Sizing the correct wire for any electrical device is regulated by the National Electrical Code (NEC). Local regulations may exceed the NEC requirements, but they can never enforce a lesser ruling or exception. Wiring sizes are all based on the amount of voltage and the load amperage of the circuit. Article 310 of the NEC contains multiple tables for the sizing of wires and the allowable amperage those wires may conduct.
Things You'll Need
Nameplate data for the air conditioner
NEC book (optional)
Locate the nameplate data tag on the air conditioning unit. This metal tag will be near the electrical connection box for the unit. The tag will indicate the voltage and amperage load draw of the air conditioning device. Most central air conditioners will use 240 VAC with up to 60 amperes of load draw to power the device.
Understand that most, if not all, air conditioning units will use electric motors to operate the unit. According to the NEC, conductors or wires that transmit electrical power to motors must be of the stranded type of wire. No solid wires may be used. Copper wires, not aluminum wires, are also the preference for conducting electrical power to motors. Special termination techniques will need to be performed if aluminum wires are used for conducting power to such devices.
Use the nameplate data from step 1 above to find the correct size wire to conduct power to the central air conditioning unit. Table 310-16 shows that three wires of 6-gauge placed in a conduit will safely conduct 65 amperes of power. The voltage range for this wire falls between 0 volts and 2,000 volts.
Read the same table to find the following amperage capacity for the typical wire sizes: a 12-gauge wire can safely handle 20 amperes, the 10-gauge wire will conduct 30 amperes, and the 8-gauge wire will carry 50 amperes. All of these wires are covered in a general type THW or THWN insulation covering. "THW" and "THWN" are wire insulation classifications by the NEC. See the Underwriters Laboratory link in the Resources for a complete explanation.
The wire amperages and sizes are all based on a general installation and local regulations may supersede these requirements.
Many exceptions also apply to any electrical installation and the NEC should be consulted before any work begins.
Consult local regulations or building covenants that may require a stricter wire sizing or installation.
Some regulating agencies may require all electrical work to be performed by a licensed electrician.