DIY Flat Cable Repair

Written by steve sloane
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DIY Flat Cable Repair
Strip off the cable's outer sheathing with a utility knife. (Utility knife image by Andris Daugovich from

Flat cables are inside wires placed together side by side in a line and then coated with plastic sheathing for their protection. These types of wires are permanently installed in homes, many times running along the inside of wall cavities. However, if these cables become damaged, they need to be repaired or replaced so that electrical fires or shocks do not occur. If only the outer sheathing has been damaged the cable can be repaired with electrical tape, but if the copper wires are exposed then wire nuts must be used in the repair.

Skill level:

Things you need

  • Utility knife
  • Electrical tape
  • Cable cutters
  • Wire strippers
  • Screwdriver
  • 4-inch metal junction box
  • 1 1/2-inch screws
  • Screw gun
  • Wire nuts

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  1. 1

    Turn off the power to the broken electrical cable at the circuit breaker in the breaker box that controls the cable. Locate the cable break. If the sheathing is broken but the wires are intact with no damage to their plastic coatings, wrap electrical insulation tape several times around the outer sheathing to cover the damaged sheathing. Start wrapping the tape onto the cable 2 inches from one side of the damage, and continue wrapping until the tape covers the break and extends 2 inches on the other side of the damage. However, if the inner wires are damaged, the cable will have to be cut and a junction box installed.

  2. 2

    Cut through the cable at the damage using cable cutters. Strip back the outer sheathing 3 inches on each side of the cut, using a utility knife. Strip back the plastic coating from the wire ends 1/2 inch, using wire strippers. Use a screwdriver to remove a knockout hole on each side of a 4-inch metal junction box, and screw a 3/4-inch cable clamp into each of the holes. Attach the box in position against a wall stud with 1 1/2-inch screws. Tighten each clamp by using the screwdriver to tighten the clamp nut on the inside of the box.

  3. 3

    Push each set of wires through the cable clamps so that the cable's outer sheathing rests in the clamps. Tighten the clamps to the cables' outer sheathing with the screwdriver. Place the ends of each set of wires together, colour to colour, and screw wire nuts onto each set of wires. Make sure all the bare wire ends are inside the nut, except for the ground wires, which aren't insulated.

  4. 4

    Push the wires into the box so that the wire nuts are spaced apart. Place the cover on the box and screw it in place. Turn on the power.

Tips and warnings

  • Wire nuts come in various sizes to fit different wires. Make sure you use the correct size nuts to fit the wires.
  • Wrap the wire nuts to the wires with electrical tape after they have been screwed into place. This will help ensure that the bare copper ends of the wires do not short against the metal box.
  • If the cables are held in position against a wall with tacks, the tacks may have to be removed so that the cable ends can be stripped and pushed into the box. The tacks can then be reinstalled by placing the sharp ends of each tack above and below the cable and knocking the ends into the wall with a hammer until the tack's bridge rests evenly against the side of the cable. Never tack through the cable.
  • If a wall stud is not in the junction box's location, hold the junction box against the drywall and mark the position of its screw holes. Knock the holes through the drywall with a small screwdriver, insert wall plugs into the holes and screw the junction box to the wall plugs.

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