Lacking X-ray vision, you can't know exactly what's behind your home's walls or above its ceilings. That's why fishing an electrical cable to a new outlet or switch, like any fishing expedition, takes time, patience and a certain amount of trial and error.
But, given the standard spacing of studs and joists typically, 16 or 24 inches on center there's a good chance you can predict where the obstructions will be. You can also use a stud finder to locate framing you can't see. Thus, with patience and a little luck, you can extend cable from an existing power source to virtually any new location, without chopping a bunch of unnecessary holes in your walls, floors or ceilings. Or, you can move the new outlet slightly so that running cable to it is easier.Three more caveats: First, always turn off the power before handling electrical cable or devices. Second, get help. Typically, one person feeds metal fish tape down into a wall cavity as a second person, with another fish tape, tries to catch the first. It's a pretty futile exercise trying to do it by yourself. Third, whenever you drill or notch into framing members to run cable, protect that cable with steel nail plates attached to the edge of joists or studs. Otherwise, when you cover up the framing again, you could drive a nail or screw into your new wiring.
How to Run Cables From AboveIf you have an unfinished attic, you can probably route cable down into a wall outlet immediately below. Locate the wall's top plate and, with your spade bit, bore a 3/4-inch hole through it. Thread fish tape through that hole into the space between two studs. If the tape hits something partway down the wall, you've probably encountered a fire stop, a wood block nailed at a right angle to a stud or joist to seal off the air passage and help prevent fire from spreading. To get cable through a stop, you'll need to open up the wall above it and notch the block. Use a hammer and chisel, and wear safety glasses.
If there are no stops, your helper downstairs (who's already cut an outlet hole in the wall with a drywall saw) can yell when he or she sees the fish tape coming down from above. To grab hold of that fish tape the metal is stiff that person will need a second fish tape or, in a pinch, an unbent coat hanger.Connect the cable to the fish tape with electrical tape as shown in the animation, then reel in the tape and cable. To wire the outlet below, you'll need at least 7 to 12 inches of cable to protrude from the outlet opening.
How to Run Cables From Below If your basement has an unfinished ceiling, you can run cable from there to a first-floor wall outlet. Just below the opening for the new outlet you want to wire, right where the wall meets the floor, drill through the floor at an angle with a 1/4-inch bit. Shove a coat hanger into the hole, then go to the basement. The coat hanger is aligned with the wall's surface, so if you mark a point 2 1/2 inches from the coat hanger, that mark should put you right in the middle of the wall cavity. Using a spade bit, drill a 3/4-inch hole straight up through the subfloor and bottom plate.
You can probably push a new length of cable up through the 3/4-inch hole to the outlet opening in the wall. If pushing doesn't work, try fishing it from above. Again, to wire the outlet, allow at least 7 to 12 inches of cable to protrude from the opening.
How to Run Cables Across Joists and StudsIf you can't fish from above or below, however, sometimes you just have to cut into finished surfaces and either notch into or drill through framing members. It's a messy business.
In plaster (top), open up the ceiling along the entire cable run, and remove parallel strips of lath. Bore 3/4-inch holes through the joists. After running the cable, patch the opening with strips of drywall. To match the thickness of the plaster, you may need to build up a couple layers of drywall. This is a good time to repaint the ceiling.With drywall (bottom), make an opening at each framing member and cut a 3/4-inch notch into its edge. After cutting the notches, run the cable and shield it at each notch with nail plates. Patch the holes.
By the Book
According to most building codes, you may not cut or drill into any joists smaller than 2-by-6s. For joists that are 2-by-6 or greater, you can notch no deeper than one-sixth its total depth (e.g., 1 inch deep for a 2-by-6). Finally, never notch a joist in the middle one-third of its span.
Tips From the ProsIf you have lots of corner notches to cut, consider renting a reciprocating saw. Its replaceable blades can get into spaces where nothing else will saw. If there's any evidence of nails in the wood you're cutting, use a metal-cutting blade and always, but always, wear safety glasses.
How to Run Cables for a Ceiling LightIf the floor above is finished and you can't feed cable down from above, you'll need to make a few selective cuts. First, thread fish tape from the new ceiling outlet through ceiling joists until you come to the top plate of the adjoining wall.
Cut into the juncture of the wall and ceiling as shown. Notch the wall's top plate to accommodate the cable. After you run the cable, protect it with nail plates wherever it passes through notched framing. Run the cable down the wall.At the base of the wall, you'll need to cut back finish surfaces across the edges of several studs. Happily, this channel can be covered by a baseboard. But again, before replacing finish surfaces, add nail plates to protect the cable.
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