Installing a new electrical box in a finished wall requires a special type of box, often called a remodel, cut-in, or old work box. The old work boxes used most commonly by homeowners today are plastic boxes that have built-in ears or wings that secure the box to the drywall. To install the box, you cut a hole in the drywall, slip the box into place, then turn a couple of screws to pull the ears or wings tight to the backside of the drywall.
Traditional metal old work boxes used by electricians (typically on commercial jobs) are installed similarly but use a different attachment system. They rely on thin strips of metal, called Madison straps or "Mad bars," that are tucked into the gap along both sides of the box. The straps have rear tabs that rest against the backside of the drywall as well as front tabs that are folded over the edges of the box to hold the straps in place. Like plastic old work boxes, metal boxes have flanges on the front edge of the box that prevent the box from falling into the hole. The Madison straps keep the box from slipping out of the cutout opening.
Madison straps are easy to install. For a professional-looking job, it's important to cut the drywall hole carefully so that the box fits and is supported properly and so that all gaps are hidden by the outlet cover plate.
Equipment / Tools
- Tape measure
- Torpedo level
- Drop cloth
- Drywall saw
- Linesman pliers
- Metal old work electrical box
- 2 Madison straps
Mark the Hole for the Box
Measure up from the floor and mark the wall where the top of the electrical box will be located. Hold the electrical box against the wall so the top is flush with the mark, and use a torpedo level to adjust the box so it is level. Use a pencil to trace along the outside of the box onto the wall. This will be the outline for the cutout.
Note: Do not trace around the flanges, or metal tabs, at the top and bottom of the box. The cutout should be a rectangle; do not cut the area behind the flanges.
Cut the Hole
Cover the floor below the work area with a drop cloth to catch dust. Punch the point of a drywall saw through the drywall just inside of the marked outline, then use the saw to complete the cut. As you finish the cut, try to grip the cutout piece to extract it from the opening (although it's not a problem if it drops back into the wall cavity; you can leave it inside the wall).
Pros often make this kind of cutout with an oscillating multi-tool with a vibrating blade designed for drywall, which makes a clean, easy cut. If you happen to own one of these tools, try this technique.
Insert the Electrical Box
Fit the electrical box into the hole. If the fit is too tight, you may have to trim the opening slightly to enlarge it. Push the box fully in so that the box flanges are tight against the face of the drywall.
If you're ready to run cables, it's usually easiest to feed the cables through the box opening, add clamps to the cables, and thread the cable ends into the box as you insert the box. Once the box is installed, use the cable ends to pull the threaded nipples of the clamps into the knockouts, then add the clamp nuts from inside the box.
Install the Straps
Insert the metal straps, one on each side of the box, with the longer end facing up, and slide the strap in past the back surface of the drywall. Then, slide the strap down until it is centered in the hole. Pull the strap outward so it is tight against the back of the drywall.
Bend the Straps
Fold over both tabs on each strap while simultaneously pulling outward on the tab and pushing inward on the box. Bend the tabs over the box edge, either by hand or with linesman pliers, than bend the tabs flush against the inner surface of the metal box with the pliers. Secure all four tabs to complete the installation. The box should fit tightly into the opening with a minimal amount of wiggling.
Once you've run the wires, you are ready to install the outlet, switch, or other device in your new electrical box.