RESIDENTIAL BY DAVID SHAPIRO
SHAP IRO, author of “Old Electrical Wiring: Evaluating, Repairing, and Upgrading Dated
Systems,” is a contractor, consultant, inspector and writer/editor based in Colmar Manor, Md.
He also is affiliated with IAEI. He can be reached at firstname.lastname@example.org. IST
For a number of irrelevant reasons,
job completion was on hold for close to
a year. Consequently, about a year after
this cable was installed, the luminaires
weren’t in. As I walked through, I noticed
something funny about the overhead pigtails. I discovered the nylon was peeling
away from the BX wires’ thermoplastic
insulation. Opening device boxes, I found
every accessible inch of the BX conductors on two of the circuits and every inch
of the home run on a third circuit had
brittle nylon. It was cracked and peeling, similar to old rubber insulation. This
was not true of the conductors of 12/3
BX in the same rooms. Consequently, it
appeared to result from one bad cable
coil rather than some odd environmental feature.
I asked the manufacturer and the
cable listing agency, a well-known and
respected Nationally Recognized Testing
Laboratory. Lee Cetrone, the lab’s senior
market surveillance engineer, suggested I
send in a sample. The investigation took
six months, and it even involved the cable
manufacturer and the nylon supplier.
The nylon company found no defect in
its resin, and the cable people found no
mistake attributable to their process.
They tested the sample under a number
of conditions. Ultimately, the lab was not
able to reach a conclusion, at least none
it was able to share.
I thought I would be able to learn
whether the manufacturer or the dis-
tributor was responsible. Why the
distributor? While I’ve never heard of
counterfeit BX, I couldn’t rule that out.
However, the cable turned out to be
legitimate, albeit unique, judging from
the fact that the lab had no record of any
One positive finding from the inves-
tigation was that the damage did not
extend to the thermoplastic, so I ques-
tioned what type of thermoplastic
conductor is used inside this BX (or
Type ACHH) cable. Apparently, the con-
ductors inside are—more or less—Type
THHN. Why more or less? Because I
cannot strip out the last few feet of the
conductors in BX or another cable that
earns its listing as an assembly and say,
“What a clever boy am I! I can finish this
run in a raceway without a junction box,
with no more transition than two con-
nectors and a coupling.”
UL Senior Engineer John Cangemi
(retired) said that conductors sold for
pulling inside raceways are marked
with identifying information every 2
feet; conductors installed inside cables
are not marked and are not intended nor
investigated for separate use.
That issue might not be relevant here.
My problem was that the conductors’
nylon had died. But what was the purpose
of the nylon? I learned that THHN means
thermoplastic, high-heat, nylon-coated.
Take off the nylon, and what do we have?
THH: thermoplastic, high heat. The prob-
lem is I find no type THH in the National
Electrical Code, so the fallback had to be
not redefining the cable or its conductors,
but accepting that the cable was compro-
Happily, the September/October 2013
issue of IAEI News covered nylon damage. The feature “UL Question Corner”
was headed: “If the nylon on THHN is
scuffed, is the insulation damaged?” The
person asking the question had scraped
the nylon covering in the course of pulling THHN through a raceway.
UL’s answer stated, “Damage to the
nylon jacket only does not constitute
damage to the insulation. The nylon
jacket does provide mechanical protec-
tion to the insulation during and after
installation and also provides gasoline
and oil resistance for the wire.”
In other words, because the thermo-
plastic underneath the nylon showed no
harm, and it was not exposed to oil or
gasoline, it maintained its listing. An engi-
neer I know with a lot of cable-evaluation
experience speculated that there might
have been an undetected problem with
the nylon’s extrusion.
Part of the nylon’s oddity is that it
showed no evidence of damage when
I installed it. Therefore, I concerned
myself with the future. Where the cable
fed devices, I had pigtailed the incoming
conductors with healthy THWN off a
coil to minimize the voltage drop from
daisy-chaining. Consequently, there
would be no need to significantly disturb the thermoplastic when installing
or replacing devices.
By the way, I replaced the funky cable
Case of the Self-Stripping Cable
What I learned from a strange misfortune
FIRST I’LL BRIEF YOU ON THE BACKGROUND. On a classy rewiring job a while
back, I used only armored cable (BX) in the 20-ampere (A) interior branch circuits.
The new cable coils came from a reputable manufacturer. If you have worked with
BX, you would recognize the brand by the black marking on the 12/2 armor and the
red and black markings on the 12/3. I bought them from a respectable distributor in
downtown Washington, D.C.