Jay Ashworth <jra@baylink.com> writes:
It is exactly that: no one says you *can't* wire a 20A branch circuit with #10.
It is even *possible*, though unlikely, that if you did so, you wouldn't have to derate it to 80%. I would have to reread the Code to be sure.
It's not the conductor that you're derating; it's the breaker. Per NEC Table 310.16, ampacity of #12 copper THHN/THWN2 (which is almost certainly what you're pulling) with 3 conductors in a conduit is 30 amps. Refer to Table 310.15(B)(2)(a) for derating of more than 3 current-carrying conductors in a conduit. 4-6 is 80%, 7-9 is 70%. Plenty good for 20 amps for any conceivable number of conductors in a datacenter whip. Thermal breakers are typically deployed in an 80% application for continuous loads, per NEC 384-16(c). See the references to 125% of continuous load, which of course is the reciprocal of 80%. http://cliffordpower.com/wp/wp-content/uploads/2010/08/CPS_info_sheet_37_CB_... -r