Troy, This is my first post. We install 32 amp rated output inverters, reduce the main breaker to 175amp, or line side tap.
Phil Forest South Mountain Company > On Apr 2, 2014, at 1:40 PM, Troy Harvey <tahar...@heliocentric.org> wrote: > > Thanks Bill, Makes sense to me too. Splitting your current over two sections > of busbar, as calculated by P=I^2R, does seem like it will in reality reduce > the heat load in the load center. > > However, no good AHJ solutions today. We are finding the most customers have > been moving towards larger systems as panel prices have fallen. If typical > service is 200AMP, and that average american household usage is ~32kWh/day, > that is approx. a 8kW system in most areas. And 8kW inverter * 125% means a > 43AMP backfeed. Next size up is 50amp. That is bigger than 120%. This is most > every project we work on these days is over 120%. > > So how is everyone else dealing with this? > > thanks, > > Troy Harvey > > > > >> On Mar 28, 2014, at 8:38 PM, Bill Brooks <billbroo...@yahoo.com> wrote: >> >> Dave and Troy, >> >> I don’t think JW had his facts correct on this. The standard test for a >> busbar is to place the highest allowable breakers directly below the main >> breaker to test for overtemperature of the busbar. With the requirement for >> Article 220 compliance of the panel, a panel that actually complies with >> Article 220 could go to 200% and will likely run cooler than a panel only >> fed by the utility. >> >> Devil’s advocates state that people violate Article 220 all the time so we >> need to be conservative. >> >> Make a proposal at the meeting in Golden on April 9-10 and you may become >> famous. >> >> Bill. >> >> From: re-wrenches-boun...@lists.re-wrenches.org >> [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Troy Harvey >> Sent: Friday, March 28, 2014 1:35 PM >> To: RE-wrenches >> Subject: Re: [RE-wrenches] Busbar 120% rule >> >> Very interesting. >> >> So, it is not a overcurrent risk, but a heat issue that may lead to a >> nuisance breaker tripping issue? >> >> >> On Mar 27, 2014, at 12:46 PM, Dave Click <davecl...@fsec.ucf.edu> wrote: >> >> >> I had a nice response all typed up before rediscovering my original source. >> Simple answer: there's still a thermal load to deal with even though there's >> no point on the bus seeing a current above the busbar rating. I am a linking >> machine today: >> http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/690.64(B)(2)Load%20Side%20Connections.pdf >> >> While this situation of connecting supply overcurrent devices at opposite >> ends may be >> safe for restricted conductors, it may not be suitable for busbars in panel >> boards, even >> though this allowance is in the 2008 NEC. Panel boards are subject to busbar >> current >> limitations and are also subject to thermal limitations due to the heating >> associated with >> the thermal trip elements in the common thermal/magnetic molded case circuit >> breakers. >> For example a 100-amp, 120/240V panel board is tested during the listing >> process with a >> 100 amp main breaker and two 100-amp load breakers (one per phase) mounted >> directly >> below the main breaker. The ambient temperature is raised to 45 degrees >> Celsius, the >> input and output currents are set at 100 amps, the temperature is allowed to >> stabilize, >> and the panel must pass this test with no deformation of any parts. If we >> add a backfed >> PV breaker pair, for example 50 amps, at the bottom of the panel, and if the >> loads on the >> panel were increased to 150 amps, no breakers would trip, no busbars would >> be over >> loaded, but the thermal load in the panel would be that associated with 300 >> amps, not the >> 200 amps the panel was designed and listed for. Panel manufacturers have >> stated that >> these panels cannot pass UL listing tests with those excessive thermal loads. >> >> On 2014/3/27, 14:34, Troy Harvey wrote: >> I am wondering about the busbar 120% rule, and if there is any wiggle room >> in the 2014 NEC. >> >> Fundamentally I don't understand the 120% rule. If my solar breaker is >> installed properly at the bottom of the busbar, and the grid-tie breaker is >> installed at the top, and the busbar itself is rated for 120% of the panel >> rating, I don't see any means by which a solar breaker of a size >> substantially larger than 120% could cause a problem. There can be no place >> on the busbar under any situation (that I can think of) that would exceed >> 120% because the supply current is coming from opposite ends of the bus bar >> - even in the worst case load situation. So even if I had a huge PV system >> (100A), backfeeding the bottom of a 200A panel, I don't see a situation >> where there is more than 200A over any one section of busbar. Am I wrong, or >> is the NEC just too prescriptive for its own good? >> >> Also would you say that the 120% is based on the inverter max output or >> backfeed breaker size? >> >> >> thanks, >> >> Troy Harvey >> --------------------- >> Principal Engineer >> Heliocentric >> 801-453-9434 >> tahar...@heliocentric.org >> >> >> >> >> >> >> _______________________________________________ >> List sponsored by Home Power magazine >> >> List Address: RE-wrenches@lists.re-wrenches.org >> >> Change email address & 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 >> >> Change email address & 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 >> >> >> >> >> >> This email is free from viruses and malware because avast! 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