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NEC gives max number of conductors for certain EMT conduit. For example, it’s 16x12AWG for a 3/4” conduit.

Now it turns out this is really not the limiting factor at all. It is de-rating.

Suppose I have 20A circuits (with 12AWG wiring) and I want to pack as many as possible into a conduit. With de-rating I am practically limited to 9 current carrying conductors, or 4 20A branch circuits. I can’t even increase this by using a very big, say 2” EMT.

Is this true?

The only option I see is running multiple conduits in parallel.

Alternatively I could use 10AWG which is nominally rated up to 30A and put up to 20 into an EMT which would de-rate to 20A. This is effectively 10x20A branch circuits. I would need at least 1-1/4” EMT to satisfy “static” fill requirements.

Do I have any other options? How is this done in practice?

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  • The issue is cooling. You could use a pressure rated pipe, wet-rated THWN wire, and then circulate water through the pipe to cool it! That's exactly what they do in DC fast charging for EVs. Jun 17, 2023 at 18:59

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Welcome to your electrical education. You more or less have it right, other than your horror at what you've learned. There is an exception for conduit nipples less than 24" long which is sometimes useful.

Nipples (sections of conduit 24"/600mm or shorter between junction boxes - note that a panel is a big junction box, among other things) are exempted from the derating requirement for number of current carrying conductors NEC 315(B)(3)(a)(2)(still subject to ambient temperature derate if applicable), and permitted 60% fill for 3 or more conductors, rather than 40% fill [Note (D) in the opening of Chapter 9]

In practice, you either:

Run a feeder to a subpanel where you break out into individual circuits (and that feeder can be nice inexpensive aluminum.) You need working space for the sub-panel, but otherwise it's a nice, clean, and money-saving approach, generally. You want "as many circuits as possible" all in the same direction, which absolutely screams that "one big circuit to a sub-panel wherever those circuits separate from each other" is the right solution.

Or: If not using a sub-panel, most people run multiple conduits since steel conduit costs a lot less than oversizing copper wires - and small conduits cost less than larger ones. However, oversizing wires is an available option as well.

One trick you missed is using 20A Multi Wire Branch Circuits MWBCs - only the hots of MWBCs count as current-carrying for derate purposes, as the neutral only carries imbalance current. However, that does tend to mean you'll need two-pole GFCI or AFCI breakers to feed them these days.

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  • Thank you! Would you mind adding a few words/reference on <24” exception? This might actually apply to me and the easiest solution. I have a bunch of junction boxes not far from a sub panel. I’d like to add circuits but avoid running extra EMT over that short distance
    – divB
    Jun 17, 2023 at 15:07
  • MWBCs are a good choice for EMT because AFCI is allowed to be at the first receptacle since it's metal conduit. The only place that doesn't work is 240V circuits but they're far more interested in making those GFCI rather than AFCI. Jun 17, 2023 at 19:01

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