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Adding Electrical Outlets: How to Wire a New Outlet to an Existing One

Dan has been a licensed journey-level electrician for 17 years. He has extensive experience in most areas of the electrical trade.

Adding and Wiring New Outlets

Do you need extra outlets and you'd rather plug into a wall? Power bars are unsightly and who wants extension cords snaking all over the floor? Besides, these can be fire hazards as well as overly attractive to small children.

Maybe you are changing the location of the TV or installing an over-the-range microwave, or maybe you bought a new computer desk. Whatever the reason, there isn't a handy outlet for the equipment and you need one. This is when it becomes necessary to add a new plug-in by wiring it into an existing one.

This article will walk you through not only the mechanics of putting a new electrical outlet into the wall, but also running the wire and tapping into the existing circuit. The tools and materials will be discussed as well as the procedures and tips for doing the job in the easiest manner possible.

While the task will take some work and may involve crawling through attics or crawl spaces, it is not particularly esoteric or difficult to understand. You can do it.

Tools You'll Need

Most of these items should be in your tool box, but some are a little more specialized. If you don't have a tool, I've listed others that might be used to do the job:

  • Drill. Some form of drill will be necessary, one that's capable of drilling through about five inches of wood. A drill bit of about ¾" will do the trick. A spade bit is fine here. A second, smaller bit around 1/8" can be very useful. A cordless drill is ideal, but if yours has a cord, you will need an extension cord as well.
  • Saw. A small square will need to be cut into the wall where the new outlet is to be installed. You could use a razor knife (box cutter), a jab saw for drywall, or a jigsaw for paneling (or drywall). It will be very handy to have a metal-cutting blade for the jigsaw, or some other saw blade that will cut nails—even a plain hacksaw blade will do.
  • Wire cutters. You will need a way to cut wire and strip insulation off of it. Wire strippers are preferred, but other tools will do. Electrician's diagonal wire cutters will be useful.
  • Tape measure. You need a way to measure, because locating the place to cut into the wall or the hole to be drilled must be done with precision.
  • Screwdriver. You will need a Phillips-head screwdriver, and a flat blade will be handy as well.
  • Pliers. A pair of needle-nose pliers will be very handy to have.
  • Flashlight. You will be working in either an attic or crawl space, and you will need some light to work by.
  • Voltage detector. A non-contact voltage detector is always very nice to have. Although not strictly necessary, it is a wonderful addition to your toolbox and can protect you from shock hazards.

Before You Start

Before rushing out to the store, please read through this article first. It will be more difficult to cut two nails with a simple hacksaw blade rather than with a brand new cordless sawzall, but it's also lots cheaper. Some of these tools are nice to have and use, but are not absolutely necessary and it doesn't always make sense to spend $50 for a new tool to save ten minutes of work (unless, of course, that is the only way to get that new tool okayed by your spouse!).

Single gang box (left); two-gang box (right).

Single gang box (left); two-gang box (right).

Materials You'll Need

  • Two new plastic "old work" or "cut in" boxes (see above photo). One of the old work boxes should match the existing box. If the existing box has two outlets in it (four places to plug into), a "two-gang" box will be necessary. (Most outlets are single, though, meaning they have just two places to plug into.) The new box can be any size you want. You could even add in a 4 gang box (with 4 receptacles giving 8 plug-ins) if there is room between the studs in the wall. Try not to buy very shallow boxes in either case. A 3-inch-deep box is far preferable to one that is only a couple of inches deep.
  • Wire. Before purchasing wire, locate the circuit breaker that feeds the existing outlet. It will have either a 15 or a 20 stamped on it; this refers to the amps available on that circuit. A 15-amp circuit breaker will require 14-2 NM (Romex) wire (with ground), while a 20 amp circuit breaker will need 12-2 wire, also with a ground. How much wire you will need depends on if it will be run overhead in the attic or below the floor in the crawl space. If the old and new plug-ins are separated by studs, measure how much wire it will take to start at the old outlet, run up through the attic (or down through the crawl space), over the new location, and down (or up) to connect to the new outlet. Up, across, and back down. In my experience it is all too easy to purchase insufficient wire, and it is often that "up and down" that gets forgotten.
  • A new outlet with a cover plate. Outlets are available in 15 amp or 20 amp capacity; choose the one that matches the breaker on the circuit, just as you did for the wire. Various colors are available, in different shapes, sizes, and materials. All but the ampacity are personal choices. A note of caution, though: If the existing outlet does not accept a three-prong cord, the new outlet must be a GFCI type. The instructions further down will guide you through the installation, replacing the existing outlet with a GFCI and adding a new outlet. While GFCI outlets are considerably more expensive, don't skimp here. The National Electric Code requires that any outlet without a ground must be a GFCI, and for good reason: without a ground wire the shock hazard increases dramatically. The GFCI outlet is designed to eliminate that hazard.
  • Other supplies. You'll want a handful of wire staples with which to fasten the wire to the house structure and possibly a half-dozen wire nuts.

What Is a GFCI?

A ground fault circuit interrupter (GFCI), also known as a residual current device (RCD), is a device that detects that a current is flowing through an unintended path (like water or a person) and shuts off.

How Can I Tell the Difference Between a 15-Amp and 20-Amp Outlet?

Visible difference between a 15-amp (gray, on left) and a 20-amp outlet (white, on right); note the slot shape circled in red.

Visible difference between a 15-amp (gray, on left) and a 20-amp outlet (white, on right); note the slot shape circled in red.

Checking an outlet to see if it is "hot" with a non-contact voltage detector.

Checking an outlet to see if it is "hot" with a non-contact voltage detector.

How to Remove the Old Outlet and Box

  1. Turn off the power! Please don't begin your work by getting a nasty shock - turn off the power to the existing outlet you will be working with. Tape the breaker off so that no one else turns it back on while you're working. Make sure that you are working safely by testing that the outlet is dead by either plugging a radio or lamp into it or using a voltmeter or non-contact voltage detector. Don't turn the breaker back on until the job is complete; I, an electrician by trade, have been shocked too many times by turning the power on "for just a minute" and forgetting to turn it back off. Don't let it happen to you!
  2. Choose the existing outlet you will wire the new outlet into. The best choice is one that's almost, but not quite, directly through the wall from where you want the new one to be. In other words, you should first consider the outlets in the room on the other side of the wall. Second best is an outlet on the same or different interior wall, preferably one without insulation. It is much easier to get new wire through an empty wall than one filled with insulation.
  3. Remove screws and cover plates. With the power safely off, remove the cover plate on the old outlet and set it and the single screw that holds it aside for re-installation later. Then remove the two screws holding the outlet to the box behind it.
  4. The wires to the old outlet now to be need removed as well. Make careful note of which wire goes where. The black wire should go to the brass-colored screws, the white wire to the silver-colored screws, and the wire bare of insulation (the ground) to the green ground screw. There may be other wires in the box that are spliced together with wire nuts or that terminate on the outlet as well; note where each goes and mark them or draw a picture for future reference. Note: The old outlet could have a red wire as well. If so, this article on wiring a half-hot outlet has more information on this specialty application.
    NOTE: A common practice is to plug the wires into the back of the outlet instead of attaching them to the screws on the side. If you have this kind of outlet, there is a small slot on the back of the outlet where a very small screwdriver or other tools can be inserted; this will release the spring tension on the electrical wire and allow it to be pulled out of the hole. Alternatively, the wires can simply be cut off if they are long enough and the outlet is to be replaced.
  5. Remove the old box. Now, unless the new outlet will be almost directly through the wall from the old outlet, you need to remove the old box. Most home boxes are fastened with two nails, just above and below the box, that are driven into the stud just to one side. The best way to remove the box from the wall is to reach just alongside a metal cutting saw blade (a Sawzall, jigsaw, or even a bare hacksaw blade will do the trick). Cut the nails holding the box to the wall, but be aware that the wires entering the box are fastened to the same stud just above or below the box - make sure that you don't damage those wires, as such damage will likely require an electrician to re-wire that section of the house.
    NOTE: If you don't care to cut the nails holding the old box, you can take the box out in pieces, since you won't be re-using it. Use a pair of pliers, diagonal cutters, a hammer, and a screwdriver—whatever it takes—to tear apart and remove the plastic box. Just tear it into little pieces and get it out of the wall. Again: don't damage the wire! Bend the nails that used to hold the box as best you can; the objective is to make sure they don't interfere with the little arms on the new box that needs to be attached to the wall.
  6. As you draw the box out of the wall, the wire(s) in it will need to pass through the slots in the rear of the box and exit the box. Different boxes have different methods of holding those wires; take a good look at what you will have to do to let them exit the box. Loosen a screw, perhaps, or simply work them gently out of slots in the back.
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Read More From Dengarden

How to Run a Wire for the New Outlet

  1. Cut a hole. Start cutting a hole for the new plastic box at the new location. Make sure not to cut too large a hole, because the "ears" on the box must remain outside the wall; if they fit through the hole, the box will not clamp to the wall but will simply fall inside.
  2. Stagger the boxes. If the new location is through the wall from another existing outlet, be sure they don't line up exactly, because there won't be enough room in the wall for two boxes to fit back-to-back. Move the new one a few inches to one side. This is absolutely the preferred location for a new outlet: through the wall and not absolutely back-to-back, but between the same pair of studs as an existing outlet. It makes running the wire a very easy task (you won't be crawling the attic after all!) and removal of the existing box probably won't be necessary.
  3. Extend the wires. If you can reach into the hole where the new box will go and touch the existing box, you can skip all the hard work of running new wire through the attic; simply take one end of the new wire, reach inside the wall, and push it into the old box alongside the other wires. Push in about a foot of wire; you will cut it off to a more reasonable length later. Cut the other end a foot outside the wall, leaving a good-sized loop inside the wall to go up and back down into the top of the new box.
  4. If the two boxes aren't so conveniently located, however, it is time to begin pulling wire between the two outlets, and it is advisable to have a helper for this operation. A ¾" hole needs to be drilled, either up from the crawl space or down from the attic, into the center of the wall directly below or above the new outlet location. Begin by drilling a very small hole through the floor or ceiling right at the edge of the wall and in line with the new outlet location. Push a stiff wire (like a straightened clothes hanger) through, so that the wire is visible in the crawl space or attic. If necessary, remove a section of the baseboard or shoe molding on the floor, so that when it is replaced, the small hole will be covered. Be aware that a spinning drill bit can and will "grab" carpet and unravel it; take extreme care here. A very small hole in a ceiling can be covered with toothpaste to make it invisible.
  5. With your stiff wire poking through, carefully orient yourself into the crawl space. To determine where the wall is, measure 2½ inches over and drill a small hole up into the wall; if the hole does not come through the floor/ceiling into the living area, you are inside the wall. If your hole went into the wall, replace the small drill bit with a ¾" bit and drill again in the same place. The procedure is the same if you are working in the attic, but you should be able to see the top of the wall and that will help considerably. The 2x4 running along the top of the wall is generally visible under the attic insulation. Either way, repeat the procedure at the existing outlet location.
  6. Push one end of the new wire into the hole until it can be reached from the outlet location, and pull about a foot of wire out of the wall at that point. String the wire across to the next hole and estimate how much more will be needed to get it through the wall and pulled out. Cut off the length needed and push it into the drilled hole until it can be reached. Do not cut too little; far better to waste an extra 5' of electrical wire here than to waste all that work.
  7. Staple the wire to convenient rafters or floor joists every 4 feet or so with staples made for NM cable.