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] <mailto:[email protected]>
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