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, and thus I assume the individual conductors inside likewise are untouched.)

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'.

 

I would use the liquid tape....esp in an attic. Eventually regular tape adhesive would let go.

If you can slide some heat shrink over it...that would be even better.

Now whether there is some code rule about this...I dunno.

 

The pros here will know for sure by my understanding is it is code compliant to repair the outer sheath of NM-B as long as the inner insulation is undamaged.

 

There is not a code reference I can think of that directly addresses this issue.

I would repair it with a good quality electrical tape (3M for example) as long as the conductors are OK. Liquid tape may be OK too, I just never use it.

 

Can't say if it helps but I have occasionally coated tape with liquid insulation in an effort to keep it from eventually pealing off. Usually did it on split bolts. As I said don't know if it helped but it won't hurt.