This may sound like a dumb question, but I'm adding some electrical outlets to my garage using 10 AWG wiring in conjunction with 30 amp circuits. I am running 10 AWG THHN through conduit to multiple junction boxes. I terminated the connections at the junction boxes using romex and I'm running the romex behind the wall to the outlets that are above the junction boxes. I have been using 10 AWG romex from the junction box to the receptacles but it's caused a few of my outlets to sit crooked in the box due to the wire being so stiff. I was curious if I could use 12 AWG romex from the outlet box to the junction box and terminate it with the 10 AWG THHN. I know that 12 AWG is only rated for 20 amps but I won't be using any power tools that require more than 15 amps on one receptacle at a time, also the receptacles are only rated for 20 amps.

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    AFIK by code the wire and the breaker must match. If the breaker for the circuit is 30 A, then the circuit cannot have any 12 AWG wire in it. If it has any 12 AWG then the breaker must be 20 A. It is also prob a code violation to use a std 20-A receptacle on a 30-A circuit. I think you have to have two separate 20-A and 30-A circuits. – Jim Stewart May 29 '18 at 15:02

You can't do that

You cannot come off a 30A breaker with #10 wire (so far so good) and then terminate it at 15-20A receptacles. (nope, nope, nope!)

Unfortunately people sometimes become aware of the rule that allows 15 or 20A receptacles on 20A circuits, and they falsely generalize that to "Any receptacle <= circuit capacity". Nope. The receptacle ampacity must be an exact match to the breaker: 15 to 15. 30 to 30. 50 to 50.

Now there are two exceptions, but they are... exceptional.

  • 40A receptacles do not exist. NEMA, distributors/stockers, and electricians didn't want the burden of yet another socket type (there are like 40 varieties already when you consider every iteration of 3-phase, 208, 480, etc.) and oven manufacturers didn't want consumers stymied by a 40A range's plug not fitting an installed 50A socket. So the (only) correct receptacle for a 40A circuit is 50A.
  • UL requires all 15A receptacles to be internally rated for 20A. As such, 15A receptacles are allowed on 20A circuits.

And that's all, folks!

Only a 30A receptacle is acceptable on a 30A breaker.

"But I want to plug in common devices!" Then you have 3 options.

Change the breaker to 20A.

Now it becomes a 20A circuit (overbuilt with #10 wire - overbuilding is allowed). You can fit 20A receptacles. You can also fit 15A receptacles due to the exception mentioned above.

How do you handle the pinchy wires? I work in metal boxes which are annoyingly small and constantly run out of statutory cubic inches. Obviously your first defense is go with a bigger box. Or go with a box extensions or lids which give extra space. Another option is to place 2 boxes next to each other joined with a nipple -- and in one box, splice from #10 solid to #10 stranded, and in the other box fit the receptacle. Once you work with stranded wire, you won't want to go back!

Install a Power Distribution Unit

PDUs are typically used with server racks. They allow the rack to have a single power cord for the entire rack. It has a NEMA 14-30 or 14-50 plug, and splits that out to a number of 120V receptacles. Mind you, the PDU is a commercial unit that has been through the rigors of UL listing. You cannot build one out of parts except this way:

Install a Subpanel

This is conventional stuff, you bring L1, L2, N and G into a small service panel then hang any number of 120V/15A or 20A breakers off that panel, and those power your 15/20A 120V circuits. In any subpanel, neutral and ground are kept separate. No "main breaker" is needed if the subpanel is in the same building.

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    Thank you for the detailed response! You just taught me a bunch of stuff that I didn't know and potentially saved me from making a dangerous mistake. – broxhouse May 29 '18 at 16:06
  • To splice #10 stranded to #10 solid why does one need to have two boxes connected by a nipple? Couldn't one do this in one box, getting a deeper one if necessary? Does one tin the ends of the stranded wires? – Jim Stewart May 29 '18 at 18:41
  • Are there receptacles which have a built-in over-current breaker or a fuse specifically to allow hanging 20-A or 15-A receptacles on a 30-A circuit? – Jim Stewart May 29 '18 at 18:44
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    @JimStewart No on the fused receptacles. "need" answered above. Don't tin, use splice devices listed for solid AND stranded wire. Stranded wire is not weird. – Harper - Reinstate Monica May 29 '18 at 19:12
  • "So the (only) correct receptacle for a 40A circuit is 50A." This is interesting. Doesn't this fly in the face of the rule that the wiring has to be the heaviest gauge part of an entire circuit from receptacle to breaker? Or is that why it's exactly right to consider it a special exception? – TylerH Mar 24 at 19:24

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