How to run underground electrical wires
Use a GFCI tester to make sure the outlet at the house is GFCI protected.
If you need to install an outdoor electrical outlet, because you need to power a pond pump or other device, think safety. There are several different methods for safely running the underground wiring, but the 12-in. deep PVC conduit method we show is about the easiest and most practical for running a typical 15-amp line. A second method, 12-in. deep direct burial of type UF (underground feeder) cable, is another easy alternative. We recommend the PVC conduit because it offers more protection against physical damage. Both methods have to be GFCI (ground fault circuit interrupter) protected before the wires enter the ground to guard against electrocution in case the wire is accidentally cut while you’re digging.
No matter which method you choose, you’ll have to get an electrical permit first. Review your plan with the inspector and get instructions for inspections.
Tying into an existing garage outlet is usually the simplest way to power your outside outlet for two reasons. First, the electrical boxes are usually exposed for easy hookups, and second, any outlets in garages built after 1981 are supposed to be GFCI protected, which would automatically protect the new outside outlet as well. Unfortunately, there are plenty of unprotected garage outlets out there, so you’ll have to check the outlet before beginning your project. The safest way to check is to buy a GFCI receptacle tester and test the garage outlets. If they’re not protected, you’ll need to install a GFCI as part of the job. You’ll find all the electrical supplies you need for the new outlet project at a home center. Begin by installing a permanent post for mounting the pond outlet box and digging the 12-in.-deep trench. You’ll need these electrical supplies and parts for doing the electrical work:
- Two weatherproof electrical boxes: one for mounting the receptacle at the pond and a second for the garage to splice the transition between the cable and individual wires. Metal electrical boxes must be grounded to the bare or green insulated ground wire(s) contained in the box.
- A cable clamp for running cable through the back of the box from the garage outlet. Use duct seal putty to seal the wall penetration hole to keep moisture out of the box.
- “Schedule 80” PVC (1/2-in.) conduit, elbows and couplings for containing the wires and connecting the exterior boxes. Use PVC cement for joining the parts.
- PVC adapters for connecting the conduit to the weatherproof boxes.
- Type THWN moisture-resistant insulated wire to pull into the PVC conduit between the two weatherproof boxes. You’ll need three different colors (green for the grounding wire, white for the neutral wire, and black or red for the hot wire). Match the wire gauge to the existing wire in the electrical box you’re powering the new outlet from.
- Conventional cable (Type NM-B) to connect the garage outlet to the individual THWN wires within the outside garage splice box. (Again, match the existing garage wiring when choosing the gauge.)
- Rain-tight weatherproof covers to protect the plug and receptacle.
In addition, you’ll need a receptacle, cable staples, conduit straps and wire connectors. Mark the garage outlet plate cover you’re tying into with a “GFCI Protected” sticker (or handwritten notice). The inspector will want to examine the below-grade depth and connections, so don’t backfill the trench until your work is examined. Otherwise you may be forced to redig the trench.
Underground Electrical Wires in PVC Conduit
Bury PVC conduit at least 12 inches down. Check all details with a local building inspector.
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