Powering heavy appliances such as air conditioners usually requires a 20-amp 250-volt outlet. These outlets typically have different configurations than the standard three-prong outlet. Wiring them 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 to a 250-volt outlet; a new circuit is usually run from the panel to the location of the outlet.
Locate the wires in the receptacle box. There should be a pair of wires, one black and one red. In some cases, there will be a black wire and a white wire. In this case, recode the white wire with red tape to indicate that this is a 250-volt circuit. There will also be a ground wire, which will either be green or an uninsulated wire.
Make sure that no power is present in any of the wires. Shut down power at the panel and use a voltage tester to be sure that there is no power.
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.
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.