How to Splice Into an Existing Electric Circuit

Written by jerry walch Google
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How to Splice Into an Existing Electric Circuit
All branch-circuit circuit breakers are labelled according to the circuit they protect. (Jupiterimages/ Images)

Splicing into an existing electric circuit becomes necessary when you want to add receptacles to a branch circuit. You will need to splice into a circuit when adding new light fixtures to an already installed lighting circuit. These are just two of many situations where you may need to splice into existing circuit wiring. Splicing into circuit wiring is not difficult, but the new wiring must be done in accordance with the rules and regulations specified in the National Electrical Code, National Fire Protection Associations Publication 70 (See Reference 1).

Skill level:

Things you need

  • Non-contact voltage tester
  • Screwdrivers
  • ROMEX® cable
  • Razor knife
  • Wire cutters
  • Wire strippers
  • Electrician's pliers, also known as Lineman's pliers
  • Wire nuts
  • Black electrical tape

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  1. 1

    Locate the point on the wiring system where you want to splice into it. All splices must be made in an electrical box, outlet box, or junction box. If there is no box present at the point where you want to splice into the existing wiring, you will need to install one If you do install a new junction box, it must be accessible. Code requires that junction boxes may not be concealed inside walls, floors, or ceilings. In most cases, you will be able to make your splices in an existing box.

  2. 2

    Locate the main service panel and turn the circuit breaker protecting the circuit you will be splicing into to the "off" position. Close the panel door and place a note on the panel warning other occupants not to turn the breaker on.

  3. 3

    Turn on the non-contact voltage probe's power switch and bring it near the cable at the point where you intend to make your splices. If you have turned off the correct circuit breaker, the probe will remain quiet and dark. If there is still voltage present, the probe will emit a steady beeping sound and the probe's light-emitting diode will blink. If there is voltage present, you have turned off the wrong breaker.

  4. 4

    Remove the cover on the box where you will be making your splices. Install the new cable through the side of the box far enough so that it extends six inches beyond the front edge of the box. Remove the cable's outer insulation with the razor knife. Separate the black, white and bare copper wires. Strip 3/4 inch from the ends of the black and white insulated wires.

  5. 5

    Remove the wire nuts from the wires in the box. Hold the stripped end of the new black wire next to the stripped ends of the existing black wires and twist them tightly together in a right-hand twist with the electrician's pliers. Complete the splice by screwing the wire nut back on the splice. Splice the white and bare copper wires together in the same way.

  6. 6

    Reinstall the cover on the electrical box and turn on the branch circuit breaker.

Tips and warnings

  • Always turn off the power to a circuit before working on it. Electricity can kill you.
  • Always double check to make sure the circuit is safe to work on, using a non-contact voltage probe before touching the wires. Safety is your first concern.

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