I am replacing an existing fiberglass double-gang electrical box with a PVC triple-gang old-work box (a Carlon B355R box: http://carlon.com/Brochures/7F61.pdf). The new triple-gang box has internal plastic cable clamps of the type that apply constant pressure to the NM cable sheathing to hold the cable in place. You can see these clamps pretty clearly in Carlon's brochure.
With new cable, these clamps seem to work pretty well. It's easy to pull the cable through the clamp, and it holds the sheathing tightly and safely (to my eye). However, I have to bring the existing NM cables (which are 14/2 of circa 1986 vintage, in good shape) into this box, and of course they already have the sheathing stripped back. The sheathing just barely reached into the old box, and the new box is deeper. The different geometry means that the sheathing on the old wires doesn't reach the new box's cable clamps (by about an inch). Instead, the clamps would clamp on each individual wire's insulation, even biting into the insulation a bit (the plastic clamps have sharp edges). And of course the individual wires don't want to come through the clamp properly centered, they want to sneak around the sides and generally misbehave. This seems like bad news to me.
Any advice on dealing with this?
With new cable, these clamps seem to work pretty well. It's easy to pull the cable through the clamp, and it holds the sheathing tightly and safely (to my eye). However, I have to bring the existing NM cables (which are 14/2 of circa 1986 vintage, in good shape) into this box, and of course they already have the sheathing stripped back. The sheathing just barely reached into the old box, and the new box is deeper. The different geometry means that the sheathing on the old wires doesn't reach the new box's cable clamps (by about an inch). Instead, the clamps would clamp on each individual wire's insulation, even biting into the insulation a bit (the plastic clamps have sharp edges). And of course the individual wires don't want to come through the clamp properly centered, they want to sneak around the sides and generally misbehave. This seems like bad news to me.
Any advice on dealing with this?