Electricity: Outdoor Wiring

By bringing power outdoors, you can extend the usable hours of your backyard or patio, do away with awkward, and sometimes dangerous, extension cords and enhance your home’s security. Basic wiring techniques are the same indoors and out, but because receptacles and switches are exposed to the elements, they must be housed in weatherproof boxes. Outdoor light fixtures must be rated for use in wet locations. Outdoor receptacles must also be provided with ground-fault circuit-interrupter (GFCI) protection.

Working underground. When buried under ground, outdoor wiring can be either individual Type THWN heat- and moisture-resistant insulated wires installed in metal or plastic conduit or Type UF (under ground feeder) cable. The code permits direct burial of UF cable in a trench at least 24 in. (60 cm) deep. Some local codes may require that conduit protect all under ground wiring. Above-ground outdoor wiring must be run through conduit for protection from physical dam age. PVC plastic conduit is easy to cut and join with PVC cement, but it must be buried at least 18 in. (45 cm) deep. Rigid metal conduit is joined with threaded couplings. It’s harder to cut than plastic and bending it takes some effort, but it requires only a 6-in.-deep (15-cm-deep) trench. Thin-wall metal conduit corrodes easily and isn't suitable for direct burial.

Planning and permits. Before starting an outdoor wiring project, check building and electrical codes and obtain needed permits. Determine where and how to run power from inside to outside the house. You can tap an existing general-purpose circuit as long as it’s not already operating near capacity, or you can run a new circuit from the service panel if you plan extensive outdoor wiring. Next, map out an efficient route for the outdoor circuit. When planning outdoor lighting, be aware of glare and its effects on neighbors and passersby. Use shielded fixtures placed well out of sightlines.

Caution: Before working on a circuit, turn off the power at the service panel and use a voltage tester to make sure the power is off.

Installing an Outdoor Outlet

A convenient way to bring power outside is to run cable between back-to-back boxes on opposite sides of an exterior wall. Locate an existing receptacle on a general-purpose circuit with excess capacity inside an exterior wall, near the desired site of a new outdoor GFCI receptacle. (Do not connect to dedicated kitchen, bathroom or laundry-room circuits.)

To add an outdoor outlet, begin by turning off the power at the service panel. Remove receptacle cover of existing indoor receptacle and use voltage tester to make sure power is off. Disconnect receptacle; remove a knockout from box. Locate studs, then mark new exterior box location; trace box outline on wall. Drill pilot holes through siding; carefully cut outline using a jigsaw with a short blade. Feed cable from indoor box to opening for outdoor box. Run cable through cable connector into new box and secure box to wall. Make connections at indoor receptacle and attach new wires to outdoor GFCI receptacle. Reinstall indoor receptacle. Fasten GFCI receptacle to new box and install an in-use-type weatherproof cover. (Code-approved remodeling-type box, Neutral wires, Existing interior outlet)


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Weatherproof Boxes and Fittings

A weatherproof outdoor switch box has an external lever that activates the toggle of a standard snap switch. The cover’s gasket seals the box against entry of exterior moisture and water.

Outdoor lamp-holders have captive lamp-sealing gaskets and require outdoor-rated bulbs made of shock- resistant glass that won’t shatter if they come in contact with water.

LB fittings with weatherproof gaskets are used with metal or plastic conduit. Removable covers let you pull wire around corners or make a transition through a wall into the ground.

Mounting a Soffit Light

With the power off, run cable from switch-controlled circuit to soffit. Secure cable to outdoor fixture box using cable connector. Secure box to soffit. Connect hot and neutral wires to fixture wires, grounding wire to grounding screw in box. Mount fixture on box.

Tapping an Existing Outdoor Fixture

Turn off power to fixture and disconnect it. Attach box extender to fixture box. Effectively seal around extender box to pre vent water intrusion. Install metal or plastic conduit from extender box to the other surface-mounted switch boxes, receptacle boxes or underground trench location, as desired. Secure conduit within 3 ft. (1 m) of boxes and every 3 ft. (1 m) thereafter.

Install Type THWN insulated above-ground wire or Type UF underground cable as required. Connect new wiring at extender box and replace fixture.

Running Underground Wiring

PVC conduit is easy to install and can be cut with any good saw. Use PVC cement to glue conduit and fittings together. A variety of boxes, fittings and preformed elbows are available. Type UF cable is another easy alternative. However, PVC conduit offers better protection from physical damage and does not have to be buried as deeply as direct-buried UF cable. Standard Schedule 40 PVC conduit can be used in horizontal trenches, but heavy-wall Schedule 80 PVC conduit is required where you move from underground to above ground or where the conduit is subject to damage, such as an exterior wall. Type THWN moisture-resistant wire is used in the conduit: black or red for hot wires, white for neutral wires and green for grounding wires.

How Deep to Bury Cable

Wiring that emerges from underground must be installed in rigid metal conduit or Schedule 80 PVC conduit for protection from physical damage. This protection for direct-buried non metallic (NM) cable must extend from at least 18 in. (45 cm) below grade to the aboveground termination point. The minimum amount of cover (from top surface of finished grade to top surface of any wiring) is as follows:

  • Rigid metal conduit: 6 in. (150 mm)
  • Residential 120-volt GFCI-protected branch circuits rated 20 amps or less: 12 in. (300 mm)
  • Rigid PVC conduit: 18 in. (450 mm)
  • Direct-buried cable: 24 in. (600 mm)

Hints on Working Down Under(ground)

No need to tear up your sidewalk to run cable. Flatten the end of rigid metal conduit, dig a trench on both sides of the walkway and use a sledgehammer to drive conduit horizontally under the walkway. Cut off both ends of the conduit using a hacksaw, file any sharp edges inside the conduit and add connectors to protect the cable. Feed Type UF cable through the conduit.

Last modified: Friday, 2020-05-01 11:00 PST