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: [email protected] [mailto:[email protected]] 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 <[email protected] <mailto:[email protected]> > 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%20 Connections.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 [email protected] <mailto:[email protected]> _______________________________________________ List sponsored by Home Power magazine List Address: [email protected] <mailto:[email protected]> 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 <http://www.re-wrenches.org/etiquette.htm> Check out participant bios: www.members.re-wrenches.org <http://www.members.re-wrenches.org/> _______________________________________________ List sponsored by Home Power magazine List Address: [email protected] <mailto:[email protected]> 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 <http://www.re-wrenches.org/etiquette.htm> Check out participant bios: www.members.re-wrenches.org <http://www.members.re-wrenches.org> --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com
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