On 03/19/2014 06:33 PM, Rob Seastrom wrote:
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%.
Actually, there is no NEC 384.16 any more, at least in the 2011 code. The current relevent, perhaps even replacement, article seems to be the exception listed to article 210.20(A). Now, 210.21(B)(2) indicates that, for each individual receptacle on a multi-receptacle, the total cord and plug connected load cannot be above certain values (which are 80% of the branch circuit rating for 15, 20, and 30A circuits) regardless of overcurrent protection device rating. If you have a 100% rated overcurrent device you could connect a total load on multiple receptacles beyond 80%, it appears.
While 210.21(B)(1) requires receptacles on single-receptacle branch circuits to be rated for the full load, any one piece of utilization equipment on a 20A or 30A branch circuit cannot be rated to draw more than 80% of the branch circuit's rating (210.23(A)(1) for 20A, 210.23(B) for 30A). So even if you have a single receptacle on the branch circuit you can't have any single piece of equipment use 100% continuously. The idea is to give the branch circuit some 'headroom;' in the ideal world, we don't load networking links past a certain percentage, depending on link technology, for similar reasons.
Tracking code changes fuels an entire industry, and several websites..... :-) Not to mention continuing education and license renewals for electricians..... and headaches for those who think they understand the code but then get a surprise at inspection time (been there, done that, go the t-shirt and the NEC Handbook so I'll halfway know what I'm talking about when dealing with these things.....)
A new NEC Handbook is in my budget every three years due to the substantial changes that are made by the committees. The physics of electricity don't change, but our understanding of those physics and our ideas about how to deal safely with electricity do. And what is allowable and available can change in a moment; I'm still a bit puzzled how the L6-30P to L6-20R adapters can actually be on the market in the first place, given that they can easily create an unsafe condition. Well, I'm puzzled from a technical viewpoint, but not from a marketing viewpoint.....if it makes money, it is marketable, until pulled or recalled.....
