A breaker box in a detached garage is usually a sub-panel to the main circuit panel in the home. The advantage of installing a breaker panel in the garage is that it allows you to have multiple circuits available while only needing to run one line to the garage from the house. This line can be buried or overhead depending on personal preference and community rules. Within the garage, you can have 120 or 240 volt circuits. Each circuit can be installed with the proper sized breaker and wire to accommodate the need.
Make a list of the type of electrical devices that you will be using in the garage. Pay special attention to the big items that use 240 volts. Use this information to help determine the correct amp rating for the breaker in the main circuit panel. For most garages, you will want to use a 50 amp breaker with 8/3 wire between the two buildings to handle the load. Install a sub-panel with its own main breaker so that the power can be shut off inside the garage for servicing the breakers in the panel.
Begin the job by turning off the main breaker in the main panel. Feed the end of the wire going to the garage into the panel. Use wire strippers to remove about a foot of the insulation on the outside of the wire. Continue by removing one inch of the outer coating from the ends of each of the wires inside. Attach the red and black wires to the 240 volt 50 amp breaker by sliding the ends into the slots beneath the screws and using a screwdriver to tighten them. The white and green wires will be attached to the neutral bar by inserting the ends under the screws on the bar and tightening the screws.
Use screws and a cordless electric screwdriver to fasten the sub-panel to the wall of the garage. Run the wire out of the house and to the garage. Inside the garage, cut the wire with wire cutters so that about a foot of wire remains inside the sub-panel. Remove the outer and inner insulation in the same way as before. Attach the black and red wires by pushing the ends under the lugs for the main breaker in the sub-panel and tightening the lugs with an Allen wrench. The white and green wires will attach the same way in the sub-panel as they did in the main panel.
Install the 120 and 240 volt circuits in the sub-panel as needed. For 120 volt circuits, attach the black wire to the breaker and snap it to the power bus. The white and bare wires will be attached under the screws on the neutral bus. When installing a 240 volt circuit, attach the black and red wires into the slots under the screws on the breaker. The white and bare or green wires will attach to the neutral bus. Make sure that all screws are tightened to secure the wires.