Installing a garage sub panel is an electrical upgrade almost any handy person can accomplish. First, check that there is room in the service panel for a new double-pole circuit breaker; then, secure the required wiring permit from the local building codes department. Although you are allowed to do almost anything a licensed electrician can do, you are also required to abide by the same rules. You you must have a wiring permit, and your work must pass inspection by a building codes department inspector.
Mount the sub panel to the garage wall. Secure the panel to the wall studs using the 1/4 inch, 1½ inches long wood screws that came with the mounting kit. You can mount the sub panel in any convenient location as long as it will be easily accessible at all times; accessibility is a code requirement.
Run the Type UF cable from the garage sub panel to the main service panel. The cable can run through holes drilled in structural members (wall studs, floor joists, and ceiling rafters) as long as the nearest edge of the hole are at least 1¼ inches back from the nearest edge of the structural member. The cable can also be run along the sides of framing members. The cable must be secured at 4½ foot intervals with cable straps. The cable must be setback 1¼ inches from the nearest edge. These are all code requirements.
Drive the centre knock out on the top of the sub panel inward, using the hammer and a flat-blade screwdriver. Grab the knockout with the lineman's pliers and wiggle it back and forth until it snaps out. Install a cable connector in the hole.
Push the cable through the cable connector until it reaches the bottom of the sub panel. Tighten the clamping screws on the connector, securing it in place.
Remove the cable's outer jacket, using the razor knife. Remove all of the jacket except for ¼ to ½ inch where it leaves the inner edge of the cable connector. Avoid damaging the insulation on the wires inside the cable. Separate the red, black, white, and bare conductors.
Attach the red and black wires to the two lugs at the top of the panel. Use the wire cutters to shorten them as required. Using the strippers to remove 1 inch of insulation from each of them and secure them under the compression screws. Insert the stripped ends under the compression screws and tighten the screws down on the wires.
Attach the white, neutral wire to the panel's silver neutral bar. Attach the bare ground wire to the sub panel's ground bar (the bar with the green screws). Some sub panels come with an electrical connection between the panel's neutral bar and the metal panel box. This connection may be in the from of a green screw passing through the bar and into the box or in the form of a metal strap between the bar and the box. Make sure to remove this bonding screw or bonding strap. Unlike a main service panel, the code does not allow a sub panel's neutral bar to be bonded to ground.
Turn off the main service disconnect on the service panel and remove the panel covers. Do not touch the main lugs at the top of the panel because they are still hot even with the main breaker in the off position.
Remove a panel knock out and install a cable connector. Install the new 2-pole circuit breaker. Depending on the panel you have, the breaker may simply snap in place, or it may be secured in place by screws.
Attach the red and black wires to the brass screws on the circuit breaker. Attach the neutral wire and ground wire to the service panel's neutral and ground bars, respectively.
Have the required inspection performed. Close up the main service panel. Secure the sub panel breaker in the off position. Turn the main breaker on. Do not energise the sub panel until you have wired in your branch circuits and closed up the panel.