How to Wire a 20-Amp 250-Volt Outlet

Powering heavy appliances such as air conditioners usually requires a 250-volt outlet and a 250-volt plug. These devices typically have different configurations than standard 125V three-prong outlets and plugs. A 20-amp receptacle is recommended for some types of air conditioners, but in some cases you may need a 30 amp 125- or 250-volt receptacle. Check the specs on the equipment you plan to plug into the outlet.

Wiring is fairly straightforward, provided the proper circuit has been installed. If you are unsure whether the wiring available for the outlet will be suitable, consider consulting an electrician. A standard 120-volt outlet often cannot be converted directly to a 250-volt outlet; a new circuit is usually run from the panel to the location of the outlet.

Air conditioner with remote controller
credit: Witthaya Prasongsin/Moment/GettyImages
How to Wire a 20-Amp 250-Volt Outlet

Safety Tips for Installing an Air Conditioner Outlet

As is true when doing any electrical work, It's important to make sure the power is off before you start work. Even after turning off the circuit breaker, you should test every wire with a voltage tester before you touch it. This is especially important when working on a 250-volt circuit, which for wiring purposes is the same as a 240-, 230- or 220-volt circuit. The voltage is double that in a conventional light circuit and can cause serious injury.

Basic Procedure for Wiring an Outlet

Locate the wires in the receptacle box. There should be a pair of hot wires, one black and one red, as well as a white wire and a ground, which is either bare or covered with green insulation.

Strip back the ends of each insulated wire using a pair of wire strippers. Expose about 1/2 inch of wire. Be sure not to nick or damage the copper core of the wire. Use the hole in the wire strippers to make a small loop in the end of the wire where it has been stripped. Hook the black wire's loop over the brass terminal screw on one side of the outlet and the red wire's loop over the other brass screw. Do not use the green terminal screw. It is for the ground. Tighten each terminal screw all the way, and then torque the screw down another 1/4 turn.

Hook the loop on the green wire over the ground terminal screw. Tighten this screw down in the same fashion as the other screws. Connect the white wire to the chrome neutral terminal using the same procedure. Push the outlet into the box and use the two small machine screws to attach the outlet at the top and bottom. Usually these screws are included with the outlet at purchase. If you don't have screws, #6-32 1/2-inch oval head machine screws will work.

Tips for Wiring a 240 Volt Air Conditioning Plug

The wire used for 240-volt circuits is larger than 125-volt wire, which can make it difficult to bend. Instead of risking loose connections, consider crimping ring or kook lugs onto the end of the wires to make it easier to connect them to the terminal screws.

Current NEC requirements mandate a white neutral wire and a ground wire, but if you're rewiring an old circuit, you may not find a white wire or ground wire. It's OK to wire the circuit with only three wires. If the third wire is white, connect it to the chrome terminal and it can act as the ground, and if the wire is bare, connect it to the ground terminal. If the circuit cable has only two wires, you need to run a new cable with a ground wire, and since you're doing that, make sure the cable also has a neutral wire.

Charles W. St.Clair

Charles W. St.Clair has been writing professionally since 2003. He lives in Oakland, Calif., working as an electrician and carpenter. St. Clair holds a bachelor's degree in public policy from Emory & Henry College and a master's degree in city planning from University of North Carolina-Chapel Hill, where he specialized in food system planning.