Excerpt from an old IAEI meeting;
When Gil visited us previously, he made a splash by noting that the transparent nylon covering THHN/THWN, while arguably making them sexy compared to THW, is not required to maintain the conductor's dielectric properties. Therefore, if in the course of installation the nylon suffers the odd nick, or unsightly run, or even is scraped off, the conductor is fine so long as the insulation underneath remains intact. Learning this much, we had to have Dave over for more of the story.
Unless conductors are exposed to gasoline or oil, or submerged for an extended period we're talking many months--the nylon covering offers strictly a mechanical protection, Dave explained, with the underlying PVC supplying the dielectric properties as Gil had told us. The nylon actually is stripped away for the UL acceptance test. Therefore, if the nylon is scraped off, but the plastic underneath is intact, an appropriate response usually is "Good; it's done its job," and a green tag.
It's not just the nylon covering on THHN/THWN, we learned, but the sheaths on cable assemblies also often are optional. A cable installer nicked the sheath of a nonmetallic cable installed in a dry indoor location, and responded in a curious way: he turned the cable so the nick faced toward the wall. Not a bad response, Dave opined (while not wholeheartedly endorsing); this way, people glancing at it in passing wouldn't be unnecessarily concerned.
You may detect from this that Dave is quite an engaging speaker; moreover he's one with 19 years' product and Code experience under his belt to give plenty of clout to the information.
Damage is not necessarily limited solely to cable sheaths, of course. If a cable appears damaged, Dave recommends cutting a window in the sheath so you can examine the conductor insulation beneath. If that's good, you can "reinstall the window."
What should you do when you do want to, or need to, repair a cable sheath? Vinyl tape does just fine, Dave said, except in a wet location, and self-sealing mastic tape does even better. Best is a layer of self-fusing tape covered by a layer of vinyl. It even can be used outdoors on UF or SE cable, so long as it is not buried. For that, you need the sealing coverings that come with a Listed underground splice kit.
Art asked whether NM or SE cable repaired in this way still meets the UL standard. Dave commented that it is tough to say. There needs to be a dialogue between the manufacturer, AHJ, and installer. A qualified installer of repair materials should be able to make a repair acceptable to the AHJ, and Southwire's warranty will remain in effect. "Qualified?" Jim Wooten asked. "Someone who has received certified training in using the material, either in an apprenticeship class, or continuing education such as is offered by tape manufacturers." Even damaged Medium Voltage cable remains warrantied, so long as the shielded jacket, which is there for mechanical protection like that of MC, is fixed using a Listed repair kit installed by a qualified person.
