How to Install Wiring Boxes and Cables

Installing Boxes

Plastic boxes come in single-, double, triple, or even four-“gang” sizes, referring to the number of switches or receptacles you will install in them. They have integral screw holes for attaching devices.

Some metal boxes also come in different gang sizes. However, it is most common to install a “square” metal box whether you will install one or two devices. After the cable is run into the box, you add a “mud ring” (also called a device extender ring) that is either one or two gangs so you can install one or two devices. This arrangement allows for plenty of room for wires.

A hammer makes for an easy and convenient height gauge for receptacle boxes.

Determine where you want your receptacles and switches. Receptacles should be no farther apart than 12 ft., and there should be at least one on every wall that is longer than 6 ft. Think through the placement; avoid putting receptacles where they will be hard to reach, and perhaps increase the number behind a desk and other places where you use lots of power.

Also, think through switch placements. Avoid placing a switch where it will be hidden when a door is opened. Where a switch is near a door (as often happens), be sure it is not so close to the door as to interfere with the door molding. As a general rule, a switch box should be at least 3 in. away from the inside of the door’s rough opening.

To attach this plastic box, simply position it using the guide, so it extends 1⁄2 in. out from the stud, and drive the nails.

Plastic boxes are often installed with their bottoms 12 in. to 16 in. above the floor. Many electricians use this simple and quick technique for getting all the receptacles at the same height: Place the head of a hammer on the floor and set the box on top of the handle.

Plastic boxes have side indentations or other guides so you can easily install them with their front edges 1⁄2 in. out from the framing—so they will be flush with the wall surface once 1⁄2-in. drywall is installed.

Many plastic boxes come with nails attached; simply drive the nails into the side of a wood stud to attach.

Metal boxes take just slightly more time to install than plastic boxes. There are several types, but for the most common type, use its guide to hold the front edge flush with the framing. The mud ring that gets installed later will bring it out 1⁄2 in. Drive two small tabs to hold the box in place temporarily, then drive a screw into each flange to secure the box to a wood stud.

Hold this type of metal box against the stud at the correct height, and tap in the little pointed tabs to temporarily hold the box in place.
Drive at least two 11⁄4-in. screws to hold the box firmly in place.

Plastic and Metal Boxe Sizes

Plastic boxes are common, but in many areas, metal boxes are required. Some metal boxes come in different gang sizes.

However, it is most common to install a “square” metal box whether you will install one or two devices. After the cable is run into the box, you add a “mud ring” (also called a device extender ring) that is either one or two gangs, so you can install one or two devices. This arrangement allows for plenty of room for wires.

WORKING WITH METAL STUDS

  • If you have metal studs, attach by driving self-tapping pan head screws.
  • Or, screw a block of wood to the stud and attach the box to that.
  • Metal studs have holes for running cable, but the holes have sharp edges. Buy bushings to fit the hole size, and snap them in before running cable.

Running NM Cable Into Plastic Boxe

As shown in the top center photo, the cable is often run through holes drilled in the centers of studs where drywall screws cannot reach. (However, in the case of an outside wall where there is a space behind the framing, you can simply run the cable behind the framing.)

  • Drill 3⁄4-in. or larger diameter holes through studs. They should be reasonably level with each other. A quick way to gauge this is to drill all the holes at hip height.
Drill holes for the cable at hip height.
  • At a corner, drill holes through both studs, angling them slightly toward the inside corner. Bend the cable to roughly approximate the path it must take, and thread it through the two holes.
2a At a corner, drill slightly angled holes in both directions.
  • At a plastic box, strip the sheathing if you have not already done so. Poke the cable through the openings, which have flaps that keep them from backing out. The sheathing should show in the box just a bit—1⁄2 in. to 1 in. Staple the cables within 8 in. or so of the box and tuck the cable(s) into the box.
3 Poke cable into the openings until the sheathing enters the box slightly. Many electricians find it easier to strip the cable and wire ends at this point, rather than before running the cable.

Hooking Cable To Metal Boxes

Metal boxes come with various means of clamping the cable, and here we show the three most common types.

  • If the box has integral clamps, use a screwdriver to remove a tab for each cable A.
A Remove knockout tabs from a box with integral clamps.
  • Strip the sheathing, insert the cable(s) so the sheathing just shows beyond the clamp, and tighten the clamp B.
B Insert the cable and tighten the integral clamps.
  • On a box without integral clamps, check with local codes to see if you need to use metal cable clamps, or if plastic clamps are allowed. Use a pair of lineman’s pliers or a hammer and screwdriver to tap open a knockout slug and remove the slug by wiggling it out with the pliers C.
  • To use a metal clamp, unscrew its nut and slip it into the knockout hole D.
  • With the screw heads facing you so you can tighten them later, screw on the clamp’s nut. Use a screwdriver to tighten the nut E.
C1 Poke opens a knockout slug.
C2. . . then wiggle it out.
D Insert a metal cable clamp and slip on the nut.
E Tighten the nut by pushing or tapping a screwdriver.
  • Either push hard on the screwdriver or tap with a hammer until the nut is firmly connected. Slide the cable in so the sheathing is visible inside the box and tighten the clamp screws F.
  • If plastic clamps are allowed, things go faster. Poke the clamp up through the hole from the inside of the box G.
  • You may need to tap it with a hammer. Slip the cable down through the clamp, taking care not to go too far; you cannot pull it back H.
F slip
in the cable and tighten the screws to secure the cable.
Push or tap a plastic clamp up through a knockout hole.
Carefully poke cable into the box; be sure you don’t push too far.

Ceiling Boxes

A flush light with a metal box uses a mud ring like a wall box. To line it up with other ceiling lights, string lines in two directions. You’ll likely need to install blocking pieces so you can locate the boxes precisely.

Ceiling boxes may be plastic or metal, or they may be recessed canister bodies. In most respects, they attach to framing and receive cable in the same ways as wall boxes.

If you will have a series of ceiling lights, take the time to measure carefully and string layout lines, so they will be evenly spaced and lined up in neat rows. Recessed canister lights often come with brackets that allow you to slide them from side to side, so you can fine-tune their positions.

Around or octagonal box like this nails on easily but is difficult to position precisely. However, it can work well if a row of lights all align with the same joist.

Installing a Canister Light Body

Canister lights also called can or pot lights, have their own electrical boxes. Run the cable to the area. Open the electrical box, which has a cover that may be attached with a clip or a screw.

  • Strip the cable sheathing and wire ends, and clamp the cable in the box. Connect the ground, neutral, and hot leads to the cable wires and close the box.
  • (In this example, push-in connectors are provided with the unit.) Tuck the wires back into the box and replace the cover.
1 Run wires into the canister light’s electrical box and make connections.
2 Tuck the wires into the box and replace the cover.
  • Attach the canister light by driving nails through the brackets into two joists; the bottom of the light housing should be 1⁄2 in. below the framing to allow for the drywall thickness.
  • Now you can slide the light to adjust its position.
Drive nails to attach the light to the joists.
4 Slide the fixture to align it.

See “Splicing Wires,” , and “Connecting Wires to Terminals,”

Wiring With Conduit

Conduit pipe often referred to as EMT, is required for electrical work in some areas of the country. And if you will be installing wiring that is exposed, the conduit is a very good idea, because it encases wires far more securely than NM cable. Joining conduit pipe to electrical boxes is not difficult, but bending it takes some skill, which you can learn with practice.

Make bends using a conduit bender of the right size for your conduit. Take some time to practice your technique, so you can make smooth, fairly accurate bends.

Make your bends so that the conduit will end up a few inches longer than needed; you will cut it off later. Most bends are made on the floor:

  • Insert the conduit, step on the footpad, and pull or push the bar to make the bend.
1 Step on the footpad of a conduit bender and pull the bar to make bends in conduit.
  • When making multiple turns, it may be easier to work with the bender upside down.
2 Multiple bends are sometimes easier to make with the bender positioned upside down.
  • At the box, remove a knockout slug and insert a conduit clamp. Hold the conduit in place and mark for a cut.
3 Mark for cutting conduit so it enters the box’s clamp.
  • Secure the conduit, then cut with a hacksaw or a reciprocating saw equipped with a metal blade.
  • Then-and this is important- remove all burrs from inside the cut end. (Burrs will dig into the wire insulation and perhaps even strip it off.) Use a hand tool like lineman’s pliers or a screwdriver, or—better—a reaming tool attached to a drill.
4 Cut conduit with a metal-cutting blade.
5 Be sure to remove all burrs with a reaming tool.
  • Slip the conduit end into the clamp and tighten the setscrew.
6 Insert the conduit and tighten the setscrew.
  • Once all the pipes are attached to the boxes, it’s time to fish wires. Insert a fish tape into one end of a run and push until it pokes out the other end.
7a Push the fish tape…
7b. . . until it emerges at the other end.
  • Attach the wires to the end of the fish tape: Bend one of the wires over, then arrange the others in descending order, and wrap tightly with electrical tape.
8 Bend one of the wires around the fish tape, neatly arrange the others, and tightly wrap the tape around them all.
  • Have one person gently push wires through while another person pulls with force at the other end.
  • If pulling gets tough, apply wire-pulling lubricant to the wires at the front end.
  • Once the wires are pulled through, cut them off.
9 While one person pulls the tape, someone at the other end pushes wires gently.
10 Lubrication helps wires pull smoothly, especially if they must make multiple turns.
11 Once the wires are pulled through, cut them off.
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