How to Hook Up a Breaker Box to a Pre-existing Breaker Box

When a breaker panel becomes overcrowded with circuits, you may need to install a subpanel. This subpanel actually becomes another circuit in the main breaker panel. Tying this second breaker box into the house circuitry is as simple as adding another breaker to the main panel. As long as the new circuit is wired to carry enough amps to drive all of the circuits that will be installed into it, this is a great way to add new flexibility to an old circuit panel.


Step 1

Mount the subpanel either adjacent to the main breaker box or in a remote location near where the new circuits will be installed. Take the cover off of the new box by removing the screws that hold it on. Using screws and a cordless electric screwdriver, attach the subpanel box to the wall about four or five feet above the floor.

Step 2

Strip about one foot of the outer insulation from 8/3 wire, using wire strippers. You will want to use heavy wire for this box so that it can carry the extra amps needed for multiple circuits. Slide the wire into the subpanel through the opening at the top of the box. Separate the inner wires and use the wire strippers to strip one inch of the coating from the ends of the wires.

Step 3

Push the ends of the white and naked wires into the holes at the top of the neutral bus. Use a screwdriver to tighten the screws to hold the wires. The ends of the black wires go into the holes on the main breaker or the lugs if there is no main breaker in this box. Screw the wires securely in place.

Step 4

Run the wire from the subpanel back to the main breaker panel. Turn off the main breaker. Use wire cutters to trim the wire so that it is about a foot longer than what is needed. With the wire strippers, remove the outer and inner insulation in the same way as for the subpanel. Use a screwdriver and remove the cover from the main breaker panel.

Step 5

Slide the wire into the main breaker box through a port on the side of the box. Using a 50-amp 240-volt breaker, insert the black wires into the breaker and secure them by tightening the screws. Attach the breaker to the power bus along side the rest of the breakers in the panel. Attach the white and bare wires to the neutral bar in the same was as they were in the subpanel.

Step 6

Replace the covers on both boxes and install the screws to hold them to the boxes. Turn on the main breaker to restore power to the system. Do not turn on the breaker to the subpanel until you have completed installing the circuits that it will control.