How do I connect wires at an underground junction box?

Written by mark morris
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How do I connect wires at an underground junction box?
Wrap wire nuts with electrical tape for weather tight connections. (wire nuts image by Paul Moore from

Outdoor electric power can be used to improve your outdoor living space in many ways from additional lighting to power outlets for yard tools and appliances, and supplying power to a shed or garage. Proper installation of your outdoor wiring will ensure safety and keep your power working trouble free for years to come. Make sure that all wires running into junction boxes are underground rated UF cable and are buried in a trench at least 12 inches deep. All outdoor outlets and switches should be sealed and have approved GFI (ground fault interruption) fixtures.

Skill level:

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Things you need

  • Screwdriver
  • Hammer
  • Conduit connector
  • Grommet
  • Wire stripper
  • Wire nuts
  • Electrical tape
  • Silicone caulking

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  1. 1

    Locate and turn off the breaker switch to the circuit. Remove the cover from the junction box with a screwdriver. Pry it away from the face if necessary to release the seal from the rubber O ring.

  2. 2

    Use a screwdriver and hammer to punch out the knockout on the correct side of the box. Tap the screwdriver firmly against the tabs at the two sides of the knockout to knock it loose.

  3. 3

    Fit the outside nut of a conduit joint onto the cable if conduit is used. Feed the cable into the box. Fit the inside nut of the joint from inside the box and tighten, or fit a rubber grommet into the hole if UF cable is used.

  4. 4

    Separate the wires of the cable and use wire strippers to strip the insulation from each of the three new wires back from the end about 1 inch. Remove the wirenut from existing black wires. Twist the new wire in with the existing black wires. Replace the original wire nut with a slightly larger size to accommodate the new wire. Wrap the wirenut with waterproof electrical tape.

  5. 5

    Repeat the pairing and joining process with the white and green wires using wirenuts and electrical tape. Seal the grommet in the knockout with a bead of 100-percent silicone caulking. Replace the lid on the junction box and tighten the screws. Turn the power back on and test the circuit before burying the wires.

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