I currently have a 30Amp CB connected to 10 Gauge wire (Red, Black & White - no bare ground wire). I am about to connect a window AC unit that is compatible with a 20 Amp plug. My first question is this: can I replace the 30 Amp receptacle with a 20 Amp receptacle. Second question, since this system requires a ground would I need to connect the white wire to the ground bus in my panel instead of the Neutral bus?

thanks in advance for your assistance

  • What are the voltages of the existing circuit (120V single breaker or 240V dual breaker) and required by the new AC?
    – DoxyLover
    Commented Aug 4, 2016 at 18:35
  • both 220V dual breaker
    – Ron
    Commented Aug 4, 2016 at 19:17
  • Is this 10 gauge wire coming from a NM cable, some sort of metal-sheathed cable, or a conduit? Commented Aug 4, 2016 at 22:29
  • Is this in North America?
    – wallyk
    Commented Nov 7, 2016 at 23:57

1 Answer 1


Yes, you can replace the 30A breaker with a 20A breaker so long as you change the receptacles to 15-20A. The breaker must match the receptacles. The only exceptions are 15A receptacles are allowed on 20A breakers (and 50A receptacles on 40A breakers, not relevant here).

It's OK to use oversized wire. It's not OK to undersize wire. If it doesn't fit on the receptacles, use a short pigtail of a smaller but still appropriate wire size.

It is illegal to re-designate a neutral (white or gray) wire to be a ground, say, by taping it with green tape or painting it green. NFPA's logic is that a future repairman might get confused, and wire is cheap so why not use the right colors in the first place. A wire which is bare its entire length can only be a ground. I'm just gonna leave that there.

NEMA 10 receptacles are obsolete and dangerous. The correct receptacle for "240V, ground, no neutral" is NEMA 6.

  • A bit confused with your response, but think I understand...my question was can I keep the 30 Amp breaker and put a 20 Amp receptacle on. Not can I change the CBs. It's your examples that confuse me:I can undersize the receptacle (15 Amp receptacle/20 Amp CB) in one case and in the other I can undersize the CB (40 Amp CB/50 Amp receptacle) is that correct? Also, is there any allowance for the existing wires, or do I actually have to rip out the original wiring and replace it?
    – Ron
    Commented Aug 4, 2016 at 19:29
  • Because they're not EXAMPLES, they are EXCEPTIONS. An example shows how the common rule applies. An exception is a very specific case where the normal rule does not apply, and there's some very peculiar reason behind that. No wonder you'd be confused trying to fit it to the normal rule: it doesn't fit, it's an exception :) You cannot use the 30A breaker for a 20A outlet. Changing a breaker is not that big a deal. Either skill up or hire help, but be safe. Commented Aug 4, 2016 at 19:40
  • Appreciate the information. I agree, changing the CB and receptacle is a piece of cake - just trying save a dime or two and trying to keep a bit more Amps available in case something changes along the way. The hard part will be pulling 50ft of wire through the walls/ceiling just to go from a neutral to a ground...might be easier just to run a completely new circuit altogether using 20 Amp components and 12 gauge wire and just having an extra (the original 30 Amp) circuit available if needed at a later date. Thanks again
    – Ron
    Commented Aug 4, 2016 at 21:43
  • The 10AWG wire will give you a practical amps boost over 12 AWG wire as you will have less wiring loss. If you want even more total power, make it a MWBC, that will give you 40A to common NEMA 5 receptacles. You won't need to pull a whole new cable to get a ground. Under NEC 2014 you can retrofit grounds simply by running a ground wire to the nearest circuit whose ground is thick enough for your circuit (ie breaker). Its breaker must be in the same panel. The ground wire does not need to stay with the conductors. Commented Aug 4, 2016 at 21:57
  • That's good news...so if I just change my CB and Receptacle to 20 Amps I can just tie into the ground of my nearest 20 Amp 110V circuits (which are all individual circuits- one for each of my shop tools) and I should be good to go. That will save a tremendous amount of work...Thanks again!
    – Ron
    Commented Aug 4, 2016 at 22:16

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