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 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 ___ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
Re: [RE-wrenches] sizing a sub-panel used to combinemultipleinverter outputs
Jason, Now you are opening up a can of worms. It's bad enough that 705.12(D) doesn't say anything about a panel that can't (or shouldn't, or won't) have anything connected but interactive inverters. But when that is the case, a bus rating of 100% of the source circuits should apply. For PV systems a 125% factor will already apply to each inverter circuit. While it seems logical that the conductors are an extension of the bus bar and should be treated the same way, 705.12(D) isn't about the conductors - it's about the bus bars. The conductors are protected by the breakers -- 80-amp breakers and wire with 80-amp ampacity. Getting into the example further, I see flaws in it. If the inverters are 7500-watt 240-volt, the output current would be 7500/240 = 31.25 amps and 40-amp breakers would be adequate. Then there would have been no issues, even going directly into the main panel. If the inverters are 7500-watt 208-volt, the output current is 7500/208 = 36 amps and the 50-amp breakers make sense. That means the bus bars and feeder conductor have a continuous current of 72 amps. That means neither the 80-amp breaker nor 80-amp wire is sufficient because 72 x 1.25 = 90 amps. Now the example doesn't resolve the limitation of backfeeding at the main. Opps! Kent Osterberg Blue Mountain Solar Jason Szumlanski wrote: 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 doesnt change things. Even though this wire would be theoretically protected by an 80A breaker at each end, you cant 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 thats 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, Iwas thinking of a singlephase 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 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 thesubpanel 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 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 ___ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org ___ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options settings: http://lists.re
Re: [RE-wrenches] sizing a sub-panel used to combinemultipleinverter outputs
I concur with Jason and Brian's interpretation of 690.64(B)(2). I think one of the intents of the code is to ensure that if there is a fault in a busbar or conductor and the maximum amperage is being delivered to that fault from all OCPDs supplying it (solar + utility), the busbar or conductor should be able to handle the sum of those fault currents. I have encountered numerous jurisdictions that interpret it this way, and it is hard to argue with them from a strictly code perspective. However, Jason Fisher once pointed out to me that if the fault occurs in a feeder between the main service panel and a subpanel (solar accumulation panel or otherwise) it is impossible for any part of that conductor to carry more current than allowed by the largest OCPD feeding the conductor. Its the same principle behind the 690.64(B)(7) requirement to locate the solar interconnection breaker at the opposite end of the busbar from the incoming feeders. This argument obviously doesn't apply to accumulation panel busbars where you have more than 2 OCPDs supplying current, but I think it is a valid argument for basing your feeder conductor sizing on the largest OCPD protecting that conductor. For a brighter energy future, Andrew Truitt Principal Truitt Renewable Energy Consulting NABCEP Certified PV Installer™ (ID# 032407-66) (202) 486-7507 http://www.linkedin.com/pub/andrew-truitt/8/622/713 [image: 24 copy.jpg] Don't get me wrong: I love nuclear energy! It's just that I prefer fusion to fission. And it just so happens that there's an enormous fusion reactor safely banked a few million miles from us. It delivers more than we could ever use in just about 8 minutes. And it's wireless! ~William McDonough On Wed, Mar 30, 2011 at 10:12 AM, Kent Osterberg k...@coveoregon.comwrote: Jason, Now you are opening up a can of worms. It's bad enough that 705.12(D) doesn't say anything about a panel that can't (or shouldn't, or won't) have anything connected but interactive inverters. But when that is the case, a bus rating of 100% of the source circuits should apply. For PV systems a 125% factor will already apply to each inverter circuit. While it seems logical that the conductors are an extension of the bus bar and should be treated the same way, 705.12(D) isn't about the conductors - it's about the bus bars. The conductors are protected by the breakers -- 80-amp breakers and wire with 80-amp ampacity. Getting into the example further, I see flaws in it. If the inverters are 7500-watt 240-volt, the output current would be 7500/240 = 31.25 amps and 40-amp breakers would be adequate. Then there would have been no issues, even going directly into the main panel. If the inverters are 7500-watt 208-volt, the output current is 7500/208 = 36 amps and the 50-amp breakers make sense. That means the bus bars and feeder conductor have a continuous current of 72 amps. That means neither the 80-amp breaker nor 80-amp wire is sufficient because 72 x 1.25 = 90 amps. Now the example doesn't resolve the limitation of backfeeding at the main. Opps! Kent Osterberg Blue Mountain Solar Jason Szumlanski wrote: 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.orgre-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 -- *From:* re-wrenches-boun...@lists.re-wrenches.org [ mailto:re-wrenches-boun...@lists.re-wrenches.orgre-wrenches-boun...@lists.re-wrenches.org] *On Behalf Of *Mark Frye *Sent:* Tuesday
