Nonmetallic, or NM, cable (commonly known by the brand name Romex) must be secured to electrical boxes at the point where the cable enters the box. This code requirement is an important safety feature that protects the wiring connections inside the box in case the cable is pulled. It also prevents the cable from rubbing against the edge of the box and potentially damaging the sheathing. Most plastic boxes have built-in spring tabs that secure the cable as it's pushed into the box. When it comes to metal boxes, there are several different clamping methods you can use, but first, you have to remove a knockout in the box.
Removing a Knockout
Knockouts are the little metal discs covering the holes in the sides or back of a metal electrical box. You remove as many knockouts as you need for the number of cables that will enter the box, and leave the others in place. A box should not have open knockout holes that are empty, as this compromises the protection offered by the box enclosure.
Look on the side of the box and locate the knockout where you want to install your cable. Some knockouts have a straight slot in their center. To remove this type of knockout, insert a straight-bladed screwdriver into the slot and twist back and forth until the knockout separates from the box. Another type of knockout has no slot and must be punched out with a hammer and screwdriver. Hit it hard and it will bend down away from the hole (don't worry; you won't hurt the box). Grab the knockout disc with pliers and twist it until it comes free.
Now you're ready to use one of the following types of clamps to secure an NM cable to the box.
Securing Cable With an Internal Clamp
Some metal boxes come with saddle-shaped clamps already mounted inside the box. Usually, there are two clamps that can hold two cables each. If a clamp is not next to the knockout you are using, simply unscrew the clamp and move it to the desired location. There should be a pre-drilled hole for the clamp screw near each pair of knockouts. Insert the cable into the knockout and slide it under the clamp. Tighten the clamp's screw to secure the cable.
Using a Locknut Cable Clamp
A locknut-type clamp is the classic metal cable clamp with a short, threaded cylinder and locknut on one end and a saddle-type clamp with two screws on the other end. To install this type of clamp, insert the threaded end into a knockout hole in the box, then thread on the locknut onto the threaded end from inside the box. Tighten the nut with pliers. Insert the cable through the clamp saddle and into the box, and tighten the screws on the saddle to secure the cable.
Using Plastic Push-In Connectors
The newcomer on the block is the plastic push-in connector. There are several different designs for these, but basically, they are all plastic bushings that snap into the knockout hole in the metal box. The cable is inserted through the connector and secured with a spring-tab or other device. One version includes a little gate piece that you slide down into a slot (similar to the action of a guillotine) then clamp down on the gate with pliers to secure the cable.
Tips for Securing Cable
Here are some standard practices to follow to make your wiring installations look like the work of a pro:
- About 1/2 inch of outer cable sheathing should extend into the box past the cable clamp. This ensures that the cable will be held securely and that the clamp will not compress and damage the insulation around the individual wire conductors. It is a code violation for the cable clamp to directly contact the wires themselves rather than the outer cable sheathing.
- There should be at least 6 inches of free wire extending into the box for connection purposes; 8 to 10 inches is even better. This excess wire provides the slack necessary to make various wire connections to devices and also provides enough excess wire in case the wires need to be trimmed off in the future.
- Never run two cables into the box through the same knockout opening with pre-installed cable clamps. Each cable requires its own knockout opening and clamp. For installed clamps (whether they're metal or plastic), no more than 2 wires should be installed in each connector of this type.
- Be careful not to tighten cable clamps so much that they crush the cable sheathing or damage the insulation on individual wire conductors. The clamps should be just tight enough so that the cables can't be pulled free by hand.
2020 National Electrical Code. National Fire Protection Association.