How to Install an Old Work Electrical Box in an Existing Wall

Electrical boxes come in a seemingly endless variety of styles, including metallic (steel) versions requiring grounding of the box and nonmetallic versions (PVC, phenolic resin or fiberglass) which do not require grounding of the box. Electrical boxes are really not that complicated and are designed for two basic applications:

• Where the box is being installed BEFORE the finished wall is installed (new work box).
• Where the box is installed AFTER the finished wall is installed (old work,or retrofit,...MORE box). Old work boxes have opposing corner tabs or "ears" integral to the box that press against the back side of the wall as the fastening screws are tightened

Metallic boxes can be used with either nonmetallic (Type NM-B) or metallic sheathed electrical cable. Nonmetallic plastic boxes are designed for use only with nonmetallic sheathed electrical cable.

This tutorial will show you how to easily install an old work plastic electrical box into an existing wall for use with an outlet or switch.

Difficulty Level

• Easy

Needed Tools and Materials

• Old Work Nonmetallic Electrical Box
• Multi-Head Screwdriver 
• Pencil
• Keyhole Saw
• Cable stripper
• Wire-stripping tool

Since the function of an old work box is to hold the box in place in an existing wall, the box will not be fastened to a wall stud. It will be located in a position where it has no support other than what it provides with its own mounting system.

To remain firmly in place, these boxes have a pretty interesting fastening system that uses tabs or "ears" in opposite corners that pop up as the attaching screws are drawn tight. As the screws are tightened, the tabs press against the back side...MORE of the drywall or plaster, while mounting ears on the front of the box press against the front face of the wall, holding the box firmly in place.

Now, a couple of tips to make use of the plastic old work box a little easier.

The screw that goes into the retention tab on the box may sometimes be very tight. This may require you to put too much pressure against the wall when you  turn the screw, sometimes causing damage to the wall.  

TIP: To avoid this problem, simply tighten the mounting screw BEFORE installation, drawing the retention tab up about 1/4" to 3/8". 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...MORE the wall. Once the screw is drawn up about 1/4" to 3/8", then 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 get to open up.

TIP: To make insertion of the power cable into the box easier, take a flat blade screwdriver and pry up the clamping tab about 1/4", allowing for easier insertion of the nonmetallic cable into the box.

1. Position the electrical box backward against the wall where you want it located. 
2. 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).
3. Using a drywall keyhole saw, carefully cut the hole along the traced outline and remove the drywall blank.

Make sure the power wire that will be fed into the box has the power shut off to it by ensuring the circuit breaker is turned off to the wire.

Once the hole is cut in the wall for the box, feed the power cable through the hole. Before the cable is fed through the old work box, use a cable stripper to  strip about 6" of the electrical sheathing from the NM-B cable exposing the conductors and ground wire.

This can also be done by taking a knife or razor blade and slicing the sheathing PARALLEL...MORE to the wires, being careful not to slice the insulators, then carefully cutting the sheathing off.

Thread the nonmetallic sheathed cable into the old work box, making sure the sheathing is securely under the clamping tab (PVC box) or clamping bar (phenolic resin / fiberglass box). Make sure to leave about 6" to 7" of exposed wires. You can either strip the wire ends now or later.

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.

Strip the insulation back about 3/4" to 1" back from the end of the wire conductors. You can use a wire stripping tool which makes short work of the task, as shown here. Or, you  can use a needle-nose pliers or wire cutter to score the insulation and pull it off the wires. Make sure not to cut into the metal wire conductors.

Once the insulation is stripped from the wires and the box is firmly in place, the box is ready to receive a switch, or, in this case,  an outlet receptacle. Twist wire nuts onto the ends o the wires for safety until the final wiring connections are completed.