The National Electric Code requires a circuit breaker in every commercial, industrial and residential electric circuit to prevent fires and electrocution. The "size" of a breaker is the maximum current it allows to pass without tripping, which should be 125 percent of the maximum current expected to pass continuously through the circuit. A 15-amp breaker, therefore, trips at 15 amps although the maximum continuous load through the circuit rates only 12 amps.
Circuit Breaker Operation
A circuit breaker contains an electromagnet that is energized by current running through the breaker. When the current exceeds the rating of the breaker, the electromagnetic force becomes strong enough to pull a switch and disconnect the circuit. Breakers that include ground-fault and arc-fault protection are required in many industrial electrical systems, and electricians commonly install them in residential panels. A ground-fault or GFCI breaker monitors the circuit and trips when it detects a surge, such as would be caused by contact of an electric wire with a pool of water. An arc-fault or AFCI breaker detects current leaking from worn insulation or a poor connection.
Continuous and Noncontinuous Load
The rating of the breaker controlling a circuit must be 125 percent of the continuous load and 100 percent of the noncontinuous load. The NEC defines continuous load as the "maximum current expected to last three hours or more." Noncontinuous load, on the other hand, includes current needed to power appliances and machinery plugged into outlets on the circuit and operate lights for a limited time. An important type of noncontinuous load is starting power, the extra power needed to start a cycling appliance, such as a refrigerator, water pump or garage door opener.
Calculating Breaker Size
In order to determine the size of breaker you need for a particular circuit, you sum the current draw from all devices that are on continuously, such as lights, and multiply that number by 125 percent. You then add the draw from anything else connected to the circuit. The rating for the breaker must be larger than this total. You can find a label on every electrical device, including light bulbs, that specifies how much power it uses in watts. Determine its current draw by dividing this number by the voltage at which it operates, which is either 120 or 240 volts.
Typical Panel Configuration
The bulk of the breakers in a residential panel are rated for 20 amps. These breakers typically control the electrical receptacles. In addition, most panels contain 15-amp circuits reserved for lighting. The electrical code requires 12-gauge cable when wiring a 20-amp circuit, while the cable for a 15-amp circuit can be 14 gauge, which is lighter. Panels also contain dedicated breakers for large appliances. Because these appliances operate at 240 volts, the breakers must connect to both 120-volt legs of the panel -- so there are two of them, paired together. The size of the breaker for each appliance is determined by its maximum power draw.