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I'm adding a 100A sub-panel to feed a stove and water heater. The sub panel is indoors and in the same building as the mains panel. Do I need to run the supply feeder in conduit? I'm in Michigan and want to make sure I'm following code on this modification. If it makes a difference I'm planning to use a 8-3 wire with ground for the sub-panel feeder.

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A brief visit to the code indicates that there is a bigger problem, in that the ampacity of 8Ga copper is only 55 at 90C (aluminum 45 Amps) so your wire is too small for a 100 amp service. You need a bare minimum (if everything is rated for 90C) of 3Ga copper or 2Ga aluminum, and probably larger after various derating factors are applied, or if 75C is the limit on any connection. There is a note that may permit use of 4Ga copper for 100 amp in a dwelling unit as a "service lateral" - you'll have to decide how you feel about taking that note seriously. I've linked to a handy calculator that might be instructive.

Ampacity calculator for branch circuits and feeders

Non-pickily, for a wire appropriate to the ampacity of the subpanel, no. If you are using an 8-3 w/g cable, the cable is probably fine, and being a cable would complicate running it in conduit (as opposed to 4 individual wires.)

Pickily, maybe, if the wire is running through an area where it's prone to damage, or through a floor - but in both those cases, the conduit is only needed in the affected area, and is a "sleeve" rather than a "raceway" for code purposes.

If you don't already have the 8-3 w/g cable bought, do compare costs .vs. THHN or THWN (or XHHW, etc) in conduit. Edit: Well, even if you do have it bought, you can't feed 100 amps with it, so shop around for bigger wire.

I have a personal bias in favor of conduit, but that's all it is. I've seen too many scary things in remodeling where rodents have stripped NM cables in concealed spaces. In addition, you are limited to the 60C rating when using NM cable.

  • Thanks for the through answer. One question about the wire gauge. Why is it ok for the supply line from the pole to be 8Ga Alum and running 200a feed but I need a larger gauge for half the power? – Dan B Dec 27 '13 at 17:37
  • Utilities get something of a pass on certain parts of the NEC - they have exceptions here there and everywhere. But that's truly an absurdly small size for 200A service, IMHO. IF you want to (I wouldn't; I like my house not burning) there may be an exception you can use - something about not requiring your panel wiring to be larger than the service wiring. – Ecnerwal Dec 27 '13 at 20:17
  • Certainly looks like the service should be 4/0 aluminum at a minimum, though. – Ecnerwal Dec 27 '13 at 20:44
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Ecnerwal is correct on the power companies having special exceptions, also I'm willing to bet at some point they upgraded the panel without upgrading the wire. Not sure how old the house was back then but older homes didn't commonly have 200a panels.

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