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I'm doing some work up in my attic, installing plywood decking over my garage for storage. During installation, I accidentally rubbed the edge of one plank of plywood against a run of NM-B, tearing its outer insulation. (The inner paper-like sheath is not damaged.)

If both ends of this run of NM-B were accessible I would just replace it completely. Unfortunately, it is the line running from my garage's overhead light, to the light switch mounted on the wall. I'm pretty certain this run will be stapled to the stud, a few inches above the switch gang, which means I can't pull it out. (It is also inaccessible for other reasons, such as: the point at which it descends through the plate down to the switch gang is directly below my A/C unit, in the attic.)

Since neither the paper sheath nor the insulated conductors inside were damaged, I'm wondering if it would be acceptable (NEC-wise) to just wrap electrical tape around the scarred portion. After the repair, this section will be hidden under the plywood decking that I am installing. (Not intentionally, that's just the way the cookie crumbles.)

I also found something at HD called 'liquid electical tape'.

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    Similar: diy.stackexchange.com/questions/11874/…
    – Edwin
    Mar 15, 2014 at 4:24
  • Thanks, I just now read that. In their question, the neutral conductor inside suffered damage. Mine doesn't seem quite so bad as that. (Perhaps that difference doesn't change the answer, but I won't know unless I ask.)
    – Cognitive Hazard
    Mar 15, 2014 at 4:26
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    The cable repair kit looks like what you want.
    – Edwin
    Mar 15, 2014 at 4:32
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    Yeah, not the best example of an answer on this site. People sometimes get all wonky when working in a domain where there's a strong authority. But if you're an electrician, the authority's word is reality in your world. Anyway, I think that's your answer. Not going to get much better.
    – Edwin
    Mar 15, 2014 at 5:31
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    This is the answer, there's a bit of detail about how to install it in this answer. The repair has to have the same characteristics as the original insulation.
    – Tester101
    Mar 15, 2014 at 17:43

2 Answers 2

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I'd trust the old reliable electrical tape over the liquid stuff.

Section 334.116 of the NEC (USA National Electric Code) talks about the sheath in very generic terms: it has to be flame retardant and moisture resistant, etc. There is nothing particularly special or magic about the requirements.

I'd inspect the wire very very carefully, maybe even remove the paper to ensure nothing is going on below, wrap it, then mention and show it to the local inspector when your job is inspected. Maybe even write a note on the cable in sharpie.

The worst case is you have to splice the cable into a junction box that forever more must be accessible.

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  • I now think I might have to do the splice-in-a-box, anyway. I needed to reroute this particular run to zig around a truss 2x4 (instead of zag,) and then found there was no longer enough slack for the NM's cut-back sheath to enter the light fixture j-box. I think that's likely a code violation. I really need to get me a copy of this NEC.
    – Cognitive Hazard
    Mar 15, 2014 at 7:23
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    See nfpa.org/freeaccess (For NFPA 70). The insulation does indeed need to reach past the clamp into each box.
    – Bryce
    Mar 15, 2014 at 16:37
  • There are so many good comments here that I wish had been given as formal answers, so that I could accept one of them. This one answer is reasonably good though, so I'll accept it (but my favorite still remains, "ask your local code enforcement office."
    – Cognitive Hazard
    Mar 16, 2014 at 3:01
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    @RyanV.Bissell the blue plastic tab in the photo is the box's clamp. You're not meant to fully remove the tab, instead thread the wire through and the spring action of the tab is the clamp.
    – Bryce
    Apr 27, 2014 at 20:33
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    Not really a big deal. The plastic clamps suck anyway, and I have been known to sand off the sharp edges so they don't knick my cables.
    – Bryce
    Apr 27, 2014 at 23:12
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Our local inspectors ask us to cut off a piece of sheathing, wrap it around nicked surface and then electrical tape.

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