I have an existing 240 outlet wired into the wall of my shop. It's there to run a phase converter that I use about 2 hours a week. I want to use this circuit to run a 240 window air conditioner when the phase converter is off.

The air conditioner arrived today and has only 3 conductors on the power cord. I looked up the configuration and determined that it's a 240v 15 amp set up.

I want to make a short extension cord, about 6 ft, to connect the 3 conductor plug from the AC to the 4 conductor 240v wall receptacle. The wall side would be an 4 conductor plug that fits the receptacle. The AC side would be a 3 conductor receptacle that fits the end of the AC power cord. So my extension cord would only be 3 conductor (no ground) going into the 4 conductor wall receptacle.

I've spent enough time around electricians to know how to connect wires and such safely. Just not sure if omitting the ground will work. The manufacturer of the AC apparently thinks so or they would not have made it with only 3 conductors.

  • 2
    What type of plug is on the A/C unit cord? Is it a NEMA 6 (hot, hot, ground) or a NEMA 10 (hot, hot, neutral)? (If you can't tell, here's a pictorial guide). Jul 20, 2015 at 0:23
  • 1
    @ThreePhaseEel beat me to it-- your AC could have a ground with two conductors.
    – mjohns
    Jul 20, 2015 at 0:27
  • 3
    There is NO way a new A/C would have a NEMA 10 plug on it. They were only older ranges and dryers. This unit will have a 6-15P plug on it. If you were to make a cord you'd be omitting the neutral, NOT the ground. Jul 20, 2015 at 0:33
  • Note that the chart @ThreePhaseEel posted shows the receptacle layout—the plug on the appliance will have the opposite configuration.
    – Hank
    Jul 20, 2015 at 0:36
  • @SpeedyPetey -- not sure if it's a brand-new or a used AC, but I agree that it's most likely a 6-15. Jul 20, 2015 at 0:45

1 Answer 1


To do this properly, You'll likely have to run a new circuit. Since you haven't posted the make, model, or nameplates of the equipment, it's impossible to say for sure if these two devices can be on the same circuit. If they can, the solution is to extend the circuit using approved methods and materials. If they can't, the solution is to run a new circuit.

As for making an extension cord, that's not the best option. Extension cords are for temporary use only, and should not be used in place of permanent wiring.


Using an extension/adapter cord like this is dangerous, and can lead to loss of property and/or life. This is a theoretical device, that should never exist. DO NOT make, use, or even think about a device like this.

If you're dead set on making and extension/adapter, you'll want to use proper sized conductors and protect the cable from physical damage.

Since you're plugging it into a 50 ampere circuit, you'll have to size the conductors to be able to carry 50 amperes. This is not typically the way it's done, but this is not a typical solution. If the A/C overloads, you don't want the cord bursting into flames. So since the overcurrent device is set to 50 amperes, you'll have to have appropriately sized conductors.

A safer option would be to build a smaller overcurrent device in to the adapter, to protect the cord and A/C unit. In this case you'd have 50 ampere conductors attached to a NEMA 14-50 plug, with the other end of the conductors connected to some form of 30 ampere overcurrent protection. Then from the 30 ampere overcurrent protection, you'd run 30 ampere conductors to a NEMA 6-15 receptacle. Then you could plug the A/C into the 6-15 receptacle.


Using an extension/adapter cord like this is dangerous, and can lead to loss of property and/or life. This is a theoretical device, that should never exist. DO NOT make, use, or even think about a device like this.

The safest option would be to install a new panel, and have it being fed by the conductors that are currently feeding the 14-50 receptacle. Then you can branch to each of the devices from the panel, using proper methods and materials.

  • Info on the phase converter, and the existing receptacle are still required.
    – Tester101
    Jul 20, 2015 at 11:59
  • The phase converter and the AC unit would not be in use simultaneously...there is not enough head room left on the service panel...hence my desire for plugs. I had hoped I could create an extension cord that would have NEMA 10-50 on one end for the hot receptacle (but with no neutral connected) and NEMA 6-15 on the other for the AC unit.
    – Jim
    Jul 20, 2015 at 14:59
  • From the manual: "American Rotary rotary phase converters can run any type of load, resistive, inductive, multiple motor, and in any combination and or number." Here's a link to the .pdf for the phase converter in case it helps. I have the 20 hp model.
    – Jim
    Jul 20, 2015 at 15:10
  • @Jim NEMA 10-50 is a three prong receptacle (hot, hot, neutral). I thought you said it was a four prong receptacle
    – Tester101
    Jul 20, 2015 at 15:35

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