How to Install an Old Work (Retrofit) Electrical Box

An old work box is designed to be installed after-the-fact—on walls that are already finished. They are often used in situations where you are adding outlets or extending an existing circuit in a room that is already finished. Because these situations don't allow you the opportunity to attach the box directly to studs, these boxes need a different means of anchoring. 

To anchor them in place, these boxes have a clever fastening system that uses retention tabs and mounting ears located in opposite corners of the box. Once the box is inserted into the wall cutout and the screws are tightened, the rear retention tabs open up and draw up tight against the back of the drywall or plaster, while mounting ears on the front of the box press against the front face of the wall. The front ears and back retention tabs "pinch" the box tightly in place against the wall surface. 

The screw that goes into the retention tab on the box may sometimes be very tight when the box comes from the manufacturer. This may require you to put considerable pressure against the wall when you turn the screw, which may damage the wall. 

To avoid this problem, simply tighten the mounting screw BEFORE installation, drawing the retention tab up about 1/4 to 3/8 inch. This effectively precuts the threads in the plastic retention tab, making the screw easier to turn later when you are actually securing the box in the wall. After the screw is drawn up slightly, back the screw out to allow the retention tab to go back into its original folded position.

Old work electrical boxes made from PVC use an integrated quick-clamping system to hold the nonmetallic NM-B cable to the box. These clamping tabs can be a little difficult to open up.

To make insertion of the electrical cable into the box easier, take a flat-blade screwdriver and pry up the clamping tab about 1/4 inch. This will loosen the clamp and make it easier to insert the cable.

The first step in installing an old work electrical box is to create the wall cutout. Position the electrical box backward against the wall, in the exact position you plan to install it. Take a pencil and trace the outline of the box on the wall. (Note: Some styles of boxes may require you to enlarge the hole slightly at the point where the retention tabs are located. If so, this will become apparent when you try to slide the box into the wall).

Using a drywall keyhole saw, carefully cut the hole along the traced outline and remove the drywall blank. A sharp blow on the heel of the saw will usually cause the blade to puncture the drywall so you can begin cutting. For plaster, you can drill an access hole in the cutout area to provide space to insert the saw blade.

Before the old work box is installed, one or more NM-B electrical cables will need to be run to the wall cutout. If the new outlet is a simple circuit extension, there will likely be just one cable running from the last outlet location to the new box location. More complicated wiring scenarios may call for two cables. Make sure the new lengths of cable are sized appropriately for the amperage of the circuit. 

Run cables to the new box location. There should be 8 to 12 inches of excess cable at the new wall opening. At this point, the new cable should be unattached to any electrical devices. If the cable is already connected on the source side of the circuit, make sure the circuit breaker supplying power to it has been shut off at the main service panel. 

Once the hole is cut in the wall for the box, feed the cable (or cables) out through the wall opening.

Before feeding the cable into the old work box, use a cable stripper to remove about 6 inches of the outer vinyl sheathing from the NM-B cable, exposing the conductors and the ground wire. If you don't have a cable stripper, This can also be done by taking a sharp utility knife or razor blade and slicing the sheathing PARALLEL to the wires, being careful not to slice the insulators. Carefully cut the sheathing off.

Thread the stripped nonmetallic sheathed cable into the old work box, making sure the sheathing is secured under the clamping tab (for a PVC box) or the clamping bar (for a phenolic resin/ fiberglass box). Make sure to leave about 6 to 7 inches of exposed wires. Properly installed, there should be 1/2 to 1 inch of cable sheathing extending into the box, secured under the clamp.

Next, simply place the old work electrical box into the hole in the wall. Hold the box so the mounting ears on the front of the box are tight against the front of the drywall while you tighten the two retention screws. Turning the screws will flip the retention tabs so they can then be drawn against the back of the drywall, securing the box to the wall.

Using a wire stripper, strip 3/4 to 1 inch of insulation off the ends of each of the wire conductors. A wire stripper is the best tool for this since it has different slots sized to match different wire gauges. If you use a utility knife to strip wires, make sure not to nick the metal conductors.

Your box is now ready to receive an outlet receptacle or switch. For safety, screw wire nuts onto the ends of the wires until you are ready to make the final connections