How to Wire a Subpanel in a Detached Garage

Installing a subpanel in a detached garage is a straightforward project but it is labour intensive. The most difficult part of this project will be digging the trench and running the underground feeder cable from the main service panel to the subpanel. The Type-UF cable is a direct burial cable but the National Electrical Code (NEC) requires that you protect it against damage where it leaves the ground and enters the structures. Use electrical conduit where it leaves the ground and enters the building.

Drive wooden stakes into the ground at five-foot intervals to mark out the route the trench takes between the structures. Connect the stakes together using Mason's cord or heavy twine.

Use the trenching shovel to excavate a trench with a minimum depth of 20 inches and a width of at least 12 inches. Table 300-5 of the NEC requires that the direct burial cable is covered with at least 18 inches of soil.

Utilise the hole-saw attachment to drill two and one-half inch in diameter holes where the conduit enters the garage and house.

Measure the thickness of the walls and then cut two pieces of polyvinyl chloride (PVC) conduit the thickness of the walls plus one and a half inches in length. Attach the PVC LB condulets to one end of each of these lengths of conduit. To attach the conduit to the PVC LB, coat the outside of the conduit and the inside of the fitting on the back of the condulet with PVC primer.

Apply a coating of PVC cement to the conduit and condulet fitting while the primer is still wet. Shove the conduit into the condulet fitting and twist it back and forth several times to spread the cement out evenly around the conduit. Slip the conduit through the walls and give the cement a few minutes to set up before proceeding.

Place the factory 90s in the trench. Using the tape measure, measure the distance between the factory 90s and the condulet fittings and add three inches to the measurements. Cut lengths of PVC conduit to theses lengths and cement the pieces together as you did the condulet and conduit nipples.

Mount the subpanel to the wall above where the conduit enters the garage using the mounting kit that came with the subpanel.

Run the cable from the main service panel's location to the subpanel's location. Remove a metal knockout from the bottom of the subpanel and install a cable connector. Slip the cable through the connector and tighten the screws securing the cable in place.

Use the razor knife to remove the cable's outer jacket. Remove one inch of the insulation from the ends of the red and black wires and slide the stripped ends of the wires into the panel's main lugs. Tighten the compression screws down on the wires. Connect the white wire to the panel's neutral bar. Connect the bare, copper ground wire to the subpanel's ground bar, the bar with the green-coloured screws.

Turn on the battery-powered work light and shine it on the main service panel. The room lights will go out when you turn off the main breaker. Turn off the main breaker and remove the panel covers.

Install a cable connector in the side of the main panel box. Install the two-pole breaker into the panel. The breaker may be fastened to the bus bar with screws or simply snap in place, depending on the style panel you have.

Insert the cable in the panel box as you did at the subpanel. Connect the red and black wires to the brass screws on the circuit breaker. Connect the white and bare copper wires the same as you did at the subpanel. Replace the panel covers and turn on the main breaker.

Things You'll Need

  • Hand sledge hammer
  • Wooden stakes
  • Mason's cord or heavy twine
  • Trenching shovel
  • 2-inch diameter PVC electrical conduit
  • 2 two inch diameter PVC factory 90s
  • 2 two-inch diameter, PVC LB condulets
  • Tape measure
  • 2 and 1/2-inch hole saw
  • 3/8-inch drill-driver
  • PVC pipe cutter
  • PVC primer
  • PVC cement
  • Subpanel with mounting kit
  • Type UF direct burial cable
  • Screwdrivers
  • Hammer
  • Lineman's pliers
  • Cable cutters
  • Razor knife
  • Wire cutters
  • Wire strippers
  • Cable connectors
  • Battery-powered work light
  • Two-pole, 240-volt circuit breaker
Cite this Article A tool to create a citation to reference this article Cite this Article

About the Author

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.