Potential Problem with 12/3 NM and Metallic Clamps?

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George Stolz

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Location
Windsor, CO NEC: 2017
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Service Manager
I just read the newsletter, thought I'd pass it along.

Click here to see it.

I have had very little experience with metallic clamps, the majority I have used on NM were the grey plastic push-in style.

Any thoughts on this? I kind of find it remarkable that the cable would be so prone to faulting even under "light" pressure.
 

stickboy1375

Senior Member
Location
Litchfield, CT
George, I have had this happen to me with the use of Twin connectors and 14-3 RX... I didn't think I overtightened them, I have done this procedure a thousand times... but when I turned the breakers on I found a problem very quickly... the connectors ARE rated for NM, I have only made this mistake once, but I still feel the connectors were not that tight....
 

memyselfandI

Senior Member
Sometimes I use the plastic ones, sometimes I use the metal ones. With the metal ones, it is a fine line between secured and loose. The plastic ones are really tight when you think about it, but they do the job. I would prefer the plastic ones because they don't have any moving parts and are kind of easy to install. I never tightened the metal ones down to a point where they would pinch the wire, but I did make sure that they were secured in the clamp. It's that fine line again.
 

hardworkingstiff

Senior Member
Location
Wilmington, NC
Thanks George.

I'm wondering how much the "cheaper" made wire is contributing to the problem. From the article:
From my limited personal experience, this failure did not occur with older Type NM cable using metal clamp connectors. The older cable was of larger diameter and provided more resistance feedback to the installer as the screws were tightened.
 

allenwayne

Senior Member
I prefer the snap in type connector over the set screw type.I have had them (set screw type) go boom on power up.Usually from an over zealous helper with a cordless that didn`t set the clutch right.
 

lowryder88h

Senior Member
Location
Massachusetts
12/3 Nm

12/3 Nm

Let me throw this out there most guys in this area are using utility knifes and "Crocadile Dundee" style knifes, maybe just maybe their getting a little to aggressive.
 

TommyC

Member
Location
Westchester, NY
I had a problem with that type of connector once on a service upgrade. There are a few types of that connector that I have seen. The one that I had a problem with had a sharp little dimple in the middle. I've only seen that crummy type of connector on that one job. It actually shorted out a 12-2 it wasn't even 12-3.
 

peter d

Senior Member
Location
New England
The company I work for never buys the 2-screw NM connectors for this very reason. They always buy the "T&B style" NM connector. I believe it's Arlington NM841.
 

peter d

Senior Member
Location
New England
Don, I'm sure the steel tube and metal box manufacturers would love that. Maybe they can lobby the CMP's just like the AFCI manufacturers did.
 

iwire

Moderator
Staff member
Location
Massachusetts
don_resqcapt19 said:
Just one more reason to adopt the Chicago code nationwide:D:D

buriedemt2.jpg


Yeah EMT is always the answer. :D
 

iwire

Moderator
Staff member
Location
Massachusetts
don_resqcapt19 said:
Bob,

We could always go to PVC coated rigid for the crawl spaces and basements.
Don

Yes we could, we could also concrete encase everything. :D

But we might want to do a cost/benefit analysis first. :wink:
 

infinity

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Location
New Jersey
Occupation
Journeyman Electrician
My guess is that it's the newer NM cable that's really the problem. The jacket is super thin and the layer of paper material that used to surround the conductors on round NM cables is gone. So now instead of compressing the paper filler you end up compressing the conductors.
 
I would be willing to bet all of Mike's money that the type of connector is not responsible, but the installer.

The percentage of installations that have failed due to the manufacturers product as opposed to the installation method is probably quite small and I think that it is not a problem we need to be so worried about.
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Pierre C Belarge said:
I would be willing to bet all of Mike's money that the type of connector is not responsible, but the installer.

The percentage of installations that have failed due to the manufacturers product as opposed to the installation method is probably quite small and I think that it is not a problem we need to be so worried about.
Ditto (LOL)....
I have installed 10's of thousnads of the metallic connectors and never had a problem. I don't doubt that some manufacturers have left small burrs on the connector and that may be what caused some of the problems, but I have seen many put on so tight I was suprised it didn't hurt the wire. I am amazed that no one is complaining about the plastic connectors. I personally would rather use the metallic except in certain situation like uc lights. In panels I use metal--- you can't put two wires in those plastic ones without tearing up the wire. If you have to take them out I find them a complete annoyance. I guess, to each his own.
 
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