How do I calculate circuit breaker size?

Divide the adjusted wattage by the rated amperage to find the breaker size. The result isn't usually a circuit breaker size. You can round up the next larger size of the breaker.To calculate the size of the circuit breaker, simply divide the 240 volts adjusted wattage to find the nominal amperage needed for the subpanel. Often, the result is not a common circuit breaker breaker and you can simply roll to the circuit breaker breaker.