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I understand the derating of (4-6 current carrying conductors)80%X 40A for the 90 deg wire I’m using for 2ea 240V 30A dryer circuits in one 3/4” EMT conduit. My questions are: Are my neutrals counted as current carrying? And is there anything else to consider for calculating or derating for my wire size or conduit fill? I plan to use 10awg THHN.

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  • No need less than 9.
    – Dan D.
    May 24, 2018 at 6:56
  • Neutral counts as current carring (at least according to CEI), PE doesn't. Anyway 30A for 5mm^2 isn't that good, it's better for you to go to a slighty thiker wire (10mm^2), I think AWG8, surely if your run is long (>10m).
    – DDS
    May 24, 2018 at 7:05
  • @DanD.it works out that way for #10-14, but it's not automatic for sizes above that. May 24, 2018 at 12:13

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Your neutrals don't count

NEC 310.15(B)(5)(a) governs this situation:

(5) Neutral Conductor.

(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a).

You'll be fine with the 10AWG

This brings you down to 4 current carrying wires, so you will be using an 80% derate off the 90°C column in the ampacity table as you mention in your question. 10AWG copper can carry 40A at 90°C, and 80% of 40A is 32A, which is greater than the 30A you're asking it to carry (and the 30A maximum it's allowed to carry by 240.4(D) small conductor rules), so you'll be fine.

Conduit fill will be fine too

You are using 14mm2 per 10AWG wire, and 6 wires, so 84mm2 in total. This is well below the 137mm2 usable fill in a 3/4" EMT, so you're fine on that front as well.

And continuous loads aren't a problem either.

220.54 allows one to use a 5kVA assumed load for dryers that do not have a larger nameplate load rating -- given that 24A at 240V is 5.76kVA, you should be fine on the continuous-load-derate front for most clothes dryers.

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  • Thank you so much. My boss mentioned resistive loads being an issue for sharing conduit so I thought I was missing something needed to calculate if sharing this 3/4" EMT. I was over complicating this and confusing myself. These dryers are in a new "crew house" and will likely operate at the same time, for more than 3 hours on some days. Will the continuous use 125% need to be considered?
    – E.Howell
    May 24, 2018 at 18:07
  • The run is short. less than 30feet.
    – E.Howell
    May 24, 2018 at 18:10
  • @E.Howell -- addressed the continuous load business in the answer. May 26, 2018 at 4:32
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    @E.Howell I know this is an age ago, but for other readers... keep in mind "30A" dryers aren't actually 30A. They are deliberately 23A so that even after the continuous derate they still fit on a 30A breaker. Water heaters are that way too. Feb 2, 2022 at 6:37

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