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Install a cable clamp in an electrical box.

Electrical Basics:

Installing A Cable Clamp In A
Breaker Panel

Wires Entering The Breaker Box Must Be
Secured With A Cable Clamp

In This Article:

A knock-out is removed from the metal breaker box and a cable clamp installed.

Related Articles:
Skill Level:
2 (Basic)
Time Taken:
About 2 Minutes
, Editor
Project Date:
March 2000
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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 ?!


Removing knock-out in breaker panel.

I used a screwdriver and a hammer to loosen one of the many "knock-out" pre-punched holes in the top of the breaker panel.




From the inside, I used a pair of Channel-Lock pliers to twist off the metal slug. It is held in by a small tab of steel.

Removing knock-out slug in electrical panel.



Note that breaker panels have many different sizes of knock-outs, to accommodate a variety of cable and conduit sizes.

Some thoughts on sizing:

  1. The size of the cable is determined by the electrical current needed for the project. This case is a simple bedroom circuit, using 14-2G (14 gauge wires, 2 bundled together, plus a Ground wire) non-metallic wire (NM-B), which has a capacity of 15 amperes.
  2. The cable clamp size depends on the cable diameter. In this case I used a 3/8" clamp, which is probably the most common size.  A typical household breaker panel has many clamps of this size.
  3. The next biggest size of cable clamp is 3/4 inch, which is big enough for a 6-3G cable. 6-3G can carry 50 Amps, and is often used to supply power to an electric stove or a 50 Amp sub-panel.
  4. The size of knock-out used depends on the clamp size.


Inserting a cable clamp in a hole in an electrical panel.

The clamp (with lock-nut removed) is slipped into the hole in the panel.




From below, the lock-nut is installed and tightened by striking it with a screwdriver and a hammer. This tends to turn the clamp slightly.

Tightening lock nut on cable clamp, using a screwdriver and hammer.



Turning a cable clamp with a pair of Channel-Lock pliers.

Then I used Channel-Lock pliers to turn the clamp back in place. This tightens the fitting, because the lock-nut has teeth that dig into the sheet metal.




Then I threaded the cable through the clamp, leaving plenty of length.

Pulling electrical cable through cable clamp into breaker panel.



Tightening screws on cable clamp to secure wire.

I tightened the clamping screws with a cordless drill-driver (set on a very low torque setting).  The cable needs to be held firmly, not sheared in two!



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Before you hurt yourself, read our disclaimer.











This brings to mind an important point:  Cable clamps must not be over tightened.  I have a little procedure that I use to ensure that the clamp is just snug enough:  

  • I wiggle the cable back and forth and see how much the cable is moving on the other side of the clamp.  
  • If the other side moves about one-half to one-third as much, then the tightness is okay. 
  • If the other side barely wiggles at all, then the clamp is probably too tight. 
  • If I can pull on the cable and it moves downstream, then the clamp is too loose.


More Info:
Tools Used:
  • Cordless Drill/Driver
  • Screwdrivers
  • Hammer
  • Channel-Lock Pliers
Materials Used:
  • Cable Clamp, 3/8"
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