Understanding the wiring of a light switch is useful for making minor alterations to your home's lighting without hiring an expensive electrician. The set-up is a basic switching circuit in which moving the light switch to the "up" position completes a circuit, sending power to the light bulb.
Turn off the light switch's circuit at the breaker box. Use a circuit testing device to make sure the power to the switch is off before you begin work.
Determine the circuit's amperage of your circuit and obtain the appropriate switch and wiring. Use 12 gauge wire if your circuit is 20 amps or 14 gauge wiring if your circuit is 15 amps. Your switch rating also should match the amps of your electrical circuit. If your home has aluminium wiring, purchase a "CU-AL" designated switch.
Loosen the switch plate and switch so you have access to the terminals at the back. If mounting a new switch, install the switch box in your wall. Measure the height of the other switches in your room and mount the new switch at the same height, usually about 45 to 48 inches from the floor. Orient the switch's "up" and "down" positions properly.
Cut and strip the end of the positive, or "hot," black wire from the power source and run it to the bottom terminal of the light switch. Loosen the terminal screw, wrap the bare end of the wire around the terminal and tighten the screw. This will close the power circuit when the switch is moved to the "down" position.
Cut and strip a section of wire and attach it to the top terminal of the light switch by tightening the terminal screw. Attach the free end of this wire to the positive terminal of the light fixture wire. When the switch is in the "up" position, the light will be on.
Cut and strip the end of the white negative wire from the power source and splice in a wire running to the negative terminal of the light fixture. This will complete the lighting circuit.
Connect the ground wire from the power source to a piece of metal in the switch box. Splice an additional grounding wire to the original and run and attach the end of this wire to a metal point in the light-fixture mounting box.
Ensure that all connections are secure and turn on the circuit's power at the breaker box. Test the switch by moving it between the "up" and "down" positions. If the light does not turn on, power down your circuit and check your connections. If the light still does not work, consult an electrician.
Always be sure your circuit is completely shut off before attempting any electrical work. Failing to do so could result in electric shock and personal injury. If you come across a light switch in your home that appears to be wired in some other fashion, do not attempt to rewire it as described in this article. Consult an electrician for additional wiring circuits and considerations.