Home wiring: how to extend a cable run

Updated February 21, 2017

One of the more convenient ways to extend your electrical cable run is to tap into an existing junction box in the attic or basement. The attic or basement is also a more convenient location to extend your coaxial cable run. Extend your cable run for room additions or to add extra electrical sockets, cable connections or lights to a room. Some prior electrical knowledge is helpful in making these extensions.

Turn off the circuit breaker that supplies electricity to the junction box containing the electrical cable you need to extend.

Remove the screws holding the cover to the junction box to expose the electrical cable connections.

Turn the dial on a multimeter to 250 alternating current (AC). Insert one probe of the multimeter into a wire connector holding the black wires together. Touch the remaining probe to the metal on the junction box. No voltage should register on the multimeter with the correct breaker turned off. Repeat the test if the junction box contains red wires for a 240-volt circuit.

Place a screwdriver against a knockout on the original junction box. Strike the screwdriver with a hammer to remove the knockout from the junction box. Grasp the metal knockout with pliers and twist the knockout from the box if it doesn't release from the metal box with the screwdriver.

Insert the threaded end of a nonmetallic (NM) connector in the knockout. Choose a 1/2-inch, 3/4-inch or 1-inch NM connector to fit your knockout size. Screw the locknut provided with the connector onto the threaded end of the connector to secure it to the junction box.

Slide the NM electrical cable extension through the NM connector and into the junction box. Size the cable to match the current cable size inside the junction box. Locate the cable size printed on the exterior of the current cable.

Untwist the wire connectors holding the bare copper ground wires together and pull them apart. Remove the connectors holding the black wires together and the white wires together and pull them apart. Remove the connector from any red wires if you're working with a 240-volt circuit.

Remove the exterior sheath from the NM cable with a dual NM wire cutter/stripper. Strip 3/4 to 1 inch of insulation from the black and white wires inside the NM cable. Remove the insulation from the red wire if you're extending a 240-volt circuit.

Connect the original white wires inside the junction box to the new white wire from your electrical cable extension and twist a red wire connector onto all the white wires, connecting them together. Repeat the connection for all the black wires, bare copper ground wires and the red wires if you're connecting a 240-volt cable.

Tighten the screws on the connector to secure the NM electrical cable to the junction box. Replace the junction box cover and turn on the circuit breaker once you terminate your electrical cable extension.

Cut your existing coax cable to split it into two pieces with an all-in-one coaxial cable tool. Strip 1/2 inch of insulation off both ends of the cut cable using the same cable tool, exposing the interior copper conductor.

Trim 1/8 inch of the outside jacket from the cable to expose the braid beneath the jacket. Slide the F-connector crimp ring over the cut and stripped ends of the coaxial cable. Fold the exposed braid back over the exterior jacket.

Slide the end of the F-connector over the ends of the cable until the connector sits flush with the interior white insulation of the coaxial. Push the crimp ring up over the F-connector, and use the coaxial cable tool to crimp the ring around the end of the F-connector to secure it to the end of the cable.

Install the two cable ends you created on a two-way video splitter. Connect a premade coaxial cable to the final connector on the splitter to extend your coax cable.

Things You'll Need

  • Phillips or slotted screwdriver
  • Multimeter
  • Hammer
  • Pliers
  • NM connectors
  • NM electrical cable
  • Dual NM wire cutter/stripper
  • Red wire connectors
  • All-in-one coaxial cable tool
  • Two-piece F-connectors
  • Two-way video splitter
  • Premade coaxial cable
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About the Author

Cecilia Harsch has been writing professionally since 2009. She writes mainly home improvement, health and travel articles for various online publications. She has several years of experience in the home-improvement industry, focusing on gardening, and a background in group exercise instruction. Harsch received her Certified Nurses Assistant license in 2004. She attended Tarrant County College and studied English composition.