How to Run Electrical Wiring Underground

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How to install direct burial electrical cable or an underground conduit to provide power to a shed or some other location in your yard.

At some point you may want to have power in a garden shed or detached patio or some other area in your yard. Instead of running an extension cord to that particular location you may want to install an underground conduit or direct burial electrical cable to provide a more permanent solution. Consult your local building department concerning electrical regulations before attempting any underground installation. Some local agencies may require a permit to be issued before any trench is dug and you may want to have a survey done of your property to locate any exiting underground electrical, water, and gas lines. Most states have a Call Before You Dig hotline or you can contact the service department for your local utility company for assistance.

Direct Burial Cable Installation

This is a more simple approach as special electrical cable can be placed directly in the ground. Consult your local building inspections department to determine if this type of installation is allowed and acquire the appropriate permits.

This article does not describe how to terminate either end of the cable at the circuit breaker or outside receptacle or structure.

Supplies and Tools

Marker flags or landscaping spray paint

Trenching Shovel

Type UF (underground feeder) wire

1. Lay out the route of the cable run with marker flags or paint to indicate the beginning and end of the underground trench location. If the trench is not a straight line, a marker flag should be placed at every deviation or turn from the straight line.

2. Use a shovel to dig the trench. Keep in mind that the type of underground wires and voltages will play a key role as to the depth of the trench. The National Electric Code (NEC) Table 300-5 is used to determine the overall depth of the trench. In residential installations the underground cable is carrying less than 300 volts. This would require a minimum depth of 12 inches of soil covering the wire or cable. Branch circuits rated 20 amps or less with GFCI protection at their source are allowed a minimum cover of 12”.

Cables that are carrying irrigation circuits and landscape lighting voltages of 30 volts or less, will need a trench of no less than 6 inches deep.

3. Remove all loose soil and rocks from the underground electrical cable or wire trench.

4. Lay the underground feed wire, Type UF, into the trench. Some regulatory agencies may require an inspection after the wire is laid into the trench, but before any soil is placed back into the trench. Obtain the proper inspection.

5. Backfill the trench soil that is free of any rocks. Do not place any type of rocks back into the trench, as the hard edges can and will damage the wire's outer insulation over time.

You may also want to install plastic caution tape about 6 inches above the cable and then fill in the remaining 6 inches of soil on top of the tape.

Various Underground Warning Tapes

Underground Conduit

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.

Additional Tools and Supplies

PVC conduit

PVC Cutter or hacksaw

PVC couplings

PVC 90 degree elbows

PVC Threaded fittings for receptacle box, if needed

PVC Primer

PVC Cement

Spools of Red, Black, White, and Green THWN copper building wire or UF cable

Electrician’s Fish tape

Electrical tape

1. For installing conduit you will want to keep the run as straight as possible. Typically you can only pull wire through four 90 degree bends, including the ones that turn up out of the ground and into the panel. If you have more than 4, you will have to install a hand pull box in the ground to allow you to pull the wires to this point and then feed them back into the conduit and continue to the end of the run.

2. 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 trenching machine.

3. Begin laying the conduit where the connection to the panel will be. You will need to core a hole into the basement wall or wood sheathing to do this. You can install an elbow and come out of the ground to allow you to enter the home through the wall instead of the foundation.

Sweep in Trench

4. Lay out the conduit end-to-end next to the trench. Cut the final length so that the last elbow is positioned correctly. Using a PVC cutter assures you will have a square cut, but you can also use a hacksaw to cut the conduit.

5. You can purchase conduit that have a flared female end on one end of the pipe so that you will not need couplings. If you have straight conduit 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.

6. Install the 90 degree sweep elbow at the end of the run and a piece of conduit to bring the conduit to the proper finished height.

90 degree Sweep

Receptacle Box

7. Wait 30 minutes for the joints to set up before pulling the wiring through the conduit. Feed the fish tape from the access on the outside of the house to the end of the run.

8. 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. Note: You can run UF cable in the conduit, but it is difficult to pull through due to the outer layer of insulation.

9. Pull enough wire through the conduit to reach through the wall and the inside conduit to the shop's subpanel.

10. Cut the wires long enough on the house end to pass though the wall and inside conduit to the service panel.

11. Seal around where the conduit passes through the walls using spray foam insulation or caulking.

1 comment

Jerry Walch
Posted on Nov 21, 2010