By this argument, wouldn't it be necessary to check every conductor in the backfeed path adding potential PV amps to their required ampacity including the utility conductor from the transformer? Kris
Legacy Solar, LLC 864 Clam Falls Trail Frederic, WI 54837 715-653-4295 sol...@legacysolar.com www.legacysolar.com -----Original Message----- 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 1:08 PM To: RE-wrenches Subject: Re: [RE-wrenches] sizing a sub-panel used to combinemultiple inverter outputs Here is a 690.64(B)(2) opinion from Wiles that may help. http://www.fafcosolar.com/download/409/690%2064%28B%29%282%29TECH-1.pdf It's still my contention that the conductors need to be sized for 150A in this example, unless an exception is made by the AHJ. If David Brearley's post about Wiles' clarification is accurate, them the 80A main breaker in the subpanel can be used for calculations, but the conductor still needs to be sized for 80A + 80A =160A / 1.2 = 133.3A. Jason From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Kent Osterberg Sent: Wednesday, March 30, 2011 12:57 PM To: RE-wrenches Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multiple inverter outputs Al, How about a 15-amp single-pole breaker for power line carrier communications equipment to talk to the inverters? Technically, I think either way they are loads and that changes everything. But in my opinion they are minor and should be allowed without considering the sum of all sources. Kent Osterberg Blue Mountain Solar Al Frishman wrote: Is a monitoring system installed in the Load Center to measure the cumulative kWh's of the inverter's considered a load? The type of monitoring device I am talking about has CT's that go around the conductors and the Voltage ref is taken by attaching conductors to each phase, the Neutral and the ground bar. The digital Meter has a 3-pole 5A breaker in it so the conductors in the load center can be connected to lugs on the bus bar directly. Al Frishman AeonSolar (917) 699-6641 - cell (888) 460-2867 www.aeonsolar.com <http://www.aeonsolar.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 8:18 AM To: RE-wrenches Subject: Re: [RE-wrenches] sizing a sub-panel used to combinemultipleinverter 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 <http://www.berkeleysolar.com/> 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 <http://www.berkeleysolar.com/> 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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