How to Fasten Romex NM Wire in Plastic Electrical Boxes

Plastic electrical box


Yenwen / Getty Images

Metal electrical boxes can have a much more secure hold than plastic, but for most purposes, plastic boxes work fine. So how do you secure NM or Romex brand electrical cable wire inside of the plastic electrical box?

Basics of Plastic Electrical Box Holders

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. Code states that you must staple on a solid surface (the stud) 8 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 kind of 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 eight inches. Still, the more you can secure a wire, the better. 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.

Project Metrics

  • Working / Working Time: 5 minutes
  • Skill Level: Beginner
  • Material Cost: $2 to $5

What You Will Need



Stabilize the Box

Place the box in its final location and attach it to the stud so it's stable while you work.

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.

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.

Press the Cable Into the Box

From the back, force the electrical cable through the clamp and into the box. There should be a sufficient amount of friction but not so much that it is difficult to insert the cable. 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.

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, and you can always cut off extra cable if needed.

Rip and Strip the Wire

With the electric 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.

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.