[RE-wrenches] Busbar 120% rule

[Dave Click]

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
> taharvey at heliocentric.org <mailto:taharvey at heliocentric.org>
>
>
>
>
>
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