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I have a 120v 20a receptacle that is measuring 220V at the outlet:

enter image description here

In the panel this circuit is controlled by a double pole 30amp breaker:

Breaker

I was using a 120V window AC unit in this outlet and it was working fine for years. How is this possible? The unit recently started having problems and I was looking to replace it with a 220V unit. Can I do that with the current configuration? What else do I need to consider (wire gauge and amperage needs of the new unit come to mind)?

Here is the wiring of the receptacle:

enter image description here

Edit: Here is the cord the cord on the unit I'm replacing it with showing it as 25amp.

And this is a link to the replacement unit in question

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    Did the old unit accept either 120 or 240V, possibly via switches or jumpers? It sounds like someone re-wired the outlet for 240V but neglected to change the cord and plug to the proper type. – Nate S - Reinstate Monica Jun 3 at 23:36
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    If you're able to show pictures of both the breaker panel, and the back of the outlet box (turn off the breaker and pull out the plug, but leave the wires connected as they are), that would help us figure out what's going on. – Nate S - Reinstate Monica Jun 3 at 23:37
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    Are sure it has been 120 in the past (someone could have used a 120 outlet on a 240 circuit) the normal case for a double pole feeding 120 circuits is you can have 1 circuit on the top receptacle and a different circuit on the bottom (I used to do kitchens this way). I am wondering if it has been this way or someone has rewwired a multi wire branch circuit wrong (this will destroy most 120v appliances in seconds. More info is needed. – Ed Beal Jun 3 at 23:40
  • @NateStrickland I added a picture of the back of the outlet with the existing wiring. What exactly do you want to see in the panel? – user2254963 Jun 3 at 23:47
  • @EdBeal The wiring on the outlet has not changed in the 6 years I've owned the property. For the last 4 I've been running a 120V AC unit off this receptical (a LG LW1214ER). I agree, I do not understand how the LG unit was not fried immediately. – user2254963 Jun 3 at 23:56
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As you've noticed, someone has wired this 20A 120V receptacle for 240V at 30A. That is very dangerous, and it's quite lucky your house never burned down. Most likely your old AC was designed to accept either 120 or 240V, and so it didn't notice the difference.

Based on your pictures of the current wiring, I believe you have 10 gauge wires (wire looks like orange sheathed NM) which can handle the 30A it's breakered for. It would be wise to double-check this with a wire gauge before upgrading to a larger air conditioner though.

To replace the current unit with one that requires 30A of 240V, assuming you already have 10 gauge wire in the wall, you'll only need to change the receptacle to the correct one, such as a NEMA 6-30. Or, if your new AC wants 20A of 240V, use a NEMA 6-20, and also change the breaker to a 20A one.

  • Is replacing the breaker necessary, or does it just free up some additional amps for other circuits? The new unit says it wants 25 amps. – user2254963 Jun 4 at 0:35
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    Breaker must match receptacle. – Harper - Reinstate Monica Jun 4 at 0:48
  • @user2254963, If you're in the US, the window unit does not call for 25A. 20A is the largest receptacle size used for something like that. Then it jumps to a giant 30A plug for something like a clothes dryer. – JPhi1618 Jun 4 at 2:38
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    Breaker must match minimum wire gauge. – Matthew Jun 4 at 5:18
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    There's also a chance the AC uses a 20A circuit, but allows you to bump up to a 25A breaker for the startup surge, as @EdBeal mentions in the comments above. If you post your AC's instructions (or a link to the model), we can help you figure out how to power it correctly. – Nate S - Reinstate Monica Jun 4 at 16:29

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