I read in other answers with similar photos that this is tar from insulation. The insulation is not that old (30 years). Is this burned cable or tar or something else?

enter image description here

  • 2
    your question is like showing a picture of an engine to a mechanic and asking if the timing is off .... you are the only one in a position to touch the cable and determine what the black spot actually is
    – jsotola
    May 4, 2020 at 15:28
  • 5
    What the heck happened to that staple? It looks as if someone took an angle grinder to it. Whatever it was, it might've also damaged the cable and left those marks on it. May 4, 2020 at 16:16
  • Looking at this on my computer monitor I see possibly the tip of a nail sticking out just to the right of the cable , I mentioned the spacing in my answer but if that is a nail it was likely the culprit and the hot may be exposed.
    – Ed Beal
    May 4, 2020 at 17:39
  • 2
    @IlmariKaronen You must be new. That's how electrical staples used to look. Even new metal staples still have a flat top edge to provide a better striking surface.
    – MonkeyZeus
    May 5, 2020 at 18:49

6 Answers 6


First, de-energize the cable entirely until you are sure.

Dip a rag in paint thinner, not dripping, just slightly moist. Wipe the spot. If it spreads all over the place and eventually cleans off the cable entirely, it's tar. Burn marks won't wipe away.

  • If it is an arc mark from the cable/ wiring paint thinner is flammable and maybe not a good choice as any liquid that could conduct electricity. The outer covering should be inspected.
    – Ed Beal
    May 4, 2020 at 4:18
  • @EdBeal One would certainly hope the cable has been taken out of service. May 4, 2020 at 5:39
  • 3
    paint thinner (white sprit / stoddard solvent / mineral turpentine ) does not conduct electricity, and a small amount will not damage the cable, WD40 is an alternative that would also work as would kerosene (which is the solvent in WD40)
    – Jasen
    May 4, 2020 at 8:18
  • 1
    You are ill-advised to test in the real world if, pure (!) or not, organic solvents such as pure ethanol and pure methanol conduct electricity.
    – Mazura
    May 4, 2020 at 18:09
  • 4
    The reason I went for paint thinner is it's a solvent for tar, and it quickly evaporates 100% with no residue. Even if it dissolves the cable sheath, it will quickly evaporate away. May 4, 2020 at 18:47

While I would play it safe like Harper suggested (turn it off to be sure before any inspection), I would say it's possible this was caused by someone pulling on the cable after it was stapled. There's two signs pointing to it, per the photo

  1. The staple is askew, causing a potential pinch point
  2. There's a small welt on the sheath just above the spot in question, indicating a hard pull that was stopped by the cable itself

So it might indeed be burn marks, but caused by something external. If the sheath isn't anything more than scuffed, you're fine


Since the black spot seems going in the insulation material I would consider the cable burned with hot wire exposed.

Considernig the shape of the insulation above the hook I wouldn't consider the wire safe at all.

If it is possible, replace the cable. The sooner, the better. Also consider using conduit to protect new cable and use insulated mounts instead of (metal) pins.

  • 2
    Metal staples are perfectly normal and suitable for cable installation like this. There is no reason whatsoever to justify recommending they run conduit and insulated clamps.
    – J...
    May 4, 2020 at 20:34
  • @J I disagree. It looks like the OP really needs conduit and insulated clamps. Just the sight of the staple holding an electric cable made my blood run cold.
    – RedSonja
    May 5, 2020 at 6:14
  • 4
    @RedSonja It's an irrational response. Metal cable staples for double insulated cable are completely safe, to code, and ubiquitous in domestic construction. This is the way to attach romex/loomex to a stud. It's as by-the-book as it gets.
    – J...
    May 5, 2020 at 12:44
  • 1
    @J I read the code as (minimum) requirement. Another level of protection is nothing bad. Seing nails, hot wires and wood together makes me uncomfortable.
    – Crowley
    May 5, 2020 at 21:12
  • 1
    @Crowley It's an irrational response. Millions of homes are built this way. It's perfectly normal and safe. Your answer said "the sooner, the better", implying a sense of urgency at correcting this 'unsafe' problem - that's FUD and it's completely unwarranted.
    – J...
    May 6, 2020 at 12:37

Felt or tar paper can leave black marks on romex. If the right hand side of this photo is the bottom I would say this may be an arc mark and want to pull that staple and get a better inspection of the wire. Romex is usually stapled in the middle of the board so if a nail misses the 2x it won’t hit the wires that may have happened here but it is hard to tell. But I would take a closer look.


acoustical sealant on poly vapor barrier
Cropped photo House Roof and details Sept 18 2007 001 by G MacRae (CC BY-NC-ND 2.0)

Appears to be acoustical sealant, aka Tremco or "black death". This messy, tar-like substance is used to seal poly vapor barriers. It never really dries and gets all over the place if you're not careful including your clothes, framing, insulation, and of course electric wiring.

You can clean it up with xylene or mineral spirits (see data sheet), but I wouldn't bother.


It kinda appears that this was in fact tar from the insulation. I wiped most of it off with water and a rag. Some remains. The question now becomes, WHY did the wire become so hot that tar leaches from the insulation to the wire?

enter image description here

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.