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]> 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 >> [email protected] >> >> >> >> >> >> _______________________________________________ >> List sponsored by Home Power magazine >> >> List Address: [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 >> >> Check out participant bios: >> www.members.re-wrenches.org >> > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: [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 > > Check out participant bios: > www.members.re-wrenches.org >
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