Adding an Outdoor Receptacle

Most older homes were built with few, if any, outdoor receptacles. But it's not that difficult to add one. If you don't already have the tools for this electrical work, don't worry: they're inexpensive and easy to find. You'll use most of the same parts you'd use for installing an indoor receptacle; the main difference is that you need a wiring box and cover rated for outdoor use. Here's how to get the job done.

To add a single receptacle that's mounted on an exterior wall, it's usually best to tap into a nearby interior receptacle, such as one in a bedroom or living room. Do not tie into circuits for heavy-duty units like dishwashers, dryers and heaters; those circuits are already dedicated to these high-draw appliances. To get the power you need for an outdoor receptacle without risking overload, it's best to feed from a 20-amp circuit rather than a 15-amp one.

One more detail worth noting: there are two kinds of exterior box covers. One has a spring-type lid that are weathertight only when shut. The other cover's lid is rated to be watertight while in use. If you intend to keep something plugged into your outdoor receptacle Christmas lights, for example the cover must be labeled "watertight while in use." It's against code to keep something plugged into the spring-lid type.

Step by Step
1. Tap into a receptacle. Turn off the power to the receptacle, and check it with a plug-in tester to verify that the power is off. Remove the cover plate and unscrew the two screws that hold the receptacle to the outlet box. Pull the receptacle out of the box as far as it will go, and disconnect it by removing all the wires from its terminals.

If the box is metal, remove the mounting screws from the wall and gently pull the old outlet box a few inches out of the wall and push it to the side. If the existing wires are too short to let you do that, detach them from the box. Then use a slotted screwdriver with a broad blade to break off or pry open a knockout anywhere in the box. This is where your new cable will come in to feed the outdoor receptacle.

If the box is plastic, you may need to use a hacksaw to cut the mounting nails that extend into the adjacent stud. Removing the box could be so messy that you should find another source of power, like tapping into a cable in the basement.

2. Run cable to the new box. With the old outlet box pulled off to the side, you should now be able to see into the wall cavity. Pull apart the insulation and make sure that there's nothing inside the wall such as plumbing that you could damage by drilling into the wall from the outside. For ease of installation, the new outside outlet box should be in the same wall cavity as the box you're tapping into, or just below that cavity, in a joist space that's accessible from the basement or crawl space. Walk outside and try to find the exact spot where the wall cavity is. If you're not sure, drill a tiny (1/8-inch-diameter) hole from inside to outside to provide a reference point. (Seal it later with clear silicone caulk.) You can use the same strategy to locate the joist space below the box, this time drilling through the floor. Use a 1/2-inch spade bit to drill up into the wall cavity from below, as shown.

Pick a spot on the exterior wall that will allow the back of the new outlet box to fit flat against it. It doesn't matter if the wall siding is angled as long you can mount the box flat. At this spot, drill a 1 1/4-inch-diameter hole that's 5/8 inch deep; this hole provides clearance space for the cable clamp on the backside of the box, so it can lie flush to the siding. Then, using a 1/2-inch spade bit, finish drilling through the exterior wall and into the wall cavity or joist space. (You could, of course, drill all the way through the wall with a 1 1/4-inch spade bit, but a 1/2-inch hole is easier to caulk and big enough to accommodate the cable.)

Push the length of new cable through the hole and into the wall cavity or joist space. Then fish the cable up through the old receptacle cutout. For now, leave 8 to 10 inches of slack cable sticking out beyond the siding and interior wallboard.

3. Mount the new watertight box outside. Bring the new cable through a hole in the back of the outdoor outlet box, securing it to the back of the box with a cable clamp. Fill the hole with silicone caulk and screw the box into place (the clamp will fit into the 1 1/4-inch-wide hole), then caulk all around the joint between the outlet box and the siding. For a joist space installation, use the same caulk to fill the voids around the cable where it penetrates the joist.

4. Hook up the outside wires. Now you're ready to hook up your wiring. Cut away any cable that extends more than 4 inches beyond the front of the box. Strip all but 1/2 inch of the sheathing off the cable; that 1/2 inch section is the portion resting beneath the clamp. Then strip 1/2 inch of insulation off the individual wire ends. Find the two LINE terminals on the GFCI receptacle. Ignore the LOAD terminals, because you will not be extending the circuit beyond this receptacle. Attach the black wire to the LINE gold screw and the white wire to the LINE silver screw. If the outdoor box is metal, you'll need to ground it: Loop the bare copper grounding wire under the box's ground screw before you attach it to the receptacle, and tighten the box's ground screw. Then attach the grounding wire to the receptacle's green screw. Once done, stuff the receptacle into the box, fit the gasket over it and install the weatherproof cover. You're done outside.

5. Rewire the interior receptacle. Back at the interior outlet box, use a cable clamp to connect the new cable to the box (some old metal boxes won't need a clamp) and remount the box in the wall. You'll likely have two or three cables now. If there are only two (one original cable feeding the box, plus your new cable), you can strip and secure the black and white wires from the new cable to the receptacle (black to brass, white to silver).

If there are three cables (two existing cables and the new one) and the receptacle's terminals are filled, use a wire connector to splice all the black wires together along with a short "jumper" wire that can be attached to one of the brass-colored "hot" screws on the receptacle. (The jumper wire provides a bridge between the spliced wires and the receptacle.) Splice the white wires and the ground wires in the same manner and attach both of these jumper wires to their respective screws, as shown. Once all splices and connections have been made, gently fold the wires back into the outlet box, screw the receptacle back into its place and reinstall its cover plate.

6. Test your work. Turn the power back on, and check both receptacles for proper wiring with a plug-in tester. At your new outside receptacle, press the GFCI's TEST button. It should trip off with an audible click, and the lights on the plug-in tester should go out. Press RESET. You're done.