Installing an underground conduit wiring system today is much easier than it once was. Not so long ago the only type of electrical conduit available was made of steel or heavy-walled aluminum, and to work with these you needed expensive tools. Today, you have an alternative with rigid polyvinyl chloride (PVC) conduit. The National Electrical Code (NEC) permits you to use this conduit for direct burial installations.
Before beginning this project, you need to determine the size of the subpanel that you plan to install in your shop. The loads the project will serve will determine the size of the underground service---the size of the individual conductors and wires---which in turn will determine the size of the conduit that you will need to run.
Things You'll Need
- Wooden stakes
- Layout cord
- Hand sledgehammer
- Trenching shovel or power trencher
- 1/2-inch electric drill
- Hole saw attachments
- Core drill
- PVC conduit
- PVC Cutter or hacksaw
- PVC couplings
- 2 PVC Factory 90s
- 4 PVC "LB's" Condulets
- PVC Primer
- PVC Cement
- Spools of Red, Black, White, and Green THWN copper building wire
- Fishing tape
- Diagonal pliers/wire cutters
- Wire strippers
- Black, plastic electrical tape
Lay out the route the conduit will take from the house to the shop. Try to pick a point outside the house as close to the location of the service panel/circuit breaker panel as possible. Drive stakes in the ground along this path and tie them together with Mason's layout cord. This will keep your trench straight as you dig it.
Dig the trench 24 inches deep. Table 300-5 of the NEC requires that nonmetallic conduit be buried at least 18 inches below the surface. Depending on the length of the trench you are digging and the type of soil that you have, you may want to consider renting a power trencher.
Drill a hole through the outer walls of your house and shop. If you are cutting or drilling a hole through wood, vinyl, or aluminum siding, use a hole saw in the 1/2-inch electric drill. If you need to drill the hole in a cement block or poured concrete wall, you will need to rent a core drill. Core drills as well as power trenchers are available for rent at most home centers and tool rental centers.
Cut a length of conduit to go through the shop's wall. Measure the thickness of the shop's wall and add 3 inches to get the length to cut. Repeat for the house wall.
Lay out the conduit end-to-end next to the trench. Cut the final length so that the Factory 90s are against the house wall and shop wall. Factory 90s have a gentle sweep, which makes passing a fishing tape past them easy. Using a PVC cutter assures you a square cut, but you can use a hacksaw to cut the conduit if you don't have a PVC cutter.
Assemble the conduit lengths using couplings. Prime the facing ends of the conduit and conduit couplings. Prime the inside surface of the coupling and the outside surface of the conduit to the depth of the coupling. Immediately coat the inside surface of the coupling and the outside surface of the conduit with the PVC cement and force the coupling and conduit together, giving them a quarter-turn twist to distribute the cement evenly around the joint. Repeat until all the conduit lengths are joined together.
Attach a Factory 90 to each end of the conduit run using the same priming and cementing procedure as you did for the straight lengths of conduit.
"Dry fit"---don't glue them together yet---the Condulets to the wall nipples and pass the nipples through the walls. Measure between the bottom to the socket on the Factory 90s and the bottom of the sockets on the Condulets and cut two pieces of conduit to those lengths.
Assemble these short lengths of conduit to the Factory 90s and their matching Condulets with primer and cement. Assemble the wall nipples to the back of the Condulets. Remove the covers to the Condulets.
Wait 30 minutes for the joints to set up before pulling the wiring into the Condulets.
Feed the fishing tape from the Condulet on the outside of the house to the Condulet on the outside of the shop.
Remove 6 inches of insulation from the ends of the insulated wires. Loop the stripped ends through the eye on the fish tape, wrapping the stripped ends around the wires coming from the spools. Tape them in place.
Pull enough wire through the conduit to reach through the wall and the inside conduit to the shop's subpanel.
Cut the wires long enough on the house end to pass though the wall and inside conduit to the service panel.
Seal around where the conduit passes through the walls using spray foam insulation.
- Underground Devices: Solvent-Cementing Joints for PVC Rigid Nonmetallic Conduit, Duct, and Fittings
- Anixter: Conduit Fill
- "National Electrical Code"; National Fire Protection Association; 2008
- Photo Credit electrical spools image by jimcox40 from Fotolia.com
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