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I think I get this one, but just to verify or clarify... (USA NEC)

When running multiple circuits (different breakers) in a single conduit, you have to derate after (is it 4?) sharing the one conduit.

But, if a single circuit (one breaker) is going to a bunch of light switches, and then the switched lines from those switches (and likely the hot line going on to other switches, LED lights do wonders for number of lights on a circuit) are sharing a conduit before they reach a junction box where they split off to the individual light strings, that's still one circuit (but more wires for the fill calculation, of course) from the "do not need to derate the wires in conduit" view?

I've seen too much rodent damage to be happy using NM cables...

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If a group of wires, together, are splitting power from one circuit, such that they together could not exceed the breaker rating without a trip -- then those wires together count as 1 wire for this purpose.

Let's suppose you have box A and box B. Supply comes in to box A. B has a GFCI deadfront and a bunch of switches, protected by the GFCI.

  • Supply hot goes from A to B.
  • Supply neutral goes from A to B, to supply the GFCI.
  • Five separate protected-switched-hot wires double back from B through A onward to their destinations.
  • Protected neutral doubles back from B to A, where it splits to feed each switched load.

This situation is annoying, because we must haul neutral from A to B and back again because of the GFCI. At 15A draw, how many wires "count" for derate?

  • supply hot is carrying 15A to the GFCI
  • supply neutral is carrying 15A to the GFCI
  • protected neutral is carrying 15A to the loads
  • Five protected switched hots, totaling 15A.

So four "wires" for derate, which in a 120/240 split-phase context we can simply count as 2 "circuits". The actual limit is 9 wires, but it's impossible to have an odd number of wires in split-phase.

If it wasn't for the GFCI, neutral could splice at box A and need only carry smart-switch loads A-B, which means it would be splitting the 15A with the switched hots, and we would only count 1 circuit.

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I'm not going to pretend to be a code expert, but I thought I would post this as my interpretation.

If wires in a conduit are on one circuit then the total power (and heat generating ability) of the wires will be limited to 20A (assuming 20A circuits). If you have wires from 3 circuits, you could have up to 60A of power flowing through that same run of conduit which would lead to more heat. This is my understanding of why the fill is derated at a certain point. There is the potential of various circuits pulling a lot of power, but any number of wires in one circuit can only possibly pull 20A.

So, in your example, you could have 20 LED light fixtures with individual wires, but the total power passing through all the wires would be 20A because they all lead back to the single breaker that limits them. You have a spaghetti of wires, but not much power or heat to worry about so no derating.

  • It's actually 15A, but yeah, that's also my thinking. – Ecnerwal Sep 17 at 15:43

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