How to Wire a Pass Through the Light Fixture
As you install the electrical circuitry for you home, you may have several opportunities to wire a pass through a light fixture. You may do this when you are wiring more than one fixture to a switch. A slightly more complicated situation arises when you want to pass the wires through the fixture to a switch that will control the light. Although it's important to keep track of the wires when you do this, the procedure is one that any electrically inclined handyperson can handle.
Wiring an Extra Light Fixture to a Switch
Run a length of 12/2 circuit cable from a switch to the light fixture through which you will be passing the circuit. If the wires are connected to the switch, make sure it is off. Run a second length of 12/2 cable to the second light fixture.
Pull the ends of both cables through the back of the electrical box for the first fixture. Strip 6 inches of sheathing from both with a utility knife, then use a set of wire strippers to remove 1/2 inch of insulation from the ends of the black and white wires.
Twist the bare ground wires from both circuit cables together with the ground wire from the lamp, which may be bare or green. Use pliers to twist them together clockwise, then screw a wire cap onto the end to keep them together.
Twist the white wires from the circuit cables together with the white wire from the light in the same way, then screw on a wire cap, making sure no bare wire shows outside the cap. Repeat the procedure with the black wires.
Passing Wires through a Light Fixture to a Switch
Turn off the power to the circuit from which you are drawing power by turning off the breaker that controls it in the main panel. Pull the circuit cable through the back of the electrical box for the light.
Run a cable the same size as the circuit cable to a switch and pull one end through the electrical box for the light. Strip sheathing from the ends of the cables in the light fixture box, as well as the end of the cable in the switch box. Expose the ends of all insulated wires with wire strippers.
Twist together all the ground wires in the light fixture box and screw on a wire cap. Twist the white wire from the light together with the white wire from the circuit cable and cap them. Do not twist the white wire going to the switch together with the other two. Twist the black wire going to the switch together with the black wire from the light and cap them. Do not twist the black wire from the circuit together with them.
Connect the white wire going to the switch together with the black circuit wire and wrap black electrical tape around the end of the white wire to signify that it is hot. Twist the wires together and screw on a wire cap.
Wrap black tape around the end of the white wire in the switch box, then connect it to the top brass terminal of the switch. Connect the black wire to the bottom brass terminal and the bare wire to the ground terminal.
- You can pass wires through a light fixture to power any circuit element, like an outlet, but if the light is controlled by a switch, the extra element will be also. To avoid this, pass wires through the switch instead of the light.
- Do not do any wiring unless you are sure the power to the circuit cable is off. If you are not sure, test the ends of the wires with a voltage tester.
Chris Deziel has a bachelor's degree in physics and a master's degree in humanities. Besides having an abiding interest in popular science, Deziel has been active in the building and home design trades since 1975. As a landscape builder, he helped establish two gardening companies.
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