How to Run Underground Wiring to a Garage – This Old House

Posted by in Home Improvement Tips, on October 15, 2015

Electrician Scott Caron shows a homeowner how to bring power to a detached garage. (See below for a shopping list, tools, and steps.)

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Shopping List for How to Run Underground Wiring to a Garage:
– UF (underground feed) electrical cable
– ¾-inch-diameter plastic conduit, used to house cable
– ¾-inch conduit outlet bodies, boxes, and assorted fittings
– Exterior-grade acrylic-latex caulk, used to weather-seal conduit outlet boxes
– Sand and red caution tape, used to warn of the buried cable
– 1 5/8-inch galvanized or stainless-steel screws, for attaching conduit outlet boxes
– Cable staples, used to secure cable to floor joists and wall studs
– GFCI breaker, installed at electrical panel

Tools for How to Run Underground Wiring to a Garage:
– Trenching machine, for excavating trench
– Cordless drill, used to drill holes and drive screws
– 1½-inch-diameter hole saw, for boring hole in house and garage walls
– Plastic pipe cutters, used to cut plastic conduit
– Propane-powered heat gun, used to heat and bend plastic conduit
– Rake and shovel, for backfilling trench
– Hammer
– Wire strippers
– Screwdriver

Steps for How to Run Underground Wiring to a Garage:
1. Call the local utility company and have them check for buried pipes or wires.
2. Rent a gas-powered trenching machine and dig a trench at least 12 inches deep from the house to the garage.
3. Bore a 1½-inch-diameter hole through the house wall and into the basement with a drill and hole saw. Position the hole directly above the end of the trench.
4. Cut a length of ¾-inch-diameter plastic conduit to extend from the bottom of the trench up to the hole in the house wall.
5. If necessary, use a propane-powered heat gun to bend the conduit to conform to the foundation.
6. Attach a plastic conduit outlet body to the end of the conduit.
7. Push a length of UF (underground feed) electrical cable into the conduit, then feed the end of the cable through the hole in the house wall.
8. From inside the basement, pull the cable all the way to the main electrical panel. Secure the cable to the overhead joists with cable staples.
9. Go outside and lay the cable into the trench, running it all the way to the garage.
10. At the garage, repeat Steps 3 through 7 to install the plastic conduit and cable.
11. After feeding the cable into the garage, screw the conduit outlet body to the exterior wall of the garage. Screw the other conduit body to the house wall.
12. Apply exterior-grade acrylic-latex caulk around the conduit outlet body to seal out water.
13. Cover the cable with about 2 inches of sand, then lay caution tape on top of the sand along the entire trench.
14. Backfill the trench with the excavated soil.

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15 Have a licensed electrician make the final wire connections in the garage and at the main electrical panel.
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