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When learning how to change circuit breakers, you will discover that your circuit breaker panel has something called the hot bus bars. These bars are fed by the heavy black wires coming into the box from the electric company. When changing a circuit breaker, you may need to replace a single-pole circuit breaker or a double pole breaker. A single pole breaker sends 120-volt current to a circuit, where a double pole circuit breaker handles 240 volts. Single pole breakers connect to a single hot bus bar, double pole breakers connect to both hot bus bars.
Changing the breaker is different than installing a new one--new breakers require installation of the wire first, attaching the ground wire to the breaker box ground, and the hot wire to the breaker box's hot bus. When replacing the circuit breaker, this work has already been done and the only thing you need to do is to attach the wire to the circuit breaker. Never change circuit breakers with the power on. Always kill the main power and never touch the big black wires that go to the mains. They are always live.