The size of circuit breaker in a main panel varies depending upon all of the devices to be supplied by the circuit. The amp load of all devices should be added together, explains The Home Depot. If the load of a device is expressed in watts, divide the value of watts by volts to equal the amps. Circuit breakers are sized to operate at 80 percent of their rated capacity.
Circuit breakers are divided between single-pole and double-pole types, depending on the voltage level required for the circuit. Single-pole circuit breakers carry 120 volts, rated in both 15 and 20 amps. Single-pole breakers are commonly used for receptacles and lighting. Double-pole circuit breakers carry 240 volts and are available in various amp ratings. Double-pole circuit breakers are typically used for hot water heaters and appliances, such as dryers and stoves. Double-pole circuit breakers connect to two slots on the main panel's bus bar, carrying two legs of current over two energized wires.
The choice of breaker is also limited by the gauge size and type of wire that feeds the designated circuit. Proper wire size and amp limitations can be determined using an ampacity chart. For example, 14-gauge NM-B copper wire is limited to a current of 15 or less amps. However, 12-gauge NM-B copper wire can carry up to 20 amps of current. As explained by Cerro Wire, the correctly sized wire must be used for the amperage requirement of the circuit to prevent the wire from overheating.
Working within a breaker panel is very dangerous and best left to licensed electricians.