1

Someone else is wiring up an on-demand heater. They need three 50A breakered circuits and they want to run the kaboodle in one conduit.

They want to use #8 THHN wire because it's allowed to 50A @ 75C (all terminations are 75C). However, we run into the conduit fill derate in 310.15(B)(3)(a).

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There will be 6 wires, so 80% derate.

OK, being THHN the wire is rated for 90C so the derate comes off the 90C column in 310.15(B)16. That is 55 amps for #8 Cu at 90C.

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80% of 55A is 44A.

Now what? Am I obliged to round down, and use a 40A breaker (won't work)? Or am I allowed to round up to the next breaker size (50A)?

2

Derating doesn't prohibit rounding up

The rule that lets you "round up" nonstandard breaker trip requirements to the next higher standard size is NEC 240.4(B):

(B) Overcurrent Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads.

(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).

(3) The next higher standard rating selected does not exceed 800 amperes.

Note that this rule says nothing about not being applicable to conductors that have been derated. So, based on the "shall be permitted" language here, without condition on the presence of the derate, we can safely say that the answer is "yes, you can round up to the next higher standard breaker size, even if you are applying temperature or fill derates to the conductors."

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