Am I just ham-fisted or something?
Intro: I'm an electronic engineer who's done lots of electrical work under homeowner/builder permits. I've been reading this board for quite a while now, and I've thoroughly enjoyed the education (love those photos!) So...
I mostly use EMT or flexible conduit, but I just finished installing some recessed lighting fixtures with short lengths (10") of NM into adjacent steel boxes. I'm embarassed to say I had to wire them three times to get it right.
The first time, there was a hot-to-ground short at one of the fixtures where the built-in screw-down clamp pinched through the jacket and insulation. I decided that the clamp's protruding "bump" was a little too agressive, so I closed out that opening, installed a plain steel NM connector in the KO next to it, and installed a new length of cable.
Since I than had NO confidence in the safety of the other fixtures, I of course did the same at all of them.
BZZZP! Another short, where I'd just installed fresh cable and a new connector (not using the built-in clamp at the fixture box). Again, it seemed that the clamp just "squeezed" the jacket and insulation right through to the conductor.
Third time's the charm: I found some 10-year-old, 600-volt NM-B with very tough-looking insulation, and replaced everything again, being a little more conservative with the screwdriver. No problem.
Was I just tightening those clamps too much, or has Romex(tm) just gotten ridiculously thin-skinned? It seems to me that running the screws down all the way shouldn't cause this kind of damage, especially since anything less just doesn't seem snug enough.
Is this a common problem in the trade? How tight is tight enough? And, how do I know I'm not just a hair's width from a short that might develop later?
Thanks!
