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 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 insulation. 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 a few different clamping devices you can use, but first, you have to remove a knockout in the box.
Removing a Knockout
Knockouts are little metal discs covering holes on any or all sides of a metal electrical box. You remove as many knockouts as you need 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.
Some metal boxes come with saddle-shaped clamps 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.
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 the clamp, insert the threaded end into a knockout hole in the box, then thread on the locknut 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. Tip: Orient the clamp so the saddle screws are facing you. This makes it easy to tighten the screws when installing the cable. You can also secure the clamp onto the cable first, then install the cable and clamp into the metal box.
Plastic Push-In Connectors
The newcomer on the block is the push-in connector. There are several different designs of these, but basically, they are 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.