Installing cable clamps on a metal surface-mount junction box. Electrical Basics:

Installing A Cable Clamp
On A Junction Box

In This Article:

A knock-out is knocked out and a cable clamp is fastened in place.

Related Articles:
Skill Level: 1 (Very Basic) Time Taken: 2 Minutes

By , Editor


Take a look at almost any electrical panel and you'll see cable clamps. These little metal devices are crucial to a proper and safe wiring job. Evidently many people do not understand this... as there are too many electrical installations where cable clamps are omitted. There are at least two purposes for clamps.

1. To hold the wire. If somebody tugs on the cable the connections inside the electrical box will not be stretched or harmed in any way.

2. The clamp prevents the wires from rubbing against the sharp edges of the metal electrical box. Otherwise, the sharp edge would eventually wear through the insulation and cause a short circuit, or, if the box was not grounded properly, the metal box could become energized and give somebody a shock, possibly a lethal shock. Is there any more need to stress the importance of using cable clamps in metal boxes ?!


The first step in installing a clamp is to remove the knock-out on the electrical box. A screwdriver is used to bend the slug down. Note the tiny bridge of metal at the right of the hole. That is all the metal the that holds the slug to the box, so it does not take much effort to remove the slug. Normally. Surface-mount junction box with knock-out being removed.


 The knock-out slug is twisted off with a pair of pliers.


The cable clamp has several parts. The locking ring is a nut that holds the unit to the metal box.

The pair of long screws acts to sandwich the cable between a thin metal plate and the die-cast body of the clamp.


The locking ring is removed and the clamp is inserted into the box. The two smaller screws go on the outside of the box.

The ring is installed, on the inside of the box.


 A pair of needle-nose pliers is a good tool to tighten the locking ring.

Next, a pair of heavy pliers is used to rotate the clamp to the desired position.


 Finally, a screwdriver is placed on one of the lock ring's tabs and given a good hit with a hammer. If installed right, the ring will only rotate about 1/12 to 1/6 of a turn before it is very tight, and the body of the clamp won't turn much at all.




Tools Used:

  • Screwdriver, Flat Blade
  • Needle-Nose Pliers
  • Channel-Lock Pliers
  • Hammer

Materials Used:

  • 3/8" NM-B Cable Clamp


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Copyright © 1999, 2005

Written May 10, 1999
Revised January 7, 2005