Dear Mr. Electrician: I want to install an outlet in an existing wall on a dedicated circuit for my window air conditioner. I’ve figured out how to run the 12/2 NM-B cable from the basement’s main circuit breaker electrical panel up through the wall under the living room window. How do I install an electrical receptacle box in the wall containing the wires and supporting the receptacle?
Answer: To install an outlet in an existing wall requires a lot of measuring. Assuming that you will be drilling a hole from below and fishing the cable into the outside wall, the best place to cut a hole and mount a box is adjacent to a wall stud. The electrical box can be screwed through its side directly to the wood to make it solid.
Table of Contents:
- Locating A Wall Stud
- How To Install A Plastic Old Work Electrical Box
- How To Use Madison Bars
- How To Install An Outlet Box In Wood Lathe and Plaster
- Tools and Materials For Outlet Box Installation
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LOCATE WALL STUD TO INSTALL OUTLET BOX
Locating a wall stud may not be that easy. I usually use an existing wall receptacle or switch as a reference. They are most likely mounted on wall studs if installed when the house was built.
I would remove the wall plate from the existing receptacle or switch and poke a long, thin screwdriver into the electrical box’s right and left. I would also push the box a little to see if it pivots. Usually, the side that is mounted to the stud will not move.
To install an outlet in an existing wall in newer homes with drywall, I also look for indications of nail holes that were spackled over to locate wall studs. I use a flashlight by shining it parallelly close to the wall surface. This shows up all of the blemishes, dimples, nail pops, and patches.
A magnet is also helpful in finding the nails and screws in the wall studs.
After I spotted a few studs, I measured over to the area where I would like to cut a hole for a receptacle. In most home construction, the wall studs are sixteen inches apart, but that is not always the case.
Sometimes, wall studs are twenty-four inches apart. On some taller condominium buildings, I have found the outside walls to have studs twelve inches apart. Measurement is from center to center, and I always measure twice. If I am lucky, my measurement will correspond with a spackled nail hole, which confirms that I have a stud. Tool and material list below.
One trick I learned from my dad is to measure 48″ from the inside corner of any wall. There should always be a wall stud because drywall and paneling are 48″ wide and need a stud to land their edge on for support and nailing.
Finding a wall stud in older homes with plaster walls is not as easy. There are no indications of nail holes, and sometimes outlets have been installed in the baseboard molding.
The effectiveness of electronic stud finders on plaster walls can be limited because of the inconsistencies of the old plaster keys and wood lathe. In this situation, the window itself may be the best indicator. There should be at least one stud on each side of the window that goes from floor to ceiling.
For walls with regular drywall, you can use a one-gang plastic old work box or a 2″x3″ metal old work box. Consult article 314 and table 314.16(A) and table 314.16(B) in the National Electrical Code for the proper size box for the wire you will use.
INSTALLING A PLASTIC OLD WORK BOX
An old plastic work box is probably the simplest to install in drywall, but it requires a bigger hole in the wall and may require a mid-size or a jumbo-size wall plate to cover the box and hole fully. Hold the face of the box up to the wall where you want the receptacle to be. Draw a line around it using a pencil.
Using an awl or a long, thin screwdriver, poke a hole in the center of your mark. Push the awl or screwdriver in all four directions to see any obstacles and how close you are to a stud. If you don’t feel the stud with the awl or screwdriver, use a fish tape or a piece of a metal coat hanger (Click here for my post on using a coat hanger) to probe further into the hole.
I like to use a metal fish tape to push further in all directions to get an idea if there will be anything I should be concerned about before drilling my hole from the basement, such as water pipes or air ducts.
When you have the side of the stud located, and you are confident that there aren’t any obstacles in the wall, you can cut the hole so that the side of the electrical box will be against the side of the stud. You can use a compass saw or a keyhole saw for this.
Ensure the plastic old work box fits into the opening against the stud. It is okay to have small gaps not more significant than 1/8″ on the side, but the top and bottom should be snug to allow the box ears to have a good place to sit against.
The old work box will have plastic cable clamps inside the back to grab the NM-B cable. You should pry one open (Do not remove it!) with a screwdriver and bring your cable into the box. Strip part of the outer sheath off and get at least eight inches of wire into the box. Now push the box into the wall opening and tighten the screws that bring the box clamps against the wall using a screwdriver or a drill.
Do not over-tighten, as the plastic ears will bend and lose their grip on the wall. Ensure the box is straight, and then drive one 1″ or 1 1/4″ x #8 sheet metal screw through the side of the box and into the stud. This will make the box solid and not wholly dependent on the drywall for support. Do not drill more than one screw, as the box will distort and will not look straight.
Alternatively, a single gang metal electrical receptacle box can support your receptacle, but the installation is slightly different. You will need to drill a hole in the side of the metal box before installation to drive a screw into the stud. Not all old-work metal electrical boxes have adjustable clamps to tighten onto the drywall, but external support can be used instead.
USING MADISON BARS TO INSTALL A METAL OUTLET BOX
My preferred method for mounting old work metal boxes is using what I call “Madison Bars.” After the old metal work box is fitted into the wall, one edge is pulled slightly out about 1/4″. The other edge is pushed flush with the wall, and a Madison Bar is inserted between the wall and the box.
Insert the long end of the Madison Bar alongside the old metal work box and slide it down. Now, push the short end of the Madison Bar into the wall and slide it up. While pushing that same edge against the wall with a screwdriver, the wings of the Madison Bar are bent over the box edge and pinched tight using needle nose pliers.
Click here to read my article with photos of Madison Bars being used to install an electrical outlet box.
On the other side of the metal box, insert the long end of the Madison Bar upwards, and when you push the short end into the wall, slide the Madison Bar down. Push the box edge against the wall with a screwdriver, bend over the Madison Bar wings, and pinch them with needle nose pliers. If you did it correctly, the box will now be held to the wall tightly.
You can unbend the Madison Bars and do it over if it is loose. Tap the box with a screwdriver and pliers to get it straight. When the box is held tightly by the Madison Bars, drive one #8 x 1″ or 1 1/4″ sheet metal screw through the previously drilled hole into the stud, and the box will be quite solid. Do not over-tighten the sheet metal screw, as that could cause the outlet box to become distorted.
A metal box must be grounded. This is accomplished using a 10-32 ground screw threaded into a hole in the back of the box. The bare ground wire from the NM-B cable is wrapped around the 10/32 screw before attaching it to the receptacle. You can squeeze the wire around the 10/32 screw using needle nose pliers. Tighten the screw.
INSTALL OUTLET BOX IN WOOD LATHE AND PLASTER WALL
Use a metal old work electrical box to install an outlet in an existing wall if you have wood lathe and plaster walls. Mark the wall as above, but instead of using an awl or screwdriver to punch a hole, use a tiny masonry drill bit in an electric drill. This will help prevent excessive damage to the plaster.
Use the hole to probe for obstacles and find the stud’s location. Also, you want to locate the wood lathe strips and ensure that one entire strip will be lined up with the center of the box. There should be a partial strip for the box’s upper and lower ears to be supported.
Once you know the exact position of the stud and where your box will be inserted, you should mark the opening on the wall. However, the height may need to be adjusted according to the wood lathe location. Ideally, you only want to remove one full section of the center lathe and a portion of the upper and lower slats.
I usually tape around the intended opening using 2″ masking tape to support the plaster. I then score the edges with a razor knife. I have heard that others have used a cement board scoring tool successfully to cut out the plaster.
You must cut the wood lathe as gently as possible when installing an existing wall outlet to avoid significant damage to the surrounding plaster and keys. A fine tooth blade such as a hacksaw blade works well, though it can be tedious. Cut the center lath through on the side furthest from the stud.
Sometimes, I will cut this side almost all the way and then use my Knipex High leverage Diagonal Pliers to cut through the last part. I do that to avoid having the lathe move too much after it has been cut, thereby protecting the plaster from breaking. Next, cut the side close to the stud.
I have not tried it yet, but my multi-function oscillating tool will probably work well in cutting holes in plaster. I expect it will wear out the blade quickly, but I think it will make a nice, clean-cut hole. I have used the multi-function tool for cutting boxes into drywall, which works well. It is also perfect for notching the wall stud to accommodate the protruding screw on the side of the metal box.
Cut part of the upper lathe and part of the lower lathe. I usually grab these pieces with my Channellock Pliers and twist them after cutting. Try fitting the old-work metal box into the opening. Make sure that it is straight.
The NM-B cable is brought into the old-work metal box in one of two ways. If the package has a built-in cable clamp, that opening is used. If the box only has knockouts, use a cable connector approved for NM-B, such as a plastic button or a two-screw metal clamp type connector.
Insert the cable clamp into the knockout. Remove at least eight inches of the cable’s outer sheath and insert the wire into the box. The ears are adjusted on metal outlet boxes so they won’t sit recessed in the wall. You can loosen the screws and move the ears. Sometimes, I have had to turn the ears around to make the box even with the wall. See article 314.20 in the National Electrical Code.
Insert the box into the wall opening. It may be possible to use Madison Bars to mount the box, but sometimes the wood lathe or plaster interferes with them sliding up or down. I usually use #4 x 1 1/14″ wood screws screwed through the holes in the box ears to fasten the box to the wall. Then I drive one #8 x 1″ or 1 1/4″ sheet metal screw through the drilled hole inside of the box into the stud. Do not over-tighten this screw, as it may distort the box’s position. Follow the instructions above for grounding a metal box.
Due to new code changes, filling gaps around the newly installed outlet box with joint compound or plaster may be necessary. This prevents air from getting in the wall to feed a fire. It will also help with energy conservation. The photo of the outlet box at the top of this page would not pass inspection today because of the gaps around it. See articles 312.3, 312.4, and 314.21 in the National Electrical Code.
You can now install the receptacle and wall plate. With metal receptacle boxes (and old work plastic boxes) that have their ears resting on the surface of the wall, it is usually necessary to cut the ears on the receptacle or switch so that it fits snugly between the outlet box ears.
Take note of the outlet in the picture at the top of this page. The ears on the metal electrical box were turned around to rest on the wood lathe while the box was even with the wall surface.
TOOLS AND MATERIALS
Some tools needed to install an outlet in an existing wall: Awl, razor knife, cement board scoring tool, Compass or keyhole saw, measuring tape, stud sensor, fish tape, lineman pliers, needle nose pliers, drop cloth, screwdrivers, drill, drill bits, small masonry drill bit, screwdriver bit and a long bit holder.
Some materials needed to install an outlet in an existing wall: Metal or plastic, one-gang old work electrical box, NM-B connector, #4 x 1 1/4″ wood screws, one pair of Madison Bars, 1″ or 1 1/4″ drywall screws.
Some part numbers:
Plastic one gang old work boxes – Steel City #E-16-8, Thomas & Betts (Carlon) B120r, Raco 7887.
Metal one gang old work boxes – Raco #601 or #590, Orbit #GDB-1 or #GDB-1-NM, Steel City #CY-1/2 or #CXWOW.
Madison Bars – Madison Electric Products #102, Steel City #820-D, Orbit #SBS, Raco #977, Caddy #DSB.
Extra long Madison Bars – Caddy #DS12A, Raco 8977
Some of the applicable “National Electrical Code” references for this job are: Articles 210, 210.12, 250, 300.4, 310, 312, 314, 334, 406.4(D)(4), 406.12, 440.6, 440.13, 440.31, 440.32, 440.62, 440.62(C), 440.63, 440.65.
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