1. Shut off the Power and Test the Wires
Turn off the power to the circuit you’ll be working on by switching off the appropriate circuit breaker in your home’s service panel (circuit breaker box). Test all of the wires you’ll be working on with a non-contact voltage tester. The test should confirm that no voltage is present in any of the wires.
2. Remove a Knockout (Metal Box Only)
If you’re using a metal box, remove a knockout on the box for each cable that will enter the box. Use a screwdriver and hammer to break out each knockout (metal disc), then twist off the metal knockout disk with pliers.
3. Clamp Each Cable
Install a cable clamp for each cable, as needed. Standard plastic electrical boxes do not have knockouts and contain internal cable clamps. Metal boxes usually have internal clamps; if yours does not, install a locknut-type clamp for each cable. Insert the threaded end of the clamp through a knockout hole and secure the clamp inside the box with the ring-shaped nut. Tighten the nut with pliers.
4. Mount the Box
Separate the circuit wires at the existing splice and loosen the cables as needed to make room for the new junction box. Anchor the box to the framing (or other support structure) with screws driven through the factory-made holes in the back or side of the box, as applicable.
5. Secure the Cables
Feed the cables through the clamps and into the box. The cable sheathing (outer jacket) should extend 1/4 to 1/2 inch into the box beyond the clamp, and the individual conducting wires should extend about 6 inches into the box. If necessary, trim the wires as needed and strip 3/4 inch of insulation from the end of each wire, using wire strippers.
Secure the cables by tightening the screws on the clamps, being careful not to overtighten and damage the cables. Plastic boxes usually have spring-tabs for clamps and do not require tightening.
Note: The ends of the individual wires should be clean, straight, and undamaged, with no more than 3/4 of exposed wire beyond the insulation. Trim any damaged wires and/or strip insulation as needed, using wire strippers.
6. Join the Wires
Join the wires together with approved wire connectors, following the manufacturer’s instructions:
Join the bare copper (or green insulated) ground wires together first. If the box is metal, add a pigtail—a 6-inch length of the same type of ground wire—to the ground wire connection, then connect the loose end of the pigtail to the ground screw on the box. Special green wire nut connectors are generally used to join the grounding wires together.
Join the white (neutral) wires together, then join the black (hot) wires together, using a wire nut or other approved connector for each wire pair. If there are red (hot) wires, join them together, as well. Confirm that all wires are secure by gently tugging on each wire.
7. Finish the Job
Carefully fold the wires into the box. Install the box cover, securing it with two screws. Code requires that the cover must be a solid “blank” without holes. Restore power to the circuit by switching on the circuit breaker.
