Instead of fuses, which burn out and aren't reusable, most modern residential panels have circuit breakers, which can be reset. The size of each breaker -- the amount of current it will pass through without tripping -- is determined by the load requirements of the circuit it controls.

Switches in circuit breaker box
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A breaker trips when the current passing through it exceeds its maximum rating.

Types of Circuit Loads

Although circuit breakers are designed to carry 100 percent of the load they're rated for, the National Electrical Code requires an extra consideration when sizing the breaker for a circuit, as outlined by the Electrical Construction and Maintenance website. Any load that passes uninterrupted for three hours or more, such as office lighting, is continuous and generates heat that affects breaker performance. The code requires the breaker rating to be 125 percent of the maximum continuous load plus 100 percent of the noncontinuous load that passes through it.

Calculating Breaker Size

To calculate the size of the breaker for a circuit, you need to separate the devices on the circuit into continuous and noncontinuous loads. A toaster oven is an example of a noncontinuous load. Add up the wattage of the devices in each class, which is displayed on the labels, and divide the totals by the voltage -- either 120 or 240 volts. Adding the results together gives you the maximum current the breaker needs to handle. The proper breaker is one with a rating that exceeds the maximum current by the least amount. If your calculations show a 12-volt maximum current, for example, you should install a 15-amp breaker.