I just bought a new window unit and it has a NEMA 6-15p. My previous unit had heat also and therefore was on a 30 amp circuit on 8/2. I've always been told that using a bigger wire is never a problem since the max load is whatever the breaker is so I figured it's ok to just switch the breaker and receptacle, right? I got a 15a breaker, but they were all out of 6-15r so I got a 6-20r instead. Does the same rule of the wire apply for the receptacle as well?

1 Answer 1



Wire is behind-the-scenes and totally generic. So bigger wire is OK - it will still do the job, and in fact will do the job better in a way that matters on really long runs (voltage drop). And nobody sees it so it will not affect what devices are connected.

A receptacle is a different story. In this particular case, it is not by itself a point of failure - it can handle anything plugged into it (15A or 20A). However, it can lead to two some real problems:

  • Nuisance Trips

If a future owner wants a more powerful unit and sees the 20A receptacle, they may (legitimately) buy a 20A unit, which could pull 16A continuous. That won't affect the wire (because it is oversized) but on a hot day running for long periods of time it will cause periodic nuisance trips of the 15A breaker. That will (a) annoy the owner and (b) eventually wear out the breaker, possibly in a dangerous way.

  • Extensions

A future owner could, incorrectly, assume that the 15A breaker means "14 AWG wire OK". After all, they will see the 8 AWG wire and know that is oversized. So when they decide to move the receptacle (e.g., to a different window) and extend from the existing location, they could splice in 14 AWG wire (fine for 15A) into the existing receptacle box and move the receptacle to a new box a significant distance away. Then another future owner uses the 20A receptacle to its fullest and instead of nuisance trips of the 15A breaker, ends up with overheated 14 AWG wires, possibly causing a fire.

Note that there is an exception in the other direction. You can have a 15A receptacle on a 20A circuit, provided there are at least two receptacles on the circuit. That is allowed so you can have, for example, two kitchen or bathroom appliances (one using a lot of power like a toaster or a hair dryer, one much smaller) running at the same time on a 20A circuit. All 15A duplex receptacles are designed to handle this. But you can't do the reverse - 20A receptacle on a 15A circuit.

So go back to the store and get a 20A breaker, or go to a different store and get a 15A receptacle.

  • Thanks for the response! So it's ok to use a 20 amp breaker AND receptacle even though the unit calls for 15 amp and has a 6-15p? I figured that might be a hazard in some way, making it not trip when it should and frying the unit or something.
    – Skervis
    Commented May 12, 2022 at 15:56
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    No, that's fine. 6-20 (and for 120V, 5-20) receptacles are designed to safety accept both 6-15 and 6-20 plugs. On the 120V side, that is what lets you plug your phone charger or ordinary computer (some big servers have 20A plugs) or lamp or radio or TV or whatever into a 5-20 receptacle (not that common in homes but extremely common in commercial buildings). Perfectly fine and safe. Commented May 12, 2022 at 16:09
  • 2
    Actually there is a code requirement to have a minimum of 2ea 15 amp receptacles on a 20 amp circuit. a single duplex fulfills this requirement as all 15 amp listed receptacles, switches, light fixtures, and devices are rated for use on a 20 amp circuit but in the case of a switch you cannot exceed the 15 amp load rating but they can be used in a 20 amp circuit.
    – Ed Beal
    Commented May 12, 2022 at 16:45

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