing through the sub-panel. Kent Osterberg Blue Mountain Solar
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