Installing Junction Boxes

Electrician working in junction box
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  • 01 of 08

    Why You Need a Junction Box

    Electrician junction box
    Mick Ryan / Getty Images

    The National Electrical Code (NEC) dictates that no wiring splices are allowed outside of an approved enclosure. One type of approved enclosure is a junction box. A junction box is simply a standard electrical box that is mounted securely (to house framing or another structure) and contains the splice—the wiring connection—of two or more circuit cables or wires. The cables are secured to the box with cable clamps (or conduit connectors, if the circuit includes conduit), and the box must have a removable cover to create a complete enclosure. Junction box covers must remain accessible; they cannot be covered with drywall or other surface material.

    Supplies Needed

    • Non-contact voltage tester
    • Code-approved electrical box with cover
    • Screwdriver
    • Hammer (for a metal box)
    • Cable clamps (as needed for a metal box)
    • Pliers (as needed for a metal box)
    • Drill-driver and screwdriver tip
    • Wood screws 
    • Wire strippers (as needed)
    • Wire connectors
    Continue to 2 of 8 below.
  • 02 of 08

    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 (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.

    Continue to 3 of 8 below.
  • 03 of 08

    Remove a Knockout for a Metal Box

    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 knockout with pliers.

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  • 04 of 08

    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 may 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 nut. Tighten the nut with pliers.

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  • 05 of 08

    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. Mount 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.

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  • 06 of 08

    Secure the Cables

    Feed the cables through the clamps and into the box. The cable sheathing (outer jacket) should extend one-half to one inch beyond the clamp, and the cable wires should extend about six inches into the box. If necessary, trim the wires as needed and strip a three-quarter inch of insulation from the each 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. Their insulation should be stripped one-half to a three-quarter inch from the ends. Trim any damaged wires and/or strip insulation as needed, using wire strippers.

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  • 07 of 08

    Join the Wires

    Join the wires together with wire connectors, following the manufacturer's instructions:

    1. Join the bare copper (or green insulated) ground wires together first. If the box is metal, add a pigtail—a six-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.
    2. Join the white (neutral) wires together, then join the black (hot) wires together. If there are red (hot) wires, join them together.
    3. Confirm that all wires are secure by gently tugging on each wire.
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  • 08 of 08

    Finish the Job

    Carefully tuck 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.