How to Fasten Romex NM With Plastic Electrical Box Wire Clamps

Closeup of fastening Romex NM With Plastic Electrical Box Wire Clamps

The Spruce / Kevin Norris

Project Overview
  • Total Time: 1 - 3 mins
  • Yield: One box
  • Skill Level: Beginner
  • Estimated Cost: $2 to $5

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.

What You'll Need

Equipment / Tools

  • Flathead screwdriver
  • Non-contact voltage tester
  • Wire stripper
  • Wire ripper
  • Utility knife or scissors
  • Gloves


  • NM electrical cable


Tools for Fastening Romex NM With Plastic Electrical Box Wire Clamps

The Spruce / Kevin Norris

  1. Stabilize the Box

    Place the box in its final location and attach it to the stud, so it will be stable while you work.

    Fastening the box to the wall stud

    The Spruce / Kevin Norris

  2. Punch Open the Clamp

    The door on the back of the box is hinged in only one direction; the hinges are always on the outer perimeter of the box. Stab the flathead screwdriver into the clamp. One stab is usually enough to break open the door free from its attached box. The hinge side should stay in place and should be difficult to break free, even if you tried.

    Punching open the box clamp

    The Spruce / Kevin Norris

  3. Pry Open the Clamp

    With the screwdriver still in the box's door, pry the door open to the size of the thickness of the screwdriver. Do not pry the clamp all the way open; otherwise, the clamp will lose its gripping power.

    Do not rip and strip the electric cable yet. Keep the end of the cable blunt, as this makes it easier to push the cable into the box.

    Prying open the box clamp

    The Spruce / Kevin Norris

  4. Press the Cable Into the Box

    From the back, force the electrical cable through the clamp and into the box. If the cable bends or buckles, remove it and pry open the clamp a bit more. Then try pressing the cable into the box again.

    Pressing the cable into the electrical box

    The Spruce / Kevin Norris

  5. Pull the Cable Through the Box

    From the open side of the electrical box, pull the cable through. It is always a good idea to pull the cable through at least 6 inches. More is better than less. You can always cut off extra cable if needed.

    Pulling the cable through the box

    The Spruce / Kevin Norris

  6. Rip and Strip the Wire

    With the NM electrical cable protruding from the box, rip off the sheathing with the cable ripper. Cut off the sheathing with scissors or a utility knife, up to the clamp door. Cut off the paper cover and discard it. Finally, strip the coating from the wires with the wire stripper.

    Ripping and stripping the cable

    The Spruce / Kevin Norris

When to Call a Professional

Always bring a licensed electrician into a project whenever you feel uncomfortable. Clamping NM wire into the back of a plastic box is, in itself, a simple project. But it is tied into the larger, more important issue of working with electrical current.