I have a room with a NEMA 18-30R outlet, and I'm looking to install a 14A 120V air conditioner. I can trace this outlet back to the breaker, and I'm currently comparing two options:

  1. Make a 18-30 to 5-15 adapter using a hot and neutral.
  2. Rewire at the breaker and swap out the face plate.

I prefer option 1, but I'm concerned from both safety and code compliance standpoints. Is there an issue running an unbalanced load through a 30A 3-phase breaker? More informally, would this be considered shoddy compared to rewiring at the breaker?

2 Answers 2


There is no issue running an unbalanced load through the breaker.

I would probably make the adapter, since code inspectors have no business looking at what's plugged in to an outlet. They should be concerned only with what is part of the fixed structure. You can always unplug it when they come around.

But that kind of outlet makes me think it's a business with a compliance department and lawyers. Even if the inspector doesn't have a right to complain about your bootleg adapter, your insurance company will if there's ever a fire, related or not. And personally, I don't like shared neutral-ground conductors.

What's going to really get you though is putting a 20A outlet on a 30A breaker. If your AC failed in a way that caused it to draw 30 amps, it could overheat that outlet and start a fire in the wall.

So. You're going to want to change the breaker, and when you do, I would recommend you rewire as follows:

1) Use the existing combined neutral-ground conductor for ground

2) Pick one of the phase conductors, and clearly mark both ends with white tape. Use that for neutral

3) Pick another of the phase conductors and cap it with wire-nuts on both ends 4) Take the last phase conductor and put it on a new, 20A single pole breaker. Install a 20A outlet for the AC unit.

For extra credit, take advantage of the wiring in the wall to bring TWO 20A circuits to the box where the AC will plug in. Steps 1&2 as above, then.

3) Buy a double-pole breaker. Be advised. In a 3 phase panel you need to particularly select a 120/208 double-pole breaker, as opposed to just a 208 double-pole breaker.

4) Wire the two remaining phase wires to the double pole breaker.

5) At the outlet box, install two seperate 20A outlets. One on each phase, sharing neutral and ground between them. If it's a single gang box, break the tab between the two hots.

This will let you dedicate one 20A circuit just for the AC and still have plenty of power on hand for other uses.

  • A 120/208 panel won't have a wild leg as it's a wye service. Commented Aug 10, 2016 at 0:32
  • That makes sense. I'll update the answer accordingly. Thanks! Commented Aug 10, 2016 at 0:59
  • I like the approach, but I believe it's illegal to re-mark a colored wire as a neutral. This came up before recently. Commented Aug 10, 2016 at 5:24
  • This would be a good place to put a citation. I would add it to the answer too. I know it's compliant with codes to mark a neutral wire as Line. I can't imagine the other way around is any different if it's clearly marked. Personally I'd rather violate that part of code than have an ungrounded outlet or have to demo a wall to run wire with a different colored plastic sleeve. Perhaps use white heat-shrink the full length of the visible colored insulator? Commented Aug 10, 2016 at 23:23
  • I can relate to your "can't imagine" and in fact I was there myself, and got schooled by the elders here. It's in 200.6a and b. Commented Aug 11, 2016 at 17:48

You have a NEMA 18 which has three line wires each 120V from neutral, and, alarmingly, no ground. It's basically the 208V 3-phase "wye" version of the vile NEMA 10, and for that reason, it has to go.

You are not allowed to relabel wires. The only exception is that a neutral can be relabeled to a hot.

You ARE, however, allowed to retrofit ground wires. You can run a ground from that outlet to any grounded point which is served from the same panel... Provided that ground path is thick enough for the breaker you are about to install. WAIT. It's rigid conduit, you say? All the way back to the panel? Excellent. That's your ground. I forgot, where you see 208/3ph, you also see commercial construction standards i.e. steel conduit.


If you aren't grounded, you can legally fake it with GFCI. This will protect people, but not equipment from static discharge. Install a 20A breaker back at the panel (and hole covers or unused breakers in the unused slots). Replace the 18-30 outlet with a NEMA 5-20 GFCI between one of the hots and neutral. Obviously you put the new breaker on that hot and cap the others. Label the receptacle "no equipment ground".

NEMA 19-20 with adapter cable

Retrofit that ground (or not, if unneeded) so you have 12AWG ground all the way back to the panel. Install a 3-phase 20A breaker. Then make that adapter cable you were talking about.

The advantage here is you can still run three-phase stuff if you have the occasion.


Retrofit that ground so you have 10 AWG ground all the way back to the panel. Leave the 30A three phase breaker alone. Tear out the 18-30 outlet and replace it with a sub-panel of any size (well, over 30A obviously). Any phases you can't use, just cap them. Although you could get a 3-phase panel if you wanted to get max use out of the circuit. Now put in a 20A breaker for your NEMA 5-20 outlet for your A/C.

The benefit to this method is you don't have to muck about in the 208 panel at all, don't need to replace that expensive or obscure breaker, don't have to fill holes in that panel, and can get cheap common sub-panels and breakers. And you get LOTS of room for expansion, with 60 or 90 amps of 120V right there for you.

  • I like the sub-panel idea, and it's rigid conduit behind the wall so I could easily ground to that (pending testing) or pull a wire. Thanks for the advice! Commented Aug 10, 2016 at 19:27
  • Yeah, I find old panels can be expensive to find breakers and covers for, particularly three-phase... And sub-panels are shockingly cheap these days. If that's metallic rigid conduit, good chance it is a grounding path. And even if the path is not complete, it's now legal (NEC 2014) to complete it any reasonable way. Commented Aug 10, 2016 at 19:38

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