Shutting Off Power
Shut off the power into the circuit breaker box before installing breakers. Some homes have a main switch for disconnecting power and this shuts off power to the service panel. Other houses have no central disconnect, in which case you need to turn off the main breaker. A voltage detector can confirm whether or not power is no longer running the wires that connect to the individual breakers.
Run Cable into Panel
Remove the knockout plug at the location most convenient for installing the new breaker, then install a cable clamp into it. Holding the cable up to the panel can help you to gauge how much wire will be necessary to get around the perimeter and to the location of the new breaker. After making sure to strip more sheathing than is actually needed, thread the exposed wires through the cable clamp and tighten.
Connecting the Wires
Route the ground wire to the open terminal and then cut to size. Poke the wire into the terminal and tighten the setscrew. This will also be done to the neutral wire, but take care to strip the wire end first.
Hooking Up the Hot Wire
Route the hot wire to the area of the new location of the breaker and cut it to size. Make sure that the wire will stay at the perimeter of the panel and fit as tightly as possible. Strip one-half inch of insulation and insert the wire into the breaker before tightening the setscrew.
Inserting the Breaker
Look over the breakers already in place to get the idea of how they are attached. Shove one end of the breaker onto one side of the hot bus bar and push the other side in so that they fit tightly. If the new breaker is not aligned with the older breakers, pull it out and try again.
Testing
Check to make sure no wires cross a hot bar, then arrange the wires as neatly as possible. Turn the power back on and test all the electrical outlets connected to the new circuit breaker and then make sure to label the new breaker immediately so you'll know to which outlets it is connected.
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