Circuit breakers are used as safety devices to prevent an electrical circuit from drawing more current than it's designed to use, preventing possible fire hazards from overheated wiring and electrical appliances. Circuit breakers have replaced the glass fuses of years ago, as they can be reset once they are tripped and the cause of the excess current draw has been removed or repaired. Every electrical outlet in a home is connected to a wiring system that gets its power delivered through a circuit breaker.
120-Volt And 240-Volt Circuits
Most homes and small businesses are connected to a local utility company's power grid, which supplies two AC (alternating current) voltages, namely 120-volt and 240-volt supplies. Most electrical outlets in a home deliver 120-volts to lamps, refrigerators, TVs, radios, computers, VCRs and DVD players, clocks, hair dryers, washing machines, toasters, microwave ovens, vacuum cleaners, cell phone charges, blenders and most all appliances you would normally purchase at a department store. Some appliances, however, need a higher voltage, 240-volts, as they require much more energy to operate. These household appliances include electric ranges (ovens), dryers, water heaters, central air conditioning systems and electric baseboard heat.
Circuit Breaker Ratings
Electricians design the wiring in a building so that a preset maximum safe current is known for each circuit. A single circuit may consist of several wall outlets. Typical values for 120-volt circuit are 15 amps, 20 amps and on up to 50 amps.
Circuit Breakers Control One "Hot" Wire
A circuit breaker controls one "hot" leg of a circuit. A "hot" wire, that is, the one that supplies the power, is fed through a circuit breaker before it goes through a building's wiring to wall outlets and appliances. A "neutral" or "return" line is connected to appliances and outlet receptacles to complete the circuit. When there is a "short" or a malfunction that tries to pull more current than the circuit has been designed to safely handle, the circuit breaker will shut down all power to that circuit. A circuit breaker that is connected to a 110-volt circuit is called a "single pole" breaker.
Double Pole Breakers
For those appliances that require 240-volts, two power legs are connected to them, each with 120-volts, but that are in opposite phase. The voltage from either leg to the neutral line is 120 volts, but across the two "hot" legs is 240 volts, which is double the 120-volt supply. To safely protect 240-volt appliances, a circuit breaker must be connected to each of the 120-volt legs. "Double pole" circuit breakers are used in this case, which consist of two single circuit breakers whose trip handles are physically tied together. That way, if either "hot" leg draws too much current, both breakers will trip simultaneously, effectively stopping all voltage to the appliance.
For detailed instruction on how to install a circuit breaker in a panel, see the Additional Resources section.