Branching Out to Break Up the Load

You could connect all of your 120-volt electrical loads—lights, bedroom receptacles, the refrigerator, and so on—to one big circuit breaker, and they would still function. Replacing a broken switch or installing a new light fixture then would mean turning off all the power to the house rather than just the power to one particular room. If your housecleaning service trips this giant breaker with a new 55-horsepower industrial vacuum cleaner while you are out of town and is afraid to reset it, you can kiss that frozen Copper River salmon in the freezer good-bye.
Electrical current is broken down into individual circuits—called branch circuits—for safety and convenience. You don’t want the entire house to go dark because a GFCI in the kitchen tripped due to a faulty appliance. Each circuit is designed to carry a certain amperage and to provide enough current to meet the wattage demands of receptacles, lights, and appliances. The following figure shows a standard residential distribution of circuits.
A circuit is laid out logically, or at least it should be. This means that a 15-amp lighting circuit will control lights in, say, three continuous rooms rather than in three rooms at opposite corners and on different floors of the house. Several forces work together to help a circuit do its job safely.