Context: I have a workshop used for woodworking. On one wall, I have a two-pole (240v) 30a outlet and a regular 120v 20a outlet. I'm building some cabinetry and I plan to convert the outlets in the wall into a junction box and mount the outlets onto the cabinets in a more accessible location.

I'm curious how I should set up the new boxes because I was wanting to do the following:

  • One single 20a 240v receptacle (from the 30a circuit - uses only black wires - reserved for future table saw)
  • One duplex 20a 120v receptacle (from the 30a circuit - fin removed, using black & white wires from 30a circuit)
  • One duplex 20a 120v receptacle (from the 20a circuit)

I don't think I can put them all in a 3-gang box since I won't find an outlet cover with two duplex / one single. So I guess I need 2 separate boxes.

I could do a 2-gang box with four receptacles (the four 120v ones), and a single gang box with the 240v. This would look cleaner in a way, but the wiring is probably messier since there would need to be wires going between the two boxes.

Or, I could do a single gang box with 120v duplex, and a two-gang box with a combo 120/240 receptacles, using a cover like this.

I guess I'm leaning towards the latter but it would be nice if I could have put this all together in one box.

(If it matters, one of the 120v circuits will power a shopvac and the rest of the plugs will power a tool (router/miter saw/table saw etc). Only one tool will run at once but vac/tool will be on separate circuits so they can run together).

edit: The 30a circuit is wired for a dryer but is unused. The electrician put that in when I asked him for a 240v circuit in case I wanted to re-wire the table saw.

edit: Upon further inspection, my 30a circuit is only wired with 10/2 romex, so my proposal isn't going to work without running new cable anyway.

  • won't find an outlet cover with two duplex / one single - You are in a woodworking workshop. Make one. Commented Jan 26, 2022 at 4:15
  • 3
    Your plan ignores the fact that you cannot safely directly connect either 20 amp outlet to a 30 amp circuit! You could overload a 20 amp outlet to 30 amps and burn up your wiring and/or outlet.
    – DoxyLover
    Commented Jan 26, 2022 at 4:40
  • 3
    Other reasons too. Devices which UL has certified to plug into 20A outlets are required to reliably trip a 20A breaker if they fail. May not trip a 30A. Commented Jan 26, 2022 at 4:57
  • Are the existing circuits run using cables, or individual wires in conduit? Commented Jan 26, 2022 at 5:08
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    @RyanSilva, residential 15A receptacles are usually rated for 20A passthrough, and the assumption is that nothing drawing more than a nominal 15A will be used on them. Things are different when you start dealing with power tools and industrial equipment.
    – isherwood
    Commented Jan 26, 2022 at 15:06

1 Answer 1


Can't put 20A receptacles on a 30A circuit.

If your 30A circuit is 4-wire, you can convert it to a feeder to a sub panel. Then, you can fit the requisite 20 amp 120V and 240V circuits as you wish, without any real restriction.

However a 30A feeder cannot have any receptacles at all, so if you jury rigged this off the dryer circuit as many do, that whole thing will need to be re-done (with separate neutral and ground if not already so). Once the subpanel is in, you can run a 30A circuit back to the dryer. That will use 100% of the feeder's capacity when you're drying, so don't use other tools then.

Since your 240V loads are 15A or 20A, you can use normal 240V sockets - you don't need to pay a fortune for the wackadoodle twist-locks LOL. If your community prefers those, pay them no mind. It's your shop.

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You might also think about multi-wire branch circuits (MWBC) to the cabinets. These use 4 wires and connect both 120V and 240V outlets to the 20A circuits. They even make "combo" receptacles that accommodate both 120V and 240V outlets on one yoke. You get dual 120V "half circuits" so you can run two 16A loads, one one each half-circuit. Any 240V loads add amps to both 120V half-circuits.

If the cabinets will be screwed to the wall, they can be cabled in with normal Romex as long as the Romex is physically protected.

If the cabinets will be free moving, I would use cord-and-plug connections and EMT or FMC metal conduit with metal boxes for the wiring internal to the cabinets. Have proper cordage (SO, SW, SJOOW and any of those "S" cables) for the cord/plug connections, and proper strain reliefs where they enter the metal box knockout. Design the cabinets to protect the cord from injury. No inspector will argue with that!

However if you cord-and-plug run an MWBC, you'll need a 4-wire 20A socket. Straight blade 14-20 sockets are rare, probably forcing you into the far more common but expensive L14-20 twist-locks. However you could just hard-wire it with FMC or a AC/MC wire whip.

  • It's a dryer outlet but it's not being currently for anything. When I had the basement renovated I asked the electrician to give me a 240v outlet for future use. He put in a dryer outlet. Thanks for all the detailed info. But why can't you put 20a receptacles on a 30a circuit? How is it different from 15a receptacles on a 20a circuit? And why do products like this exist if you can't do what I'm suggesting?
    – Ryan Silva
    Commented Jan 26, 2022 at 13:18
  • Re-reading your paragraph about MWBC, is that different from what I'm proposing? Except instead of the combo receptacle I was just going to put the dual "half circuits" on a separate receptacle. If the problem is just the breaker rating, I can swap that for 20a since there's no dryer. The only other unusual thing would be the oversized 10 gauge wire, which should be fine.
    – Ryan Silva
    Commented Jan 26, 2022 at 13:34
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    1 - There is a very specific exemption for 15A + 20A, with a lot of very good reasoning behind it. 2 - The product you linked is to allow 20A 120V + 20A 240V on one circuit. That is absolutely fine, but you still have to breaker at 20A. 3 - Yes, the real issue is the breaker - swap it for a 20A and you're fine. Commented Jan 26, 2022 at 13:37
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    That's a code violation in both cases. I would report it to the inspector as that guy needs a talking-to. 3-prong NEMA 10s have been illegal since 1996 for dryers/ranges and 1966 for everything else. And using 10/2 ground as neutral has never been legal. You can change it to NEMA 6-30 and run 30A/240V devices, or breaker to 20A and receptacle to 6-20 and run smaller 240V devices. Commented Jan 27, 2022 at 0:45
  • 1
    @Ryan yeah you could do a subpanel and power as much as 30A of 240V loads, however you'd have to keep 120V loads out of that subpanel. It can't support them. Commented Jan 27, 2022 at 1:12

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