NEC 240.4B1 with #10 THHN on 20A REC

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Thomas Dugan

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May I use #10 THHN with a 90 degree temp rating when there are 32 current carrying conductors in a conduit exceeding 24" in length supplying general receptacles in an office building and protect the conductors with a 20A circuit breaker.
Based on 310.16B the #10 wire is good for 40 amps and when I apply the 40% derating in 310.15 B2 I get a total ampacity of 16 AMPS.
My main question is on 240.4b (1) that applies when "not part of a multioutlet branch circuit suppying receptacles for cord and plug connected portable loads" Does 240.4 b (1) prohibit going up to a 20A circuit breaker on each hot conductor or am I reading into this too mutch.
 
For a single receptacle on each circuit you can round up to a 20 amp OCPD. For more than one receptacle on each circuit you'll need to use a 15 amp OCPD.
 
There are two things you did not tell us. First of all, is the calculated load on any given circuit at or below 16 amps? For example, if you have 12 receptacles on each circuit, each being calculated at 180 VA, you have a total of 2160 VA. Divide that by 120, and you come to 18 amps. You could not use a 16 amp rated conductor to supply that circuit. So if the total load, per circuit, is above 16, then you can?t use the conductor arrangement that you are proposing.

Secondly, you didn?t tell us whether the circuits are arranged as multi-wire branch circuits. If they are, then I would say you are not reading too much into the rules. You have read the rules correctly. You could not use this arrangement, if there are MWBCs involved.

Summary: If the 32 conductors come from 16 circuits, with two wires (ungrounded and grounded) each, and if the calculated load on each circuit is 16 amps or under, then you can use your proposed configuration. Otherwise, you cannot.

Welcome to the forum.
 
As Rob mentioned if the circuit supplies multiple receptacle outlets the calculated load will be irrelevant. All that will matter is breaker size vs conductor rating.
 
charlie b said:
Secondly, you didn?t tell us whether the circuits are arranged as multi-wire branch circuits.
Click to expand...
I see that I misread the rule myself. It talks about multi-outlet, not about multi-wire. So I will amend my earlier summary.

I still say that you can't use 16 amps worth of wire, if the calculated load is more than 16 amps. But I agree with Rob's statement, regarding the prohibition against using a higher rated overcurrent device, when the circuit serves more than one receptacle outlet.
 
Thank you for the comments. It has always been an a given by mouth trade rule that when you use #10 THHN for derating in a panel that has nipples over 24 to a trough above with OCPD all 20 Amp that it was OK. This was told to me 20 years ago as an apprentice and was an accepted practice. I understand that the amount of current carrying conductors and the calculated load are both needed to determine the amapacity and the OCPD that can be used and that the hand me down rules from elders in the industry does not always "meet code"
Based on 310.15 B (2) table then the old saying is OK if there are no more than 20 current carrying conductors in each conduit in the above example using 90 degree THHN.
Thanks for the help!!
 
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