Electrical Basics: Installing A Cable Clamp In A Breaker Panel

By Bruce W. Maki, Editor

A cable clamp is used to hold wiring in place and prevent it from rubbing against the sharp metal edges of an electrical panel or a metal junction box.

	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.

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 size of knock-out used depends on the clamp size.

	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.

	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.

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!

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.

Copyright © 2000, 2005  HammerZone.com

Written March 10, 2000
Revised January 7, 2005