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I have an unused 30amp circuit from an old dryer with 2x30amp breaker at panel and 10 gauge wire.

I read a similar question asked by someone here and very good explanation why not to use 15 or 20 amp outlets in 30amp circuit.

Can I remove the old dryer outlet and bring wires to a box to install a 20amp outlet, removing one 30amp breaker at the panel? I would use an extension cord for my 20amp welder or compressor, just one at a time. (With leaving safety note on how to use that outlet, while I own the property and if I’m selling then just remove outlet and close as jumper box!!)

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  • Can you post photos of the dryer receptacle please? Mar 30 at 11:46
  • It would be most helpful if you would edit your post to include pictures of the panel, the outlet and the wiring behind the outlet in question (turn of the breakers and do not detach any wiring, but pull the receptacle out of the wall).
    – FreeMan
    Mar 30 at 11:46
  • Thanks, pictures added. Mar 30 at 14:11
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    Also, leaving a note about safety isn't going to cover you or anyone should something go wrong. The note could be moved (just for a sec while I clean), or get lost/damaged, etc. And, of course, remembering to wire everything back the way it was before you move out after a sale will be the last thing on your mind...
    – FreeMan
    Mar 30 at 14:24
  • Can you please post photos of the actual dryer receptacle involved here, including inside the receptacle's box? The photos you've posted tell me nothing that I didn't know before they were added... Mar 30 at 23:40
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TL;DR Replace the double 30A breakers with double 20A breakers. Keep the 10 AWG wire. Replace the dryer receptacle with 2 20A receptacles wired as a MWBC - one hot to each, neutral shared. No safety notes or special procedures once the changes are complete.

And now all the gory details:

  1. Replace the double 30A breakers with double 20A breakers
  • You can't have devices designed for use with 15A/20A circuits on a 30A feed. There are certain very specific exceptions (e.g., a 15A receptacle that comes pre-installed as part of an electric range, where the range is itself connected to a 30A or larger circuit), but in almost all situations, if you are going to either install 15A/20A receptacles (whether 120V or 240V) or hardwire devices that are designed for 15A/20A circuits, you must replace the 30A breaker with a 20A (or 15A, but that gains you nothing in this case, so go with 20A) breaker.

  • Simply removing one of the 30A breakers doesn't help. It does remove power from one of the hot wires. But it does nothing to change the 30A vs. 20A problem.

  • Replacing both 30A breakers (and they are really a pair, removing just one would not be so easy anyway) with a double 20A breaker provides advantages for use as an MWBC (Multi Wire Branch Circuit) which (a) lets you get 2 20A 120V circuits without changing the wires and (b) lets you use 20A 240V equipment as well.

  1. Keep the 10 AWG wire

This may seem obvious, but it includes some important details. You can use 10 AWG wire with 20A breakers (should be no problem at all) and receptacles (if they can't take 10 AWG wire directly, you can connect using a pigtail). Leaving the 10 AWG wire in place avoids having to run new wire now (which may be trivial or may be a lot of work) and also allows future usage for a 30A circuit if desired.

  1. Replace the dryer receptacle with 2 20A receptacles wired as a MWBC - one hot to each, neutral shared.

This is where it gets interesting. The existing dryer receptacle, if original (or straight replacement) to the 1959 build, is a 3-wire connection. This includes two hots and a neutral, but not a separate ground wire. This has safety implications, which are actually easier to resolve with 20A 120V circuits than with a 30A 240V circuit! The methods include:

  • If you have metal conduit or an existing (but unused because of the 3-wire connector) ground wire, use that as the ground for your new receptacles.
  • If you have an accessible ground from another circuit 20A or larger (so that the ground will be large enough for this circuit), you can run a ground wire from the new receptacle to that circuit's ground.
  • You can install a 20A GFCI and label it "No Equipment Ground". This gets tricky (but doable) with MWBC (either only use 1/2 the MWBC, or install two separate GFCI receptacles).

Any of these solutions will take care of "old dangerous dryer circuit without ground". Note that if the receptacle is in a location that requires GFCI (which is a lot of places now, depending on your state's version of the NEC), you may be required to install GFCI, either at the receptacles or using a double 20A GFCI breaker.

As far as MWBC, this lets you use any combination of:

  • 240V circuit - black hot/red hot
  • 120V circuit "1" - black hot/white neutral
  • 120V circuit "2" - red hot/white neutral

which conveniently lets you get two 20A 120V circuits without running any new wires.

  1. No safety notes or special procedures once the changes are complete.

This is crucial. For some oddball separate device, you can get away with your own funny procedures. But you can't do that for hardwired equipment or receptacles, because procedures will be forgotten. For example:

You leave the 30A breaker in place and wire up a standard duplex 120V 20A receptacle. You sell the house in a hurry, forget to (or run out of time to) change things back. New owner has no idea. Plugs in 2 or more 120V devices at the same time rated for high power - e.g., a couple of space heaters and some tools. Total power usage goes to 40A. Not enough to trip the 30A breaker for quite a while. But enough to overheat the 20A-rated wire (especially if it is older cable).

A proper installation - with the most important piece being the 20A breaker - avoids any special requirements. What you or any future owner can use is determined at a glance by the receptacles available. Unambiguous. Safe.

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  • Depending upon how old the house is it may have a 3 prong dryer outlet fed by 2 hots and a ground. If so, can't do a MWBC bc of no neutral. Also, are we sure the welder is 120V? Could be it's 20 amp 240. Most of the equipment in my shop is on 20 amp 240 volt circuits. Mar 30 at 9:12
  • Sounds like he has(or thinks he has) two single 30 A breakers, instead of proper double breaker. He says wants to remove one, and I guess have a 120v circuit.
    – crip659
    Mar 30 at 11:16
  • Hello, Thanks for your replies. I’ve few tools that requires 20amp circuit (air compressor, welder 20amp/120V, demo hammer, SSD hammer) use mostly one at a time. This work set up is in garage. 30 amp double pole circuit was used for old electric dryer, newer one is gas and uses regular 120v. Old dryer plug is in laundry room which is near by garage. So instead of running new line from panel (about 50-70 ft wire) I was thinking if I can utilize unused circuit. Mar 30 at 13:29
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    Old dryer outlet had Blk,red and white. Grnd was connected to metal box Apr 9 at 2:34
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    Yes. I’m replacing 30 amp DP to 20 amp DP and keeping same configuration of wire red and black in breaker. White to neutral and bare to ground. At the outlet side pig tail on white (neutral) splitting for two separate 120 circuit. (Using junction box do this dividing circuit) One circuit I need for kitchen microwave (which just happened to have a new location and bringing from main penal is far then available this created circuit and bring it in kitchen. Apr 9 at 13:33
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Ok.

10 gauge wire is sufficient for reasonable length 30 amp runs, and thus is plenty good for a 20 amp run. No problem there. Larger wire than required just reduces voltage drop, a good thing.

You'll have to get a 20 amp receptacle rated for 10 gauge (stranded?) wire, no problem.

But the more typical solution is just an adapter plug, depending on what your dyer and welder need. A 20A load can be supplied from a 30A circuit. dryer to welder adapter cord

What plugs exactly on each side?

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  • Yes, I was thinking about this dogbone style adapter too. I’ve posted pictures of my outlet and make wire connector of welder, hard part is to find 3 prong old style male adapter. Also question arises, by using adapter circuit will be 220 or 230v, whichever configuration it is. But is it safe or okay to use on 20amp side? Also can I make my own adapter? If so what gauge of wire? (I don’t think so, but if it’s right way to use??) Mar 30 at 14:07
  • @HemantSurti install 20A receptacle and wire from 30A outlet.
    – user263983
    Apr 9 at 12:48

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