The correct size for the breaker depends on the size of the wire, while the size of the wire depends on the electrical load it services. Read on to learn how to choose the proper breaker and the proper wire.
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Find the cable that the circuit breaker protects and read the size printed on the sheath, for example, "10-2." The 10 indicates the wire gauge and the 2 refers to the number of wires, not including the ground wire. The smaller the wire gauge, the larger the diameter of the wire.
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Use a 15-amp circuit breaker with 14-gauge wire, a 20-amp breaker with 12-gauge wire, a 30-amp breaker with 10-gauge wire and a 40-amp breaker with 8-gauge wire. Use the next largest wire size if the cable will run more than 100 feet or alongside other cables through restricted areas where there could be a lot of heat buildup.
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Figure out if the circuit can handle the load you intend to put on it. The maximum load for a circuit is the number of amps multiplied by the number of volts; for example, 15 amps multiplied by 120 volts equals an 1800 watt maximum load. Use only 80 percent of the maximum load to avoid any possibility of tripping the breaker due to overload. For example, 1800 watts multiplied by 80 percent equals 1440 watts.
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Combine the wattage of all the loads on your specific circuit. For example, a total of 2700 watts would mean that you need a 30-amp circuit breaker on a 120-circuit with 10-gauge wire.