There are a few things that you will need to consider before you begin rewiring the garage. Are you going to be using the garage as a workshop as well as place to park your car and store your garden equipment? This is an important consideration because it will determine how many branch circuits you will need to install. When rewiring a garage that will include a workshop, you should install a subpanel because of the heavy concentration of loads.
Plan out your rewiring project on graph paper and present it along with a detailed materials to be used list to the Authority Having Jurisdiction (AHJ), your city's Buildings Permits Department, when applying for the wiring permit. Wait for the permit to arrive and post it on the garage as required before beginning this project.
Mount the subpanel in a spot where there will be free access to it at all times. Free access is a National Electrical Code (NEC) requirement. Secure it to a concrete or cement block wall by drilling holes and installing the lead anchors that came with the subpanel. Mount it to a wooden wall by screwing directly into the studs.
Remove a knockout from the top of the subpanel box. Drive it sideways with hammer and screwdriver. Grasp it with lineman's pliers and wiggle it back and forth until it snaps free. Install a cable connector in the hole.
Run the 1/3 w/Gr. Type UF cable from the subpanel to the main circuit breaker panel. You may run cable along the side of framing members or perpendicular to them through holes drilled through them. Follow all NEC requirements for drilling holes and laying cable.
Push the cable into the subpanel a distance equal to the height of the panel and secure in place by tightening the clamping screws. Remove the cables outer jacket using the razor knife.
Run the bare copper grounding wire down to the panels grounding bar identifiable by its green screws. Shorten as necessary and secure under the compression screw of the grounding bar's main lug.
Attach the white neutral wire to the panel's neutral bar in a similar manner after stripping ¾ inches of insulation from its end.
Attach the red and black wires to the two main buss bar lugs at the top of the panel.
Turn off the main service disconnect and remove the service panel covers. Remove a knockout from the side of the panel box and install cable connector.
Install the 100-ampere circuit breaker in the panel. The breaker may plug in to the panel, or it may be secured to the buss bars by two screws, depending on the style of panel that you have.
Install the feeder cable in the panel. Attach the ground wire and the neutral wires to the ground and neutral buss bars. Connect the red and black feeder wires to the brass screws on the circuit breaker. Secure the 100-ampere breaker in the off position before turning the main disconnect back on.
Install the required number of branch circuit GFCI breakers in the subpanel. Divide them as equally as possible between the two buss bars. Keep in mind when laying out your wiring that no 20-ampere branch circuit should supply a load greater than 1920 watts. Calculate the total wattage of all you power tools and divide by 1920 to get the minimum number of circuits required.
Connect the white pigtail leads on the GFCI breakers to the subpanel neutral bar. Remove the required number of knockouts and install cable connectors for the number of branch circuits that you are installing. Divide theses cables evenly or as evenly as possible between the two sides of the panel box.
Mount the device boxes for the general-purpose receptacle so that no point along the unbroken floor line is more than 6 feet from a receptacle. Mount the first box 6 feet from the garage door and then mount the remaining boxes at 12-foot intervals. Duplex receptacles count as two receptacles.
Mount the boxes for receptacles and switches 46 inches above the floor.
Run 12/3 Romex between the device boxes for the receptacles and run a cable from the last box to the load centre, the subpanel. Insert the cable ends in the boxes so they extend 6 to 8 inches from the boxes. Insert the cable into the subpanel far enough for it to reach the bottom of the panel.
Remove the cable jacket and remove ¾ inches of insulation from the ends of the wires in the device boxes.
Run a cable from the panel to the light switch just as you did for the receptacle circuits. Run another cable from the light switch to the ceiling-mounted lighting-outlet box. The number of lighting-outlet boxes you will need to install will depend on the size of the garage. You can never have too much light in a garage used as a workshop. Link the lighting-outlet boxes together just as you did the receptacles.
Connect the black wires to the brass screws, the white wires to the silver screws and the bare wires to the green screw on the receptacles.
Splice the white wires in the light-switch box together. Attach the black wires to the brass screw on the light switch. Attach the bare wire to the green screw.
Call the AHJ for the required inspection.
Install the receptacles and switches in their boxes. Install all cover plates, install lighting fixture(s), and close up panels. Turn on all breakers.