Adding an additional light to your existing old light fixture is a convenient way to add more light to a normally dark area. For example, a long hallway might have a single light at one end, while the other end of the hallway is dim. Adding the second light at the other end could provide safety by giving you the opportunity to see trip hazards in what was once a dark area. Knowing how to wire a second light fixture can keep dollars in your pocket because you didn't need to hire a professional—you did it yourself.
Turn the electrical power off to the light circuit by turning off the breaker at the main electrical panel.
Test to verify the circuit is disconnected. Remove the switch plate cover from the switch that powers the light. Use an inexpensive two prong electrical tester and touch one prong to one of the black wires on the switch and the other prong to any metal on the switch or electrical box. The power is disconnected when the tester does not light up. Repeat this for the remaining black wire on the switch. Replace the switch cover plate.
Remove the original light fixture from the ceiling. Remove the light shade and the light bulb to access the screws holding the fixture to the ceiling. Remove the screws and the fixture, and set the fixture aside.
Cut a hole in the ceiling to install the ceiling junction box. Trace around the template provided with the new "old work" ceiling junction box and cut the hole using a keyhole saw.
Pull a length of 14/2 NM cable from the new light location to the original light fixture electrical box. Leave approximately 8 inches of cable exposed at both locations.
Strip the NM cable at both light locations. Use dual NM wire cutter/strippers and remove 4 to 6 inches of exterior sheath from the cable exposing the enclosed black, white and bare copper wires. Strip 1/2 inch of insulation off of the black and white wires.
Install the new "old work" ceiling junction box. Pull the NM cable through the junction box knockout and install the junction box into the ceiling following the manufacturer's directions.
Connect the wires and reinstall the original ceiling light. Connect the white wire from the original light, the new white wire you installed and the white wire from the original electrical box using a red wire connector to connect all three wires together. Use a red wire connector to connect the three black wires together, the black from the light and the two blacks located inside the original electrical box. Connect the two bare copper wires together inside the junction box with an orange wire connector. Attach the original light fixture to the junction box. Replace the bulb and the shade.
Install the new light fixture. Assemble the new light fixture according to the manufacturer's instructions. Connect the bare copper green ground wire to the ground screw located inside the "old work" ceiling junction box. Connect the white wire from the junction box to the white wire from the light fixture by twisting an orange wire connector onto the two wires. Connect the black wire from the light fixture to the black wire inside the junction box with an orange wire connector as well. Push all the wiring inside the ceiling junction box and attach the light following the installation instructions.
Turn the power back on to the light circuit by turning the breaker on in the main electrical panel.