Step-by-Step Garage Wiring

Install a subpanel on an interior or exterior wall of the garage, screwing it to a stud or to the sheathing at eye level and at a place that minimizes the amount of wire you need to connect it to the main panel. If you install the panel outside, be sure it is rated for exterior use.

Run a length of 8/3 electrical cable between the subpanel and the main panel. If you have a detached garage and have to run the wire underground, dig an 18-inch trench, enclose the cable in PVC conduit and lay the conduit along the bottom of the trench. Connect the conduit to each panel with the proper connector after first pulling enough cable into the panel to make connections.

Turn off the main breaker in the main panel and connect the neutral and ground wires to the silver and ground bus bars respectively in that panel as well as in the subpanel. Connect one end of each of the black and red wires to the lugs of a 60-amp double-pole breaker and snap it into place in the main panel. Turn off the breaker and turn the main breaker back on to restore power to your house. Connect the other ends of the black and red wires to the two brass bus bars in the subpanel.

Plan the circuits you need in the garage. It's usually a good idea to put outlets on one circuit and lights on a different one so that if an outlet trips the breaker you'll still have lights. If you have a garage door opener, wire it to a separate circuit controlled by a breaker with a rating that matches the current draw of the door opener. Each 240-volt appliance must similarly be on a dedicated circuit controlled by a breaker with a rating that matches its current draw.

Install electrical boxes for all the lights, switches and outlets you need, nailing them to studs or rafters at appropriate locations. If you need an exterior light, install an exterior box on the siding or under a soffit.

Run 12/2 cable from the subpanel to the first box in one of the circuits, then run a second length of cable from that box to the next and so on, according to your plan. Pull the cable into the subpanel through a knockout hole in the side or bottom, and pull enough cable through the back of each box in the circuit to make connections. If the circuit will power a device that operates at 240 volts and draws 30 to 50 amps, use 10/3 cable.

Strip the cables in the subpanel, separate the wires and connect the end of each black circuit wire in the subpanel to an appropriately rated circuit breaker and install the breaker into the front of the panel. 15-amp breakers are usually sufficient to control light circuits, but use 20-amp breakers for outlet circuits into which you plan to plug power tools. You need a double-pole breaker with an appropriate rating for each 240-volt circuit for which you ran 10/3 cable.

Connect all the white wires in the subpanel to the silver bus bar by inserting the end of each one into a vacant slot and tightening the screw with a screwdriver. In the same way, connect all the ground wires to the ground bus.

Attach each circuit device, whether it is an outlet, a switch, a light fixture or an appliance, to the wires extending out of the appropriate electrical box. If you have to splice together wires in the box without attaching them to a device, such as the white wires in a switch box, twist them together and screw on a wire cap.

Switch off all the breakers in the subpanel, then switch on the breaker in the main panel that controls the subpanel. Verify that the breaker stays on, then switch on each breaker in the subpanel in turn. If any breaker trips, check the connections in that circuit.