When observing old BX cable installations (at least what can be seen inside service panels and junction boxes), sometimes the most noticeable thing is that the rubber insulation on the conductors has become dried and brittle, making you wonder what the condition of the conductors is where we **can’t **see them, i.e. under the armor inside the entire length of the cable.
I’ve read in Rex Cauldwell’s book (Wiring a House) that what can be seen does not necessarily represent the condition of the rest of the cable.
I got to see it with my own eyes. I got my hands on a piece of just-decommissioned BX cable from early 1950’s. The whole 9 yards: dirty/dusty heavy steel jacket with a few spots of rust here and there, dried/brittle rubber insulation at terminations, no aluminum bonding strip (which, I think is what separates the modern AC from the old BX), and tin-coated copper conductors which can be (and often are) mistaken for aluminum, until you scratch off the tin from the wire to expose the copper underneath.
Supposedly the tin is there to stop the sulfur in rubber insulation from attacking copper.
So, I cut the end piece of the cable off, and pulled the conductors out.
Check out the picture; It does not show the BX clamp and the “redhead” bushing, but you can see the rest.
What a contrast between what I could and could not see!
While the rubber insulation that was sticking out past the “redhead” was dry and felt brittle, inside the cable the rubber was soft, supple, probably as much so as the day the cable was made. I could see the white coloring of the neutral conductor that usually is browned and discolored inside the junction boxes. The waxed craft paper lining, spiraling around the conductors, still felt fresh and not dried up at all. It had that “new hardware” oily smell, which was captured under the armor for the past 60+ years.
Overall the length of the cable, now removed from inside the wall, was heavy, but surprisingly flexible, which now made sense, considering how “fresh” it is on the inside.
My point being… No point really, just an educational experience for me, and hopefully for others here.
I know that as HI’s, we are not in business of assessing the internal condition of electrical cable, prescribing solutions or administering fixes. However, this helps me understand something; Before, I have called out dried and brittle insulation on BX conductors inside service panels. I will continue to do so, but now when I hear that the electrician’s fix was to wrap the exposed conductors with electrical tape or shrink tubing, I’ll be less skeptical.
You can also get a choke effect using the wrapped steel jacket in some conditions
Article 320 III. Construction Specifications
320.100 Construction.
Type AC cable shall have an armor of flexible metal tape and shall have an internal bonding strip of copper or aluminum in intimate contact with the armor for its entire length.
The armor of Type AC cable is recognized as an equipment grounding conductor by 250.118. *The required internal bonding strip can be simply cut off at the termination of the armored cable, or it can be bent back on the armor. It is not necessary to connect it to an equipment grounding terminal. It reduces the inductive reactance of the spiral armor and increases the armor’s effectiveness as an equipment ground. *Many installers use this strip to help prevent the insulating (anti-short) bushing required by 320.40 (the “red headâ€) from falling out during rough wiring.
Well, AC/BX cable jackets without bonding wires were once acceptable for grounding. Those that were originally wired that way can continue being used that way. However, if you change the circuit, you can not use and old, unbonded AC/BX jacket for grounding today.
Forgive my obsession with the subject, but I hope you guys will too find it interesting.
Came across another vintage decommissioned BX, this one probably from the 30’s or the 40’s…
Cloth over rubber in a coiled steel armor.
The ends of the wires exposed inside the junction box were extremely brittle; Any attempt to bend the wire would make the cloth and the rubber insulation disintegrate into dust and black crumbs.
However, when I pulled the conductors out of the armor, their entire length that was concealed was in a very decent shape – the insulation still fresh and flexible despite the 70+ years of service.
I’ll take BX over any other wiring any day…
Great info, thanks Roy.
I read both the discussion and the case study PDF.
I have to amend my statement: sounds like BX/AC cable is the best old wiring method, but only as long as it is protected by AFCI’s.
Of the modern wiring, in my very humble opinion, I think MC is the way to go for the best and safest installation. Conduit, of course is safer still, but it’s an overkill, I think, and there is no way to do it without ripping down all the walls.
I am really leery of NM (Romex), simply because of a complete lack of any physical protection, and also because some household rodents have an unusual taste for its jackets…
Obviously I can’t say any of this on my inspection reports, but if a friend asked my advice, that’s what I would say: go with MC.
