Things You'll Need
Electronic stud finder
"Old Work" device box
3/8-inch drill or driver
12/2 w/Gr. ROMEX® cable
ROMEX® cable staples
ROMEX® cable connector
Needle nose pliers
20-amp, 120-volt receptacle
Receptacle cover plate
20-amp, 120-volt, single-pole GFCI Circuit Breaker
The average microwave ovens consumes 1440 watts of power, which is equal to a current draw of 12 amps. This load is the maximum load permitted by the National Electrical Code (NEC) on a 15-amp branch circuit. The NEC also permits a 16-amp load on a 20-amp branch circuit. Yet people often plug their microwave ovens into general-purpose receptacle circuits that support other loads as well, which results in nuisance tripping of the circuit breaker. You should place a microwave oven, whether counter top or built-in model, on its own dedicated receptacle circuit. Installing a dedicated receptacle is a relatively straightforward project, but routing the wiring back to the service panel or breaker panel may present a few challenges.
Obtain a wiring permit from the local Authority Having Jurisdiction (AHJ,) the municipal buildings department. Don't start this project until you have the permit. At this time, you are also given the proper telephone number of whom to call to schedule a future inspection of your work.
Using the electronic stud finder to make sure that you aren't drilling into or cutting into a stud, and mark the location for the new device box. Using the box as a template, draw an outline of the cutout. Drill a 1/2-inch hole in each corner of the cutout and the finish removing it with the portable jigsaw.
Cut a 12-by-12-inch hole in the back of the cabinet at floor level in the same stud space as the receptacle cutout. Drill a 1/2-inch hole down through the wall's sill plate into the basement.
Drill a 1/2-inch hole down through the wall's sill plate into the basement. Route the cable down through this hole and over to the service panel or circuit breaker panel location. Run the cable parallel to the side of the floor joists and perpendicular to the floor joists through 1/2-inch holes drilled through them. Drill all holes so that the near edges of the holes are at least 1 and 1/4 inches back from the near edge of the floor joist or other framing member. This 1 and 1/4-inch minimum spacing is required by the NEC to protect the cable from damage by screws or nails.
Secure the cable within 6 inches of the circuit breaker panel with a cable staple and at 48-inch intervals thereafter where it runs parallel to the wall or framing member. When the cable passes through holes drilled in the framing members, no additional support is required for the portion of cable supported by the framing members it passes through.
Run the cable up through the wall space and into the kitchen through the cutout for the microwave oven receptacle. Cut the cable long enough so that you can insert 6 to 8 inches of the cable into the device box. Insert the device box into the wall cutout and secure in place by turning the mounting wing screws in a clockwise direction, drawing the mounting wings securely up against the backside of the wall.
Remove the outer jacket of the cable using the razor knife. Be careful not to damage the insulation on the wires. Remove 1 inch of insulation from the ends of the insulated wires.
Make loops in the ends of the black wire, the white wire, and the bare copper grounding wire. Connect the wires to the receptacle by placing the loops facing in a clockwise direction under the terminal screws. Place the black wire under the brass screw, the white wire under the silver screw, and the bare copper wire under the green screw. Tighten the screws snugly but don't over tighten. Don't install the receptacle in the box until after the inspection by the AHJ.
Turn off the main disconnect breaker on the service panel and remove the panel's cover. Remove a knockout from the side of the panel by driving it sideways with the hammer and a screwdriver and then wiggling it back and forth with the lineman's pliers until it snaps free. Install the cable connector in the hole, tightening its lock nut securely against the inside of the panel.
Install the GFCI Breaker in the panel. Depending on the panel that you are working on, the breaker may simply snap in place, or it may be secured to the panel's Buss Bar by a screw. Uncoil the white neutral pigtail from the breaker and connect it to the panel's neutral bar.
Install the cable in the panel and secure it in place by tightening the two clamping screws on the cable connector. Remove the cable's outer jacket and route the bare copper grounding wire down to the panel's grounding bar and secure it there.
Attach the black wire to the brass-colored screw on the GFCI breaker and the white wire to the silver-colored screw on the GFCI breaker. With the GFCI breaker in the off position, turn back on the main breaker. Don't close the panel up until after the inspection by the AHJ.
Call for the inspection.
Install the receptacle in the device box. Install the receptacle cover plate. Replace the service panel cover. Turn on the new GFCI circuit breaker. Plug in your microwave oven.
Based in Colorado Springs, Colo., Jerry Walch has been writing articles for the DIY market since 1974. His work has appeared in “Family Handyman” magazine, “Popular Science,” "Popular Mechanics," “Handy” and other publications. Walch spent 40 years working in the electrical trades and holds an Associate of Applied Science in applied electrical engineering technology from Alvin Junior College.