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 coloured 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 coloured 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.
Safety tip--notice that in the procedures, you are instructed to wire the ground first, the neutral second and the two hot wires last. Whenever you work on wiring, follow this procedure and reverse it when you are removing wiring. Ground - neutral - hot for installing; hot - neutral - ground for un-installing. This maintains a ground that doesn't involve your body. You don't want to undersized your sub-panel, or oversized it by too much either. Remember that you're bringing in two lines and each can carry the rated capacity on the double breaker. So if you install a 240 volt, 30 amp double breaker, you have a total of 60 amps available to you. If you're installing circuits for shop tools, find out which, if any, can run on 240 volts instead of 120 volts. The power output and usage will be the same, but the amperage draw on your sub-panel will be halved. So if your table saw draws 13 amps at 120 volts, it will only draw 6.5 amps at 240 volts. 13A x 120V = 1560 watts and 6.5A x 240V = 1560 watts. The same power, twice the voltage and half the amperage. If you go 240, you'll have to add a plug for 240 volts and a 240 volt outlet. The tool will need minor rewiring according to the manufacturers directions. This usually means changing how the motor's wiring connects to the power cord and is simple and straight forward. The power requirements of all motors are listed on a metal tag on the motor. The voltage, amps drawn, power in watts, sometimes expressed as volt-amps or VA and horsepower are all possible listings. It's a good idea to have a fair amount of reserve power. If you're planning for a number of tools, you'll want a fair amount of power. Installing a 40 amp sub-panel won't cost much more than a 30 amp sub-panel.
Even though the main circuit breaker is turned off, the lugs on the main circuit breaker are still live and carry lethal electrical current. Never touch the main lugs on a panel board unless you know for a fact the power is turned off before it enters the box. Always test to insure power is off before touching any conductor. This procedure is simple and safe enough for most beginners to handle if they follow adequate safety precautions. However, if you are unsure about how to proceed, or are uncomfortable working inside a circuit panel, hire a licensed professional or get help from someone who knows what they are doing. Working with wiring and electricity can potentially expose you to dangerous or lethal electrical currency. Always shut off the power to a circuit you are working on and test to make sure it is off before proceeding. Always use tools designed for electrical work when working on electrical circuits or wiring. Get the right tool for the job.