enter image description hereenter image description hereI'm a DIY-er. I have a house built in 1972 with an unused 3-prong dryer outlet (I'm on natural gas now) that's on a 30 amp breaker. I want to remove the outlet and instead run wire from the box through the wall directly into the back of a NEMA 3R 60-amp AC disconnect, for use by a 240V 20-amp mini split AC condenser. I plan to use a 10-gauge whip. The mini split instructions say to use 12-gauge wire.

Would there be any issues with this mix of gauges? What parts should I use to go through the wall? Could I surface mount the disconnect, or should I inset it into the siding?

  • 1
    A picture of the wiring behind the receptacle would be helpful to ensure you've got the wires necessary to make a safe installation.
    – FreeMan
    Commented Nov 21, 2023 at 14:50
  • A couple FYIs... you're on a 120V/240V system and have been for better than half a century. Plugs are on the ends of cords--we're discussing outlets or receptacles.
    – isherwood
    Commented Nov 21, 2023 at 15:49
  • If you do not have four wires, your solution should involve a licensed electrician. At least have one review the current wiring. A mistake by the previous installer or yourself could kill someone. If your wiring started a fire, you might lose insurance coverage. Commented Nov 21, 2023 at 17:45

2 Answers 2


As far as the mix of wire sizes and components, the key steps are:

  • Determine actual circuit size needed: 20A
  • Make sure that all wires are equal to or larger than the minimum size required for that circuit (20A == 12 AWG): 10 AWG (existing), 10 AWG whip - all good. Obviously must use at least 12 AWG from existing receptacle box to disconnect, but I would recommend 10 AWG so the entire circuit can be upgraded to 25A or 30A in the future if needed.
  • Disconnect size equal to or larger than circuit size: 60A (Which may seem like overkill, but they are so inexpensive that it make sense.)

You've got conduit. Add a ground wire. Done.

Even though you had a 3-wire dryer receptacle (no ground), the box and metal conduit may already be part of a legitimate ground connection. If so, extend to the new location using metal conduit (only certain types, some do not actually qualify as ground) or start a ground wire in this box (along with the wires you connect to the existing yellow hot and (if you need it) white neutral wires) and screw it into the metal box and it is your ground. If it turns out the existing conduit (we can't tell the type from the picture) is not a valid ground path, then add a ground wire going back to the breaker panel.

Because you have conduit here, which is relatively uncommon, it is quite possible that you must use conduit everywhere. That is the case in some large cities and very much jurisdiction dependent. Check that out before adding any NM cable. If you end up using conduit, the new hot wires can be yellow to match the old wires or they can be black or red or blue - almost any color except white (always neutral in conduit) or green or bare (always ground everywhere).

Original from before picture. This is still relevant for the very similar (and probably more common across the US) situation of cables instead of wires in conduit:

The remaining question then is whether you can reuse the existing wiring from the panel to the receptacle or not. You have a 10-30 receptacle, which has been obsolete for a very long time. It may or may not have a ground wire, which is required for both a new dryer installation and for anything else (like AC). Assuming you have cables and not conduit (if you have conduit then you add or replace wires as needed and this is all a non-issue), there are 3 likely possibilities of what you will find behind the receptacle:

  • Black/white/red/bare - All set. Use the black and red for hot, white for neutral if you need it, bare for ground. This can happen if there was originally a 4-wire receptacle and it was replaced with a 3-wire receptacle to accommodate a 3-wire plug from a dryer instead of replacing the plug/cord on the dryer.
  • Black/white/bare - Bare wire (possibly correctly, possibly incorrectly) was being used as neutral. Use black and white for hot (mark the white with black or red tape on each end) and bare for ground. Everything is good as long as you don't need neutral for the outside unit.
  • Black/red/white - This is a problem. While neutral can sometimes be bare (and then turned into ground as needed), ground can never be white (it must be bare or green). If you were trying to turn this 3-wire dryer receptacle into a 4-wire receptacle then you could run a separate ground wire (this is the exception to "all wires must be in the same cable"). However, you are trying to reuse these wires and extend to a new location/new function, so that doesn't apply. You're probably stuck.

So there is a non-zero chance that you can indeed do this.

  • Yellow-yellow-white, running through BX cable. Commented Nov 21, 2023 at 21:22
  • 2
    Why do you think it's BX cable? Judging by the fitting seen in the photo and the wire colors, it looks like you have wires in conduit. Cables with multiple wires of the same color aren't really a thing.
    – nobody
    Commented Nov 21, 2023 at 21:29
  • BX (Type B eXperimental) was discontinued in the 1940's. If 1970's flexible metal you have either AC or FMC. Commented Nov 21, 2023 at 21:54
  • Picture shows flexible metallic sheathing. I understand that I can run a separate ground wire back to the panel--connected to the disconnect? Commented Nov 21, 2023 at 22:22
  • Installing an additional ground may be possible. You really need to address that as a separate question. Look for questions that have answers that reference NEC 250.130(C). Also see /viewer/minnesota/nfpa-70-2023/chapter/2/wiring-and-protection#250.130 text copied from NEC. Commented Nov 21, 2023 at 22:36

The issue is about the ground. The legacy plug and wires indicate two hots and a neutral, the code allowed installation for specific appliances by grounding via the neutral. Air conditioning is not one of the allowed applianes. Unfortunately there isn't a legal method to re-identify the white as a ground.

You may be able to us this if you have an all metal pipe raceway that qualifies as a ground. Then you just need to pigtal the ground wire to the box. If you have FMC (Flexible Metal Conduit) that type is now only good for grounding up to 6 feet. It is sometimes possible to pull new wires into existing flex, sometimes you need to use all the old wires out as pull wires to get new wires into the flex. If you have Armored Cable (Type AC) it is possible that a thin conductive wire accomplishes the ground, this is not likely with yellow wires present as hots. I think you have FMC.

Surface mount the disconnect, I'm not even aware of a recessed disconnect that would work. I'm retired now, but where I worked if the label said "Maximum Fuse Size" you needed a fused disconnect, if is says "Maximum Overcurrrent Protection" then a non-fused disconnect would be acceptible if you change the breaking feeding the wire. If fuses are required don't use a disconnect rated above 30A, smaller fuses won't fit.

How to extend depends on how much patch work is acceptable. An experienced electrician could line up from the inside a long 1/4" pilot bit through one of the existing Knock Outs, the from the outside use holesaw though the siding using the pilot hole and a holesaw 1/4" bigger than KO to accomadate fitting a connector through the siding. Then put a combination of chase nipples, rigid nipples, rigid couplings to fit between the back of the box and edge of siding into the surface mounted disconnect.

Alternately it may be best to cut out a 12" square piece of drywall next to the jbox so you can reach a knock out, then fish a piece of UF cable or some type of raceway into the wall and penetrating the siding. (Remember you can't use NM (Romex) if it penetrates the siding.) This method is probably your best option to be able to mount disconnect at a serviceble height. You may also be able to use that hole to fish in your required 120v service receptacle (and you can't use the wash machine circuit).

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