[RE-wrenches] Busbar 120% rule

[Troy Harvey]

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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