I have an 240v outlet in my garage that was wired to use with a kiln. It was never used for this, and now I want to use it to connect a window AC instead. It current has a 60-amp breaker. I assume it's a bad idea to run the AC on this, since it's much too big. Can I just replace it with a 20-amp breaker? I assume it needs a two-pole breaker since it's 240v, right?

The outlet was never finished, and it has 4 wires. Black, white, red and ground. I want to install a 6-15R receptacle. I will connect the hot black and hot red and ground, and leave the white capped and unconnected, correct?

Finally -- is it always unsafe to use an extension cord with the AC? My preferred AC model comes with a cord that is too short. If I use an appropriate extension cord (3ft, 20A, 5000W), would that really be a problem? I will probably just get a different AC model with longer cord just to be safe, but is the extension really that bad if it is dimensioned correctly?


  • Do you know what gauge the wire on the circuit is? Jul 5 at 17:15
  • Since it's wired for 60 amp I assume it is 4 or 6 gauge, but I'm not really sure. It's stiff solid copper wire. Jul 5 at 17:39
  • What make/model is your electrical panel, by the way? Jul 5 at 17:45
  • Just your standard grey panel. It's not that old -- installed about 5 years ago. It's marked GE PowerMark Gold inside the lid. Jul 5 at 17:51
  • Reason we ask is that breakers must be particular to the panel manufacturer, with rare exception. No such thing as a standard breaker. Although Eaton CL (not BR) is legal in your panel. Jul 5 at 18:21

Replacing the breaker shouldn't be an issue

Replacing the 60A breaker with a 2-pole, 20A breaker shouldn't be an issue at all; the THQL2120 breaker you need for your panel is a stock hardware-store item. However, you'll need to use fat wirenuts to pigtail the likely-6AWG wires in your cable run to some 12AWG, as the lugs on the new breaker can't handle wire that fat.

You'll have to pigtail the receptacle too

You'll have to do the same thing at the receptacle end of your run, as a NEMA 6-15R or 6-20T won't accept 6AWG wire no matter what you do. This means you need to use more of those fat wirenuts to attach 12AWG pigtails to the 6AWG hot wires in the box, then connect the receptacle to the pigtails.

Since this is a NEMA 6, you can simply cap off the neutral in the box, and unless your ground wire is a 6AWG instead of the normal 10AWG ground found in a 6/3 W/G NM cable, you can simply use a normal wirenut to connect it to some 12AWG going off to the box grounding screw and to the grounding screw on the receptacle.

Note that unless you want to install a duplex receptacle here, you'll need to use a NEMA 6-20T (T-slot) receptacle instead of a NEMA 6-15 as the NEC requires that a branch circuit with only one receptacle use a receptacle that matches the circuit's ampacity.

And a surface extension is a much better bet than a cord

For a high-amperage load like an air conditioner, you're much better off using some surface raceway (Wiremold™ 500 series or equivalent), a starter box, and a single-gang surface raceway box to extend the wiring run to a location where the air conditioner's cord can reach instead of hacking together a 240V extension cord from cordage and field-fitted parts (plug & cord-cap). Note that surface raceways are like conduits in that they are intended to have individual (12AWG in your case) THHN wires run in them, not NM cables (which won't fit very well, even).

  • Awesome! Thank you for the extensive and clear answer. Now I feel I know exactly what I need to do. Cheers :) Jul 5 at 22:46

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