A circuit breaker's breaking capacity describes the maximum current that it can carry. Below the value, which engineers also call the interrupting rating, the circuit breaker can safely short the circuit. This interrupts the current and protects the circuit's components. Above the value, current may damage the circuit despite the breaker. This may burn components and perhaps release a dangerous electrical arc. Labels on residential circuit breakers usually specify their breaking capacity. Calculate unknown breaking capacities from circuits' electrical properties.

Multiply the number of sets of electrical windings or other components by the voltage each offers. For example, if you are calculating the breaking capacity connected with a three-phase 520-volt transformer, multiply 3 by 520, giving 1,560 volts.

Divide the volt-amperage running through the circuit by this voltage. For example, if the transformer runs at 78,000 volt-amperes, divide 78,000 by 1,560, giving 50 amperes.

Divide this current by the components' impedance. Suppose that the transformer from this example offers an impedance of 5 percent. Divide 50 by 0.05, giving 1,000. This is the connected circuit breaker's breaking capacity, measured in amperes.