If you want more light in your garage, you can install additional lights on an existing circuit. As long as the combined maximum amperage rating of all the fixtures combined doesn't exceed the circuit breaker rating, you can install as many fixtures as you want. On a properly installed circuit, adding more fixtures is a simple matter of running and connecting additional wiring.
Turn off the breaker that powers the garage lighting circuit.
Remove the bulbs from an existing fluorescent light fixture in the garage. Remove the two retaining screws from the cover of the fixture. Remove the cover and set it and the retaining screws aside.
Remove the retaining screws and cover from the fluorescent fixture that you are installing. Pry one of the knockouts out of the top of the fixture with a flat screwdriver.
Place the threaded end of a cable clamp through the knockout opening with the threads inside the fixture. Thread the included knurled nut onto the cable-clamp threads. Turn the knurled nut clockwise until it is snug.
Choose a mounting location for the new fluorescent fixture. Mount the fixture in the chosen location using the included screws.
Run a length of two-conductor wire from the existing fluorescent fixture to the new fixture. Loosen the screw on the side of the cable clamps on both fixtures. Feed the ends of the wire through the cable clamps and tighten the screws until the wire is secure.
Cut the wire, leaving approximately 6 inches of wire sticking out of the fixtures. Remove the outer jacket from the wire up to the cable clamp. Strip 1/2-inch of insulation from the ends of the conductors in the wire.
Remove the wire nuts from the connections inside the existing fixture. Insert the stripped end of the white conductor into a wire nut with the white conductors in the fixture. Turn the wire nut clockwise until it is tight. Insert the stripped end of the black conductor into a wire nut with the other black conductors in the fixture. Turn the wire nut clockwise until it is tight.
Loosen the green grounding screw inside the fixture. Wrap the bare copper wire around the screw. Tighten the screw until the wire is secure. Place the cover on the fixture, insert the retaining screws and tighten them securely. Install the bulbs in the fixture.
Insert the bare end of the white conductor into a wire nut with the loose white conductor in the new fixture. Tighten the wire nut. Insert the bare end of the black conductor into a wire nut with the loose black conductor. Tighten the wire nut. Wrap the bare copper wire around the green ground screw and tighten the screw.
Place the cover on the fixture, insert the retaining screws and tighten them. Install bulbs in the fixture. Turn the circuit breaker on and test the lights for proper operation. Check the connections and breaker if the lights don't work.