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NM shorts under metal clamps -- How common? #51972 05/16/05 07:08 PM
Joined: May 2005
Posts: 177
John Crighton Offline OP
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?


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Re: NM shorts under metal clamps -- How common? #51973 05/16/05 07:37 PM
Joined: Jul 2002
Posts: 680
walrus Offline
One of the toughest things to learn in any mechanical type connection is when is it tight and when is it too tight(or loose). Good mechanics, pipe fitters, electricians etc. learn that just right feeling over time. I don't use alot of romex connectors but I can't ever remember worrying about bottoming out the screws to make sure a cable wasn't going to fall out or be loose. Seems to me you're being alittle too aggressive with the screwdriver

Re: NM shorts under metal clamps -- How common? #51974 05/16/05 07:46 PM
Joined: Mar 2002
Posts: 86
straightedge Offline
I agree with Walrus, that was one of the first lessons I learned in the field. I came into the field thinking that every screw and bolt had to be as tight as I could posibly make it. Well after time you learn that's just not true. Things seem to work better when you make them just snug enough. When you need to disasemble the thing you won't break a bolt.


Re: NM shorts under metal clamps -- How common? #51975 05/16/05 09:35 PM
Joined: Dec 2001
Posts: 444
Sandro Offline
With romex, you can use the plastic connectors that don't require any screwdriving. Just push the wire in. Fast and simple and you never have to worry about damaging the sheath.

Re: NM shorts under metal clamps -- How common? #51976 05/16/05 11:02 PM
Joined: May 2005
Posts: 706
Tiger Offline
Try tightening the connector until you feel a little resistance. Try to pull the wire out a bit. If it moves, tighten it a bit more. Then try to pull the wire out. With practice you'll get a feel for how much to tighten the connector. Don't tighten it any more than needed to keep it from pulling out of the connector (one handed, gentle pull...don't need to put your back into it!)


Re: NM shorts under metal clamps -- How common? #51977 05/17/05 01:22 PM
Joined: May 2005
Posts: 177
John Crighton Offline OP
Thanks to all who replied.

I certainly agree about gaining a feel for torque through experience, since I've been doing electromechanical assembly all my life, including spaceflight hardware. Don't laugh, but I even use a Utica "click-stop" torque driver to tighten wires under device screws! (Old habits die hard... I always act like there's a QA guy looking over my shoulder making tick marks on a clipboard.)

It's just that this is the only electrical hardware I can think of that's "squishy," and it caught me by surprise that the Romex could be damaged so easily.

It probably doesn't help that I use a zippy little Milwaukee power screwdriver to run those long screws in, but I just naturally expected the clamps to be designed to prevent damage to the cable. Live and learn, eh?

Another contributing factor I hadn't thought of: The clamps probably won't damage 14/2 cable, but this was 12/2 (because of the 20 amp branch circuit OCPD), and it's a little thicker.

I think I like Sandro's snap-in idea the best, because it's just like the entry grips in plastic boxes -- the pressure is molded in, and there's nothing metallic to cause a short even if it does pinch the jacket.

Thanks again.

Re: NM shorts under metal clamps -- How common? #51978 05/17/05 04:08 PM
Joined: Apr 2001
Posts: 507
golf junkie Offline
Another thing to consider here is the quality of the materials.

If you are using connectors that come on card packs from the hardware store or the big box store you don't have the best quality stuff and have to be more careful.

If you are using bridgeport, t&b, etc. you will find you don't have to be as careful.

Re: NM shorts under metal clamps -- How common? #51979 05/18/05 10:08 PM
Joined: Feb 2005
Posts: 693
Larry Fine Offline
Generally speaking, just enough pressure to create a visible dent in the sheath is usually sufficient, just as with stapling. That's what I teach my guys.

Larry Fine
Fine Electric Co.
Re: NM shorts under metal clamps -- How common? #51980 05/19/05 01:42 PM
Joined: May 2005
Posts: 177
John Crighton Offline OP
Golf, you may be right, but the connectors I used seemed to be pretty good quality: NEER two-piece, die-cast zinc with a steel clamp. The problem was originally at the integrated clamps in the HALO fixture boxes anyway.

I think I just need to monitor the clamp force, as Larry and others have suggested.

Nevertheless, I'm just not impressed with the design of these connectors. It would make more sense to me if the clamp was smooth or had a slight tooth to it to spread the force over a large area of the cable, rather than having that deep, narrow "guillotine blade" bump stamped in. Modern NM cable clearly won't withstand much localized pressure.

But then, you guys do this every day. I'll have to wait until the next lighting upgrade my wife requests before I can test my skills again. ;-)

Thanks, all.


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