Because every electrical circuit has its own circuit breaker, do-it-yourselfers may one day find themselves having to add a new circuit breaker to the service panel. Installing a new breaker is not difficult and requires only basic electric wiring skills. The entire job takes only a few minutes, and is probably the easiest step when adding a new branch circuit to your home. The service panel looks more complicated than it really is, so do not be afraid to tackle this project -- provided your local electrical code does not require the job to be completed by a license electrician.
Turn off the current at the main breaker, located at the top of the service panel.
Access the bus bars and breakers by removing the service panel cover. The cover is held in place by screws around the edge.
Remove a knockout cover on the side of the box to allow the electrical cable to pass into the box. A screwdriver and hammer should do the trick.
Install a cable connector and clamp assembly in the knockout hole. The connector serves two purposes: it protects the wires from the sharp edges of the knockout and holds the cable securely in place. The locking nut of the connector goes on the inside of the hole and the clamp on the exterior.
Thread the cable from your new branch circuit through the knockout. Allow about 12 inches of cable inside the service panel. Once the cable is inside, tighten the connector clamp and locknut.
Connect the white wire from the cable to the service panel's neutral bus bar. Look for the other white wires to distinguish the neutral bar. Attach the ground wire to the panel's ground bus (it has green screw terminals). If there is no ground bus, connect the ground to the neutral bar.
Connect the black wire to the circuit breaker and tighten the set screw to hold it in place. Snap the breaker into place by clipping it to the hot bus bar in a vacant slot.
Knock out the breaker slot cover on the service panel cover that corresponds to the location of the new breaker. Replace the panel cover and mark the location of the new circuit on the panel map.
Turn off all of the breakers to avoid a power surge in the branch circuits when restoring power to the panel. Turn on the current at the main breaker and reset the other breakers.
Only use a circuit breaker rated for the branch circuit. A mismatched breaker can be a fire hazard.