L6-20P -> L6-30R

Lamar Owen lowen at pari.edu
Thu Mar 20 15:05:35 UTC 2014

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 

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