Electrical – connect 12 AWG wire to 10 AWG in a junction box


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.

Best Answer

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.