Is the UL listing of thhn voided when the outer, slippery coating gets damaged during installation? If so, do the effected area(s) need a heat-shrink tube or other acceptable means of insulation restoration?
The nylon is technically a jacket installed over PVC insulation. The nylon primarily provides a decrease in pulling resistance, making pulls easier. It also does provide a small amount of mechanical protectection.
Nicks in nylon are common an are not a problem. However nicks in the underlying PVS inulation are an issue. Most specifications don't want to have to make an insulation assessment so they say "no nicks in the nylon" as a way of guaranteeing no nicks in the PVC.
I think UL covered this in an article in the IAEI News or possibly in their own newsletter sent to inspectors. As the others have stated, minor damage to the jacket is no big deal under most circumstances. However, the nylon jacket is part of the gasoline & oil resistance (if the wire is marked as such) and if the outer jacket has suffered much damage it should be replaced.
Few things are harder to put up with than the annoyance of a good example.
The reason I ask is because a local inspector requires the slippery coating to be completely intact. Assuming that "gas and oil" are not an issue, what evidence can I show this inspector that shrink tubes are not necessary as long as the inner insulation is unaffected?
Triple, the following is an excerpt from an IAEI meeting, which is what Peter was refering to. http://patriot.net/~deshap/IAEI-GW
Notice the text in bold, you might show this to the inspector.
Next Most Recent Meeting This is the report of our meeting Tuesday evening, November 18, 2003. It was held in Laurel, MD, thanks to the good offices of Chapter godfather Art Hesse, who not only arranged for the meeting space but came early and set it up despite suffering from bronchitis. We had 11 Attendees, including 5 inspectors and one non-member.
We started right off with our scheduled program, which gave us a head start asking what we wanted of jolly old Mr. Nick. Our good friend at NEMA, Gil Moniz, brought Dave Mercier, Technical Director of Southwire's electrical division, the world's largest building wire and cable manufacturer, and owner of the"Romex(R)" brand. He came to talk about how to and whether to! repair cable jackets-and lots more. Dave is an alternate member of Code-Making Panel 7, and former member of CMPs 6 and 8. He is involved in NFPA's study of aging residential wires, the Copper Development Association's study of elevated ambient, and other important projects.
Dave started out with a light touch, warning us of the risk associated with traveling with wire samples, post-9/11. "You'd better have a business card, to show the security people what you're doing with it."
He talked about the fact that in some commercial work there is a cycle of regular, planned, rewiring, but this is not the case with residential work. He figures that if wiring is 30 years old or newer, it's fine. Older, and he just can't predict status. Some folks have a rule of thumb that says if you touch wiring that's over 30 years old, you replace it. This is not based on any studies, which is why he considers the NFPA Research project so important.
The ambient temperature study also is quite important. One 2005 change coming out of it is that cables bundled for 24" or more no longer will be the only ones requiring derating due to bundling. Bundling for a shorter distance and covering with draft-stopping insulation will also require derating. Running raceways outdoors in the sun will mean adding 30 deg F to the high end of the general expected ambient temperature.
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.
[This message has been edited by Roger (edited 08-29-2005).]