A garage is a great place to have a workshop. Often, the builder only puts in a single 15 amp circuit for the lights, garage door opener and one receptacle to plug in a hedge trimmer or similar equipment. That single outlet was put there mainly because it is required by code and not out of any generosity or consideration. Adding a sub-panel to the garage for additional outlets, lighting and shop equipment is a common and fairly straightforward project.
Determine the size of the sub-panel service. Common sizes are 30, 40, 50 and 60 amps. Consider the power requirements of the equipment you plan to use and how many pieces of equipment you can use at one time. For many installations, the 30 Amp service is plenty, but don't underestimate your needs today or in the future.
Find the path for the sheathed cable between the main circuit panel and the garage. The shortest distance is preferable, but locating the sub-panel conveniently is also important.
Locate two adjacent studs where you plan to mount the sub-panel. At a minimum height of 42 inches, mount the plywood to the studs with wood screws. Use the torpedo level to make sure it is level.
Mount the sub-panel to the plywood using 5/8 inch round head screws through the box mounting holes.
Drill any holes necessary for running the sheathed cable between the main circuit panel and the new sub-panel. Run the cable between the two boxes, but do not open the main circuit panel or make any connections just yet.
Open the main circuit panel door and turn off the main circuit breaker. With the no-contact voltage tester, bring it near any wire that exits the panel. If the power is off, the light will remain off and the panel is safe to work on. Remove the panel cover.
Install the new 240V circuit breaker in the main circuit panel. The double circuit breaker will hook onto the panel, then snap into place when you place pressure on it. Put the new breaker in the OFF position.
Remove a convenient knockout hole from the main circuit panel box and install a sheathed cable bushing in it. Strip the sheathing from the cable at a sufficient length to maintain the neatness of the wiring in the box and reach all four connections. Run the cable into the box through the bushing, leaving about 1/2 an inch of cable sheathing in the box.
Route the bare ground wire to the ground bus bar. In many main circuit panels, the ground bus bar and the neutral bus bar are the same. This is NOT true for sub-panels, however, so the wiring in the sub-panel will be different. Put the ground wire through an empty hole in the bus bar and tighten the screw firmly.
Strip about 3/4 inches of insulation from the white neutral wire with the wire stripper. Run the wire to the neutral bus bar, put it through a hole and tighten the screw firmly.
Strip 3/4 inches of insulation from the two colored wires, usually black and red. Place one into each terminal of the new circuit breaker. It does not matter which one goes into which terminal of the circuit breaker. Tighten the screws firmly.
Dress the wires neatly in the panel box and put the cover back on, replacing all the screws. You'll have to remove two knockouts from the panel cover to accommodate the new double circuit breaker before replacing it. Insure the new circuit breaker is in the OFF position, and turn the main circuit breaker back on.
Remove a convenient knockout from the sub-panel box and install a bushing in it. Strip the sheathed cable to a length that gives you plenty to work with. Push the cable into the box through the bushing until 1/2 inch of cable sheathing is in the box.
Connect the bare ground wire to the ground bus bar. Insert the wire into a hole and tighten the screw firmly. In Sub-panels, the ground bus is connected to the metal cabinet and isolated from the neutral bus bar. This isolation must be maintained for safe operation.
Strip the end of the white neutral wire and connect it to the neutral bus bar. Note that in a sub-panel, the ground and neutral bus bars are always different and never connected (bonded) in any way.
Strip the ends of the two colored wires and put each into the supply lugs, usually at the top or bottom of the box. Tighten the lugs firmly. You are ready to install new circuit breakers in the new box and wire your new circuits.
Replace the sub-panel cover when you are finished working on it. Don't leave it off overnight or while you are not around.