How tight should a cable clamp be?

Question

When using metal junction boxes for electrical work, it is required to use a cable clamp to hold the cable at the edge of the box and to protect it as it passes through the potentially sharp metal. Often, a screw-clamp clamp like this is used:

^{Image courtesy of topaz-usa.com. First small image I found for what I was after}

How tight should the screws be that actually clamp down on the cable? I've always tended to tighten them until the wire cable^* is squashed between the clamps, but recent reading indicates to me that this may be too tight.

What is a reasonable rule-of-thumb for knowing when the clamp is tight enough?

^*I originally meant to say "cable", but wrote "wire". I know I've pinched the cable housing, but don't know if I've tightened things down tight enough to actually pinch the wire insulation within the cable.

Answer 1

You want to tighten the screws so that the cable's outer insulation is just compressed a bit which should leave you with the wire unable to move in or out of the box unless you apply a lot of pressure.

You do not want to compress it so much that the inner insulation is compressed or the wire itself is deformed.

Yes, the tendency is to overtighten these. My rule-of-thumb is tighten gently and check if the wire can move. If it does, tighten a bit more until you get it right.

You CANNOT "undamage" a wire once it's been damaged so proceed carefully.

Answer 2

I have not seen x2 damaged. A couple of times I have found x3 damaged and they have been in use for years but a overload that heated the wires allowed the conductors to come in contact. Currently there is no requirement other installing without damage it is much like staples, what is right? It is a judgement call and different styles like x2 with ground are flat just about impossible to mess those up without damaging the sheath but x3 can be damaged more easily. So I would take the clamp down evenly until contact then 1 additional turn.