How to Fasten Romex NM With Plastic Electrical Box Wire Clamps

When you use some types of plastic electrical boxes, you may encounter built-in plastic electrical box wire clamps on the back. How do you use these clamps? Should you punch them out entirely?

Electrical box wire clamps, for all their convenience, can be a bit tricky to use. They require some finesse and just the first amount of force. But used correctly, plastic electrical box wire clamps can be a real time-saver over metal cable clamps.

Basics of Plastic Electrical Box Clamps

When you establish a new outlet, light switch, or junction, you use an electric box. This box can be either plastic or metal.

Metal electrical boxes are sturdy and they rarely have any issues with maintaining a strong grip on the stud. On the back, they either have built-in screw-down clamps for the cable or punch-out holes that allow you to add separate clamps.

Plastic electric boxes cost less than electrical boxes and sometimes don't hold as well on the studs as metal boxes. But for most purposes, plastic boxes work fine.

Called a molded outlet box with integral cable clamp, these boxes have been around since 1972 and few substantial changes have been made in the last half-century. The original design called for a thin-walled box that had to be injection-molded. Forcing all of the plastic into a single mold saved on manufacturing costs but also made it more difficult for users to punch the fastening bracket inward.

One way to make integral cable clamps easier to use is to first loosen them with a screwdriver, then force the cable through the clamp. There should be a sufficient amount of friction to hold the cable but not so much that it is difficult to insert the cable.

Codes and Permits

Securing electric cable in the box is good practice, and it makes for a safer installation. However, you do not need to secure the NM or Romex wire inside the plastic box. The electrical code states that you must staple on a solid surface (the stud) eight inches or nearer to the outside of the box. The staple provides a secure hold because it is separate from the box itself.

But the electrical box does have flaps or doors that you break open on one end to allow the wire to pass through. You can break out one side of these flaps (instead of punching them all the way out), creating a strong door that will act as a clamp to hold the wire inside the box.

Even though clamps are provided, you do not need to use the clamp method. Electrical code only requires that you staple within 8 inches of the box. Even so, the more you can secure a wire, the better the connection. So, if clamps are provided, use the clamps.

Safety Considerations

Electrical work always requires that you shut off the circuit at the circuit breaker. Merely flipping off a switch is not sufficient. After turning off the circuit breaker, check for power within the box with the voltage tester. It is also good practice to test the voltage tester itself by first trying it out on a known live wire.