If a circuit overloads, its circuit breaker will flip off or its fuse will blow. If this happens often, it is more than an annoyance: the circuit is overstressed, and a fire could result.

This calculator refers to 120-volt circuits only. A 240-volt circuit usually supplies power only to one major appliance. If a 240-volt circuit overloads, call a professional electrician.

A few tips: Never attempt to solve the problem of an overloaded circuit by increasing the size of its fuse or circuit breaker. Breakers and fuses are designed to shut down before wires overheat dangerously.

A service panel should have an index detailing which electrical users are controlled by which circuit. To make sure you know all of the items connected to a circuit, shut off the circuit breaker or unscrew the fuse. Then, go through the house finding out which lights, receptacles, and appliances are not working. Mark the circuit with a label indicating which appliances are served by it.

The safe capacity of a circuit is its total capacity, minus 20 percent. For instance, the total capacity of a 15-amp circuit is 1800 watts; the safe capacity is 1440 watts. Electrical codes demand that all circuits be kept within safe capacity.