What happens when a conductor gets damaged

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Recentsword

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I had a wire between a couple of outlets nicked by a sawsall yesterday. The sheath was torn a small bit of the neutral conductor was damaged. I know what needs to happen, splice it in an accessible junction box or replace it. I’m curious to understand why.

I was thinking maybe it would run hotter with the reduced cross section but I ran ~20 amps through it for a while and took some photos with a thermal camera and I could not see any increased heat.
 
Recentsword said:
I had a wire between a couple of outlets nicked by a sawsall yesterday. The sheath was torn a small bit of the neutral conductor was damaged. I know what needs to happen, splice it in an accessible junction box or replace it. I’m curious to understand why.

I was thinking maybe it would run hotter with the reduced cross section but I ran ~20 amps through it for a while and took some photos with a thermal camera and I could not see any increased heat.
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1. There is no margin built into the ampacity tables. They are based on laboratory testing and computer calculations. I have seen damage on applications where the equipment runs at the limits where “1 or 2 strands” ends up eating up the cable and a breaker.
2. How do you determine how much the ampacity is affected? You can’t. Neher McGrath doesn’t give damaged cable tables. Plus there are all kinds of effects that are hard to predict with short circuits where the current flow is affected.

On nicked insulation (not conductors) for low voltage the worst that happens is exposure to moisture and a possibly short. I’ve run into plenty of failures over the years which are the result of not installing a plastic bushing.
 
paulengr said:
1. There is no margin built into the ampacity tables.
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Do you have any proof of this?

We have almost 100 years of application experience with the tables so they do work. I know testing is done in accordance with the table values. It is just I have never seen the data used to create the tables in the first place.
 
jim dungar said:
Do you have any proof of this?

We have almost 100 years of application experience with the tables so they do work. I know testing is done in accordance with the table values. It is just I have never seen the data used to create the tables in the first place.
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You have no way of determining what the effective AWG # is where the strands are cut.
 
ggunn said:
You have no way of determining what the effective AWG # is where the strands are cut.
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True, but that is immaterial to my question.

What data was used to create the NEC tables?
Yes they are proven effective over a century of use, but that doesn't mean they are anything but guessed at values.
 
jim dungar said:
True, but that is immaterial to my question.

What data was used to create the NEC tables?
Yes they are proven effective over a century of use, but that doesn't mean they are anything but guessed at values.
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That makes no difference. The NEC gives us rules to follow with respect to ampacity and AWG gauges; whether you or I agree with the way they were determined is irrelevant.
 
Taking a fresh look, and ignoring ampacities, I just want to point out that for wires subject to flexing or heavy vibration, even a small nick provides a stress riser where fracture can start and progress across the conductor.
 
ggunn said:
That makes no difference. The NEC gives us rules to follow with respect to ampacity and AWG gauges; whether you or I agree with the way they were determined is irrelevant.
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I was asking the poster, paulengr, if he had any supporting data to back up his claim that there is no margin built into the ampacity tables. He claimed they were based on laboratory testing and calculations. I have not said the rules should not be followed.
 
Biggest concern, already mentioned, is the stress riser factor.
If a 'nick' less than 1/4 the diameter, the ampacity is barely affected if one considers the heat sink on each side of the nick - OP has thermal image data for backup
:rolleyes::rolleyes:
 
Recentsword said:
The sheath was torn a small bit of the neutral conductor was damaged. I know what needs to happen, splice it in an accessible junction box or replace it. I’m curious to understand why.
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Yes, a splice and a JB are surely the way to go but if you are not able to that without causing a lot of damage you could try a product like one of these in a pinch:


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It is obvious that leaving a nicked wire installed is a bad idea. So, I was looking for a code citation that prohibits installing nicked wires, or leaving them installed. I couldn't find any reference in either the NEC or the ANSI/NEIS 1-2015 "Good Workmanship" standard. Does anyone know of any code citation that addresses proper stripping of wires without nicking the conductors? Or that speaks to replacing the nicked conductor if you find one? Thanks!
 
jim dungar said:
I was asking the poster, paulengr, if he had any supporting data to back up his claim that there is no margin built into the ampacity tables. He claimed they were based on laboratory testing and calculations. I have not said the rules should not be followed.
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I believe he was talking about margin in the rules, not margin for more amps through a wire than the tables allow. Of course you aren't going to burn the insulation off a 40A conductor by pulling 41A through it.
 
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