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.
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...
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