How to splice into an existing circuit for an outdoor light

Updated February 21, 2017

Outdoor lighting often requires creative wiring solutions to supply power to locations far from existing fixtures or outlets. However, existing indoor electrical circuits can provide a readily accessible source of power for most outdoor lights. Identification of the most appropriate circuit and installation of a proper junction box are key to safely tapping into existing wiring without causing a potential fire hazard. You can quickly and easily install an electrical junction box and splice into existing wiring to supply an outdoor light installation.


Measure the distance from the planned location for the outdoor light to the exterior walls of the home. Record the external position of the light.

Identify the most appropriate existing circuit to split to create a power supply for the outdoor light. The circuit running through an attic or crawl space is a likely choice.

Locate the rafter space that corresponds to the position of the outdoor light, using distances to exterior walls.

Map the route of the wires from the existing circuit across the attic, through the wall header plate, and between the wall joists to the location of the light. Identify the most accessible, least obstructive location for installation of the junction box.


Turn off the circuit breaker to the circuit you will splice to create a power supply for your outdoor light.

Mount the junction box to a rafter in the designated location. Pop out plugs on two or three sides of the box as needed to accommodate the wires running in and out of the box.

Strip the plastic sheathing from a 1.2 cm (1/2 inch) section at the end of each existing wire at the junction box. Insert one end of the cut (existing) three-wire cable or wire bundle into one side of the junction box. Insert the other end of the cut cable into the other side of the junction box.

Remove the plastic sheathing from a 1.2 cm (1/2 inch) section at one end of the three-wire cable or wire bundle that will power the new outdoor light. Insert one end of the new cable or wire bundle into the desired opening in the junction box.

Connect matching wires inside the junction box with wire nuts. The wires will be in groups of three: three black, three white, three green. If the existing wiring does not have a green ground wire, connect the ground wire of the new cable directly to the junction box, according to the manufacturer's recommendations.

Fold the connected wires over neatly inside the box and attach the junction box cover.

Drill holes through the exterior wall at the location of the exterior light or at the most accessible exit point for the wiring. Drill a hole through the header plate at the top of the space between the wall joists above the light, if necessary. Insert the electrical fish wire through the hole in the exterior wall and gently push it through the space between the wall joists and through the header plate into the attic. Attach the new wiring to the fish wire and pull it through to the exterior of the house to power the new light.


Use metal electrical conduit to run wiring from the junction box or an easily accessible exit point to the location where you plan to install the light. Seal around wires at the exit point with expansion foam sealant to keep out mice, snakes and other pests. Install a motion-activated light so that you do not need to install a mechanical switch for the outdoor light.


Be sure that adding the load of the outdoor light will not overload the existing circuit. If the circuit breaker for the circuit trips easily, select a different circuit.

Things You'll Need

  • 3-strand 14-gauge copper wire (or separate black, white, green wires)
  • Wire cutting/stripping tool
  • Wire nuts for 14-gauge wire
  • Electrical junction box kit
  • Drill with large drill bits
  • Screwdrivers
  • Electrical fish wire
  • Measuring tape
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About the Author

Emmalise Mac has been writing professionally since 2006 and her work has been published online, in newsletters, newspapers and scientific journals and in wildlife guidebooks. She has published on topics including wildlife, pets and pet health, science, gardening, outdoor activities and crafts. She holds Bachelor and Master of Science degrees in biology.