correct wire wrap at screw terminals

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pfriday

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"Regarding NFPA 70 (1999) Article 110-14:

There are specific recommendations and illustrations in the handbook for correctly terminating aluminum wires at wire-binding screw terminals. However, there seems to be little or no guidance for terminating copper wires at wire-binding screw terminals. Do such requirements exist or is this not an issue addressed by NFPA 70?"

I previously posted this inquiry and there were no responses. However, I believe I recently stumbled onto the answer. For those who are interested, this comes from the text "Electrical Fires and Failures", Chapter 15- Electrical Contact Resistance,

"Contrary to what one might expect, the electrical contact resistance between two conductors in contact with each other does not depend on the nominal area of contact. It depends only on the mechanical force between two conductors, apart from certain mechanical and electrical properties of the material of the two conductors."
 
G

Guest

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Re: correct wire wrap at screw terminals

Originally posted by pfriday:
<snip> "Contrary to what one might expect, the electrical contact resistance between two conductors in contact with each other does not depend on the nominal area of contact. <big snip>"
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That may be true when the connection is fresh. I'm thinking of backstabbed (speed-wired) connections where a flat spring at an angle contacts a round solid conductor. At best there is only a few molecules of contact. Over time with hot/cold expansion/contraction (and surges) the minimal contact will pit, oxidize, or otherwise be compromised. The end result will be arcing or an open circuit.

[I question whether nominal is the best word to describe the area of contact, but that's another thread :) ]

My point is initially the amount of contact surface may not be a factor, but in the long run, the more contact, the longer the contact will maintain its integrity and low resistance. Minimal contact may look good in a theoretical equation, but it won't hold up under the rigors of usage.
 
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pfriday

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Re: correct wire wrap at screw terminals

I couldn't agree with you more. So, why doesn't the code address this issue with respect to wire binding screw terminals? Certainly more than a few poor installations have resulted in fires or other hazardous conditions.
 
tonyi

tonyi

Senior Member
Re: correct wire wrap at screw terminals

A connection's contact resistance isn't the same as that connection's ability to handle any significant amperage though.

Imagine two sharp tapering conical contact points being pressed together point to point. At low amperage the connection will survive. At higher amperage, you'd effectively have a fuse and the tips would vaporize. Keep shoving them together and at some point the melting/vaporization would stop as the contact patch on the cones grew to the point where it was able to handle the amperage.

(all bets are off for superconductors) :p
 
G

Guest

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Re: correct wire wrap at screw terminals

A conducting chain is only as strong as its weakest installed link. We go to so much technical trouble to properly size conductors for anticipated ampacity and then somewhere in the chain we allow a nano-speck of copper conductor conduct. How's that supposed to hold up over the life of the installation? It's a recipe for failure (unless we are diligent in our execution and dismiss such shortcut technology).
 
tom baker

tom baker

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Re: correct wire wrap at screw terminals

Does not the UL testing procedure test the various termination methods, under load? They allow a stranded wire, up to 10 AWG under a screw terminal. There are also instructions with the devices. Is is possible that some of the fires are a failure to follow the mfgs instructions/ thus a violation of 110.3(B)?
I say the code does address "So, why doesn't the code address this issue with respect to wire binding screw terminals?", but there were about 5 changes in the 2002 cycle that were UL issues brought into the NEC as few knew them, such as:
Dimmer not allowed to control a receptacle
Use of SCh 80 PVC for physical protection
One grounded condcutor per terminal

Perhaps a code proposal for 08 cycle on how to terminate a wire under a screw head is in order.
Or include the UL white book with the NEC. At least the white book is available as a download from UL now.
 
charlie

charlie

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Indianapolis
Re: correct wire wrap at screw terminals

IMO this is just a matter of workmanship and training. The apprenticeship classes teach about torquing terminals. BTW, UL does torque all terminations for testing. I generally do not see a problem but I do know that most electricians use a standard screwdriver instead of a torquing screwdriver.

As far as I am concerned, the UL White Book is part of the Code. :D
 
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hurk27

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Re: correct wire wrap at screw terminals

By: Charlie As far as I am concerned, the UL White Book is part of the Code.
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And so are the manufacture's instruction's

And in most if not all they do show the proper way to wrap the wire around the screw terminal and the required torque.
 
tonyi

tonyi

Senior Member
Re: correct wire wrap at screw terminals

Originally posted by charlie:
..but I do know that most electricians use a standard screwdriver instead of a torquing screwdriver.
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There's two different size torque wrenches (for panel lugs and larger breakers) and a Snap On torque screwdriver in my bag - and I make sure inspectors see them laying out in an obvious place by the panel when they come by. You'll have to carry around a magnifying glass too to read the torque specs on some of these breakers though. The GE's are a PITA with microscopic fine print.

If something torches up, the lawyers would eat you alive if you couldn't demonstrate there was a correct installation.
 
don_resqcapt19

don_resqcapt19

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retired electrician
Re: correct wire wrap at screw terminals

hurk,
And so are the manufacture's instruction's
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I don't agree. Only instructions that are part of the listing and labeling are required to be followed by the code. If manufacturer's instructions were required to be followed, then there would be no permissible use for "classified" breakers.
Don
 
tom baker

tom baker

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Location
Bremerton, Washington
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Master Electrician
Re: correct wire wrap at screw terminals

Charlie-I agree the UL white book is part of the code. At least its available as a download now.At the IAEI meetings I attend UL always has copies available.
 
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