I just had our annual furnace tune-up and the guy who performed the inspection gave us a B tag violation because the electrical supply wire to our gas furnace is corroded and copper wiring is exposed. I am going to contact the company who installed it to look at fixing ASAP but the technician who found the issue said he has never seen it before and can't say what caused it.

The furnace is located in our basement near the foundation wall, and being an older house, there is some dampness/potential leaking happening (will have to wait for our first spring before we know the full extent) so my first guess would have been water damage, but could it have damaged the wiring this badly? And what is that white crusty stuff?

Furnace installed and all the electrical was redone in 2017 when previous owner flipped the house.

I am just wondering if anyone has seen something like this before and can confirm the cause, and if it's something I should be worried about reoccurring after the wire is replaced.

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  • My guess would be extreme heat on the conduit and some moisture
    – mark f
    Commented Nov 2, 2020 at 23:39
  • Is that armor magnetic? (Try putting a fridge magnet to it) Commented Nov 3, 2020 at 3:11
  • "all the electrical was redone in 2017 when previous owner flipped the house" How do you know that for sure. That sounds like something a flipper says but doesn't typically do. Commented Nov 3, 2020 at 3:15
  • 2
    Do you have a water softener nearby? If so, when you fill the salt tank you generate salt dust. In a high-humidity environment like a basement near a foundation wall that is a surefire combo for corrosion.
    – jwh20
    Commented Nov 3, 2020 at 10:02
  • 1
    I have seen BX and MC corroded that it is so localized I would guess MC that a caustic like drain-0 or some other chemical was spilled on it. Usually when BX is in bad shape there is rust.
    – Ed Beal
    Commented Nov 3, 2020 at 15:12

1 Answer 1


2017 - probably MC-Lite (aluminum armor) which you could verify by magnet, but the fact that the corrosion is primarily white leans heavily that way.

Given that much corrosion in 3 years, I'd suspect that it's getting something like acidic condensate from the furnace flue (or condensate drain) dripped or splashed on it, or there's water softener salt/brine getting onto it.

A third possibility would be if there was any concrete work associated with the "flipping" and concrete/cement got onto the cable and was not removed - concrete and aluminum are highly incompatible.

Aside from correcting whatever is contaminating the cable, or if you can't figure out what that is, this might be a place where liquidtight conduit and THWN would be a better bet than MC-Lite.

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