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

Work Gear for Electricians and the Trades
Re: NM shorts under metal clamps -- How common? #51973
05/16/05 07:37 PM
05/16/05 07:37 PM
W
walrus  Offline
Member
Joined: Jul 2002
Posts: 680
Bangor Me. USA
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
05/16/05 07:46 PM
S
straightedge  Offline
Member
Joined: Mar 2002
Posts: 86
Mesa, AZ
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.

Tev

Re: NM shorts under metal clamps -- How common? #51975
05/16/05 09:35 PM
05/16/05 09:35 PM
S
Sandro  Offline
Member
Joined: Dec 2001
Posts: 444
Stoney Creek, ON, Canada
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
05/16/05 11:02 PM
T
Tiger  Offline
Member
Joined: May 2005
Posts: 706
Crystal Lake, IL USA
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!)

Dave

Re: NM shorts under metal clamps -- How common? #51977
05/17/05 01:22 PM
05/17/05 01:22 PM
J
John Crighton  Offline OP
Member
Joined: May 2005
Posts: 177
Southern California
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
05/17/05 04:08 PM
G
golf junkie  Offline
Member
Joined: Apr 2001
Posts: 507
York, NE
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
05/18/05 10:08 PM
L
Larry Fine  Offline
Member
Joined: Feb 2005
Posts: 693
Richmond, VA
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.
fineelectricco.com
Re: NM shorts under metal clamps -- How common? #51980
05/19/05 01:42 PM
05/19/05 01:42 PM
J
John Crighton  Offline OP
Member
Joined: May 2005
Posts: 177
Southern California
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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