You're overlooking an important fact. Grid-tie inverters are current-limited devices. They lack surge capability.
Trip times for various circuit breakers vary depending in part on the overload as a percentage of the breaker's ratings and the circuit breaker temperature (for thermal breakers). A properly operating UL1741-certified grid-tie inverter that's exposed to a sudden dead short in the AC output will stop producing current before the circuit breaker would trip. Dan --- On Wed, 3/30/11, Mark Frye <ma...@berkeleysolar.com> wrote: From: Mark Frye <ma...@berkeleysolar.com> Subject: Re: [RE-wrenches] sizing a sub-panel used tocombinemultipleinverter outputs To: "'RE-wrenches'" <re-wrenches@lists.re-wrenches.org> Date: Wednesday, March 30, 2011, 3:38 PM Right, so going totally on the fly at the white board, let's say we have an inverter with a 45A AC output, it see's the short and pours it's little heart out at 45A for 0.1 sec before shutting out, delivering 4.5 A-sec of energy into the fault. Meanwhile we take out a clearing time curve for the generic 60A breaker that the inverter would be backfeed at the other end of the feeder. It will deliver 10 times it's rating or 600A for 0.5 secs before clearing. That's 300 A-sec of energy, 66 times more than the inverter. So can we say that when looking at sizing the AC side of these inverters we need more to concern ourselves with continuous overload conditions and not so much with short-circuit/ground fault conditions? 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 Exeltech Sent: Wednesday, March 30, 2011 12:04 PM To: RE-wrenches Subject: Re: [RE-wrenches] sizing a sub-panel used tocombinemultipleinverter outputs Per your question: > 1 - What does a UL1741 inverter do when it is connected > to a line that is experiencing a short-circuit or ground fault? Presuming a hard short, the voltage in that circuit would be essentially zero, and a UL1741 compliant inverter would cease producing power in less than 0.1 second. Dan Sr. Engineer Exeltech --- On Wed, 3/30/11, Mark Frye <ma...@berkeleysolar.com> wrote: From: Mark Frye <ma...@berkeleysolar.com> Subject: Re: [RE-wrenches] sizing a sub-panel used tocombine multipleinverter outputs To: "'RE-wrenches'" <re-wrenches@lists.re-wrenches.org> Date: Wednesday, March 30, 2011, 12:37 PM #yiv1838450963 UNKNOWN { FONT-FAMILY:Helvetica;} #yiv1838450963 UNKNOWN { FONT-FAMILY:Helvetica;} #yiv1838450963 UNKNOWN { FONT-FAMILY:Calibri;} #yiv1838450963 UNKNOWN { FONT-FAMILY:Tahoma;} #yiv1838450963 UNKNOWN { } That is a really good question Brian. Mostly when thinking about these sizing issues I think about overload conditions and not so much about short-circuit and ground fault conditions. So two questions come up: 1 - What does a UL1741 inverter do when it is connected to a line that is experiencing a short-circuit or ground fault? 2 - What does the feeder breaker do when it is connected to a line that is experiencing a short-circuit or ground fault? As far as number two, thanks to all the great work we do establishing low impedence fault paths back to circuirt protectors, the breaker is going to trip at it's rating and protect the wire at the same rating. But that doesn't mean that the conductor won't see more than rated amps during the fault. It will see alot more than for a very short time. During this very short time frame, the actual currents flowing into the fault from the utility side of the equation far far exceed the currents contributed by the power limited inverter side. 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 Brian Teitelbaum Sent: Wednesday, March 30, 2011 10:02 AM To: RE-wrenches Subject: Re: [RE-wrenches] sizing a sub-panel used tocombine multipleinverter outputs Wouldn’t a fault in that conductor between the main and the sub have a potential for up to 160A of current? I would think that at noon on a sunny day, the inverter system could produce 80A from one direction (albeit really only the max amperage outputs of the inverters) and 80A of grid current from the main into a partial fault. Brian Teitelbaum AEE 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 9:49 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multipleinverter outputs Al, I would say that yes the meter is a load and this goes to show the relative inadequacy of the vocabulary we are using in the discussion. In the example we have been using with the 80A breaker, wire and sub panel, you could have up to 80A of loads installed in the sub and still not create an over current condition in any of that equipment. 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 Al Frishman Sent: Wednesday, March 30, 2011 9:00 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multipleinverter outputs 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 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 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 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 -----Inline Attachment Follows----- _______________________________________________ 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 -----Inline Attachment Follows----- _______________________________________________ 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-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
