[RE-wrenches] Busbar 120% rule

[Troy Harvey]

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 <daveclick at 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
>> taharvey at heliocentric.org
>> 
>> 
>> 
>> 
>> 
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