How to Make an Underground Wire Splice

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Making an underground wire splice requires an outdoor, waterproof electrical box designed for underground use. All electrical connections must be accessible for future inspection and repair. When making an underground splice, you need the access door on the electrical box to be above ground. You also need special wire nuts designed for underground exterior use to protect the connection from moisture. Work only on electrical circuits where you can turn the power off at the main breaker.

Things You'll Need

  • Noncontact voltage detector
  • Shovel
  • Underground exterior electrical box
  • Underground exterior wire nuts
  • Wire strippers
  • Screwdriver
  • Turn off the power to the circuit you are working on at the main breaker. Confirm the power is off by touching a noncontact voltage detector to the wire's insulation.

  • Dig a hole with a shovel where you wish to make the wire splice. The hole should be large enough to accommodate the exterior underground electrical box and deep enough so that the box will be sightly higher than flush with the surface when backfilled.

  • Strip 3/8 inch of insulation off the conductors for the two wires you wish to join.

  • Insert one wire through an access hole in the side of the electrical box and pull 6 inches of wire into the box. Secure the wire in place by tightening down the wire clamp with a screwdriver. Consult the exterior electrical box's manual for the method to seal the access hole from moisture penetration. Repeat this process to bring the other wire into the box.

  • Twist together the like-colored wires with pliers and install exterior underground wire nuts on each connection.

  • Push the wires into the electrical box and place the cover on top. Secure the cover to the box by tightening the four screws with a screwdriver.

  • Place the box in the hole so the top of the box is slightly above the surface. Backfill the hole with soil, keeping the box's cover exposed.

Tips & Warnings

  • Choose a location for your wire splice that eliminates the chances of mechanical damage from lawnmowers, automobiles or people walking around the property.
  • You need to protect an exterior electrical circuit with a GFCI electrical breaker to prevent electrical shock.

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