Tapping into an existing circuit to add a light socket or fixture is an excellent project for beginning do-it-yourselfers. Because the hard work of adding the branch circuit to your home's electrical system has already been done, all you need to do is make a few cuts in the drywall and ceiling, install a light switch and junction box and hang your new fixture. This presents a great opportunity to learn how to run electrical cable behind walls and ceilings, make connections and repair drywall. Only basic tools are involved and no special skills are required.
Locate an electrical socket to use as a power source. Outlets have four terminals on the side: two brass screws and two silver screws. The terminals are arranged in pairs with one brass and one silver screw across from each other. One of the pairs is labelled "load," while the other is labelled "line." Power flows into the outlet through the load pair and out through the line to the next outlet in the circuit. Find an outlet with no wires attached to the line pair to use as a source for the new light.
Shut off the electrical current to the circuit.
Make the cuts in the wall and ceiling for the electrical cable, wall switch and fixture. Above the switch hole, where the wall and ceiling meet, make a small hole through which to feed the cable. Do the same above the source outlet.
Install the mounting hardware for the light fixture and the switch. Use a brace bar strung between two ceiling joists to support the junction box for the light. You can attach the switch box to the drywall.
Run the cable between the switch and fixture by first threading the fish tape through the hole above the switch and across the ceiling until it reaches the junction box. Attach the electrical cable to the fish tape and pull it back toward the switch. Drop the cable behind the wall to the switch hole.
Run the cable between the fixture and the power supply outlet, repeating the process above.
Connect the wires at the switch by attaching the black wire to the brass terminal and the white wire to the silver terminal. Wrap a piece of electrical tape around the white wire to indicate that it is hot, meaning that it is carrying electrical current
Connect the wires at the outlet, attaching the black wire to the brass line terminal and the white wire to the silver line terminal.
Connect the white wire coming from the light switch to the black wire on the light fixture. Mark the white wire with electrical tape to indicate that it is hot. Connect the white wire from the power supply to the white wire on the light fixture. Connect the two remaining black wires together and ground the fixture to the junction box.
Attach the fixture to the ceiling junction box. Use a drywall patch kit to repair any holes.