Dear Mr. Electrician: I want to install an outlet in an existing wall on a dedicated circuit for my window air conditioner? I’ve got it figured out how to run the 12/2 NM-B cable from the main circuit breaker electrical panel in the basement up through the wall under the living room window. How do I install an electrical receptacle box in the wall that will contain the wire and support 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. That way the electrical box can be screwed through its side directly to the wood to make it solid. NOTE: Some text links below go to applicable products on Amazon or EBay. As an Amazon Associate I earn from qualifying purchases.
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. If they were installed at the time that the house was built, then they are most likely mounted on wall studs.
I would remove the wall plate from the existing receptacle or switch and poke a long thin screwdriver to the right and to the left of the electrical box. 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.
In newer homes with drywall I also look for indications of nail holes that were spackled over in order to locate wall studs. After I have a few studs spotted, I measure 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 at 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 there because drywall and paneling are 48″ wide and need a stud to land their edge on for support and nailing.
On older homes with plaster walls it is not as easy to find a wall stud. There are no indications of nail holes and sometimes outlets have been installed in the baseboard molding.
The use 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 have a choice of using 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 that you will be using.
INSTALLING A PLASTIC OLD WORK BOX
A plastic old 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 fully cover the box and hole. 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 if there are any obstacles and to see 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.
Check to make sure that the plastic old work box fits into the opening against the stud. It is okay to have small gaps not greater than 1/8″ on the side, but the top and bottom should be snug to allow for the box ears to have a good place to seat against.
The old work box will have plastic cable clamps inside the back of the box 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 bring at least eight inches of wire into the box. Now push the box into the wall opening and tighten down 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. Make sure that 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 completely dependent on the drywall for support. Do not drill more than one screw as the box will distort and will not look straight.
As an alternative, a single gang metal electrical receptacle box can be used to support your receptacle, but the installation is a little different. You will need to drill a hole in the side of the metal box prior to installation for driving a screw into the stud. Not all metal old work 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 metal old work box is fitted into the wall one edge is pulled slightly out about 1/4″. The other edge is pushed in flush with the wall and a Madison Bar is inserted on that side between the wall and the box.
Insert the long end of the Madison Bar along side of the metal old 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 see my blog post 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 and 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.
If it is loose you can unbend the Madison Bars and do it over. 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 by using a 10-32 ground screw threaded into a hole into the back of the box. The bare ground wire from the NM-B cable is wrapped around the 10/32 screw before it is attached 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 LATH AND PLASTER WALL
If you have wood lath and plaster walls you will need to use a metal old work electrical box. Mark the wall as above, but instead of using an awl or screwdriver to punch a hole, use a small masonry drill bit in an electric drill. This will help prevent excessive damage to the plaster.
Use the hole to probe for obstacles and to find the exact location of the stud. Also you want to locate the wood lath strips and make sure that one full strip will be lined up with the center of the box. There should be a partial strip for the upper and lower ears of the box to be be supported on.
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 lath location. Ideally you only want to remove one full section of the center lath and a portion of the upper and lower laths.
I usually tape around the intended opening using 2″ masking tape to provide a little extra support for 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 need to cut the wood lath as gently as possible to avoid major 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 all the way through on the side furthest from the stud. Sometimes I will cut this side almost all of the way and then use my Knipex High leverage Diagonal Pliers to cut through the last part. I do that to avoid having the lath 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 very well in cutting holes in plaster. I expect that 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 and it works well for that. It is also very good for notching the wall stud a little to accommodate the protruding screw on the side of the metal box.
Cut part of the upper lath and part of the lower lath. I usually just grab these pieces with my Channellock Pliers and twist them out after cutting. Try fitting the metal old work box into the opening. Make sure that it is straight. The NM-B cable is brought into the metal old work box in one of two ways. If the box has a built-in cable clamp, that opening is used. If the box only has knockouts you must 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 outer sheath of the cable and insert the wire into the box. There is some adjustment of the ears on metal outlet boxes so that 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 lath 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 position of the box. Follow the instructions above for grounding a metal box.
Due to new code changes, it may be necessary to fill in the gaps around the newly installed outlet box with joint compound or plaster. This is to prevent 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 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.
Some tools needed for this job: 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 for this job: 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.