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I am preparing to wire up a 7500 W electric heater in my garage. I have obtained a 40A double-pole breaker, and a suitable length of 8/2 Romex to run from my breaker box to the heater junction box location on the ceiling, which has a 1/2" knockout on the cover plate.

What puzzles me is that I'd planned to run a 4'-6' 8/2 NM whip from the ceiling box to the heater, but every 8/2 whip I've found is 3/4". My understanding is that you can run up to three 8 ga wires in 1/2" conduit.

I have found 1/2" flex tubing through which I could just manually run my own 8/2 wire, but it occurred to me there may be a reason I don't understand that explains why there's no 1/2" 8/2 whip available. I thought I'd ask to clarify the matter.

(Yes, the obvious solution might be to get a jbox cover plate with a 3/4" KO, but I've been unable to find one at ANY local electrical supply house.)

Thanks.

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    You can run three #8 through 1/2" conduit, but that is not the same as running 8/2NM. NM takes conduit fill same as a single wire of its widest dimension (since it twists), and NM internal wires can't be removed from their sheath and used, unless the individual wires are marked with an insulation type. Jan 22, 2022 at 20:46
  • Aha! I think I get what you mean - I'm neglecting dimension of the wire with insulation. Right?
    – David W
    Jan 22, 2022 at 21:57
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    @DavidW -- NM is a cable that consists of several wires (of "mystery meat" type -- they aren't required to meet the THHN spec, so they can't be used outside the cable save for pigtailing in boxes) that have been wrapped up inside a PVC jacket, with a paper separator between the wires and the jacket. Jan 23, 2022 at 3:19
  • Thanks to you both for your comments. I understand the subtlety in play here now. At least it wasn't something entirely obvious to a relative noob. If either of you wish to convert your comment to an answer I'll be glad to accept it. Thanks again.
    – David W
    Jan 23, 2022 at 12:49

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A cable is several wires grouped together inside a sheath. *

Here's the trick with cables. (or oval wires if that ever existed). For conduit fill purposes, a cable follows the same rule as a round wire of the larger dimension.

So if you have an 8/2 cable whose sheath is 0.59" on the wide side, that fills conduit like a round wire of 0.59" diameter - bigger than a 3/0 wire alone!

A single wire can't fill conduit by more than 53%, so a quick shorthand can be computed in advance: the conduit inside diameter (ID) must be 138% of the wire size. In this case 0.814 inches conduit ID for 8/2 NM. A little too big for 3/4 so you are forced to 1" conduit.

That's the half of it. 6/3 UF cable requires 2" conduit!

For two wires in conduit, the fill can't exceed 31% (due to 2 wires' propensity for jamming). Quick calculation for 2 wires of width X is an inside diameter of 254% of X. Two 8/2NM just make it in 1.5" conduit.

"And this is why we don't like cables in conduit. Plus, pulling them."

* Unless it's not in a sheath. Some cables such as MH feeder are just individual wires glued together. Those compute as their member wires using the normal conduit fill calc that I gather you are familiar with. E.g. 2-2-2-4 MH computes as three #2 and one #4.

So you can just get three #8 and one #10 copper wire into a 1/2" FMC conduit (due to the actual ID of FMC being slightly on the large side, or other conduits if the ground is bare). 3 or more wires are allowed 40% fill. But you'll have to build that conduit yourself with FMC and connectors, not use a pre-made whip.

By the way, if you don't need a ground but have 2 large wires, throwing a ground in the pipe adds a 3rd wire entitling you to use 40% fill instead of 31%.

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