There are a couple of ways to connect NM Cable to a metal box. The first is by removing one of the knockouts that are designed to secure NM cable. To find the knockouts, look on the side of the box and locate the two knockouts with a straight slot in them. This is where you will insert a straight-bladed screwdriver into the slot and twist back and forth until the knockout separates from the box. These type boxes supply two knockouts on each side opposite of each other, giving you four NM cable openings for attachment.
After inserting the NM cable, you’ll need to tighten the cable clamp screw located inside the box between the two knockout openings on the rear of the box itself.
The other method of attachment is through a cylinder-looking knockout. This knockout is a little harder to remove and may require a sharp blow of a hammer and screwdriver or a pop of a pair of linesman pliers. Once removed, you’ll need to add a cable clamp (often called take all) that will hold the cable securely to the box. This clamp is fed through the cylinder opening and the locknut is screwed on and tightened with a pair of pliers. Be sure to leave the cable clamp screws facing you when tightening for ease of access when attaching the NM cable.
Insert the cable and tighten the two screws on the cable clamp. This connection secures the cable and keeps it from becoming damaged on the sharp metal edges of the knockout opening.
