How to Tap Into an Existing Wall Switch
Electrical switches typically have 110-volt power connected directly from the circuit panel. The switch will pass the electricity from the panel to a light or ceiling fan whenever the light switch is turned on, and break the power connection when the switch is turned off. If you need to provide power to an outlet or another switch, you can tap into either the live wire from the panel on the existing wall switch, or the switched wire, should you desire that the new element be controlled by the switch.
Locate the circuit breaker that controls the switch and turn it off.
Pull off the switch cover plate by removing the two screws with a screwdriver. Turn the switch to the on position. Verify that the power is off to the switch by touching one leg of a circuit tester to the ground wire within the box, and the other to one of the two contact screws on the switch.
Unscrew the two screws that hold the switch in the box, and pull the switch and the wiring out of the box as far as the wiring will allow.
Insert a new section of electrical cable (of the same gauge and configuration as the wiring already in the box) through the hole in the back or side of the box. Pull out about 8 to 10 inches of wire.
Strip about 6 inches of outer sheathing from the cable with a utility knife. Then expose about 3/4-inch of bare wire from the tip of both the white and black wires of the new cable with a wire stripper.
Loosen the wire nut connecting the two white wires. Twist the new white wire around the two existing white wires, and re-connect with the wire nut.
Cut an 8-inch single strand of black electrical wire from your supply. Strip about 3/4-inch from each end of this short piece of wire. Twist one end of this wire around the exposed black wire in the box and connect with a wire nut.
Remove the black wire coming from the breaker panel from it's contact on the switch. Twist this wire around the two black wires in the box, and connect with a wire nut. Then attach the exposed end of the short wire to the contact on the switch.
Twist the three bare wires together, and make sure one of the three twisted bare wires is connected to the green grounding screw within the box.
Push the switch and the exposed wiring back into the box, and tighten the two screws to hold the switch into position. Replace the switch cover and the two cover screws.
Turn on the electrical power at the breaker panel and test the switch and the power at the end of the tapped wire.
Things You Will Need
These steps are for connecting to un-switched power from the panel. Should you desire to have the switch control the tapped wire, twist the two new black wires to the black wire connected to the electrical element (the fan or light that the switch controls), rather than the black wire from the circuit panel.
Do not attempt to tap into the switch without first turning off the power to that switch at the breaker panel.
Chris Baylor has been writing about various topics, focusing primarily on woodworking, since 2006. You can see his work in publications such as "Consumer's Digest," where he wrote the 2009 Best Buys for Power Tools and the 2013 Best Buys for Pressure Washers.