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Why are these wires putting burn marks on paper-backed batts?

Peter_Rogers | Posted in General Questions on

I’ve seen two houses in the past couple of weeks where wiring is leaving burn marks on paper-backed batts (see pics). I found these because I was snooping around looking for air leaks. I’ve always been told that insulating around wiring is fine if it’s modern Romex type wiring, and that knob and tube is the only thing to watch out for. So finding these two examples is pretty terrifying.

Both of these houses have had panel upgrades in the last 1-3 years. It looks to me like in both cases the heat from the wire is bringing the paper to the ignition point, which then starts to burn the wire itself, rather than the wire burning from the inside out. But I could be wrong. There’s no sign in either case of little furry things nibbling on the wiring.

I’m obviously recommending that an electrician be send to investigate before any additional insulation is added, which would only increase the temperatures around the wiring. Is this something that anybody else has seen? Any theories as to the cause, and/or relative danger?

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  1. LucyF | | #1

    Seriously scary. I don't have a clue, but those are frightening photos.

  2. user-995912 | | #2

    Likely an overloaded circuit or undersized wire. The paper facing is adhered to the fiberglass with tar, which, when hot is seeping through the paper and onto the wire.

    That's my guess.

  3. Expert Member

    As Noah said the cause is electrical. The insulation may to some small degree contribute to things, but a wire buried in insulation should never heat up to the point where it can char paper.

  4. Peter_Rogers | | #4

    Aha, that makes sense. Now that I look more closely at the images, it appears that what I had interpreted as burn marks on the wires is just the tar from the paper melting and sticking to the wires - the dark spots on the wires matches up perfectly with missing bits of paper on the batts.

    So, probably not reaching a burning temperature, but still a concern, due to a mismatch between the wiring and the load it's supposed to carry? I'm going to send in an electrician anyway to investigate before we insulate these attics.

  5. wjrobinson | | #5

    14 wire with 20 amp breakers?

  6. charlie_sullivan | | #6

    Wire running at its full rated current can heat up substantially, and that's considered OK. The wire insulation is rated at 90 C, which means it can run continuously at 90 C without degrading. Tar melting point varies a lot but it seems that 50-60 C is common, so it could have easily melted without the wire being outside its safe range.

    So tar melting might have happened within the normal operating range of the wire.

    On the other hand, that is an indication that that wire was pretty hot, and it's worth making sure there's not a problem. In principle, the circuit breaker should trip if the circuit is overloaded. But with a slight overload, the circuit breaker responds very slowly. So if there was, for example, a refrigerator and a space heater on that circuit, it might get the wire hot when both are running, but the refrigerator might cycle off before the breaker trips. That's why it's a good idea to pay attention to how circuits are loaded rather than just loading them up until a breaker trips.

    It's also possible that the circuit breaker is oversized for that wire.

  7. Yamayagi1 | | #7

    Or a 30A fuse? If you have fuses, make sure they are all 15 amp (usually a blue label in the glass) unless you know for certain that the entire circuit is wired for anything heavier. Also check to make sure no-one put pennies into the Edison fuse sockets at your fuse panel, bypassing the fuse entirely, and an open invitation to burning down your house. "Type S" fuse adapters, if properly sized for the circuit, can be installed to prevent oversizing of replacement fuses. And yes, if fused properly, fuses are as safe, or can be safer than standard circuit breakers. EXCEPT!!! that requirements for new services and new construction require almost all circuits to be of either "Combination Arc-Fault Circuit Interrupter" (AFCI) type, or "Ground Fault Circuit Interrupter" (GFCI) type, and in many cases, BOTH GFCI AND AFCI protection is required. . Both separately or in combination offer vastly superior electrical safety protection for people and for property damage that any fuse can offer. AFCIs can detect intermittent arcing that is often a cause of home fires. GFCI principally protect people from becoming a deadly path of electrical flow; they can prevent electrocution by sensing the flow of electricity where it is not suppose to go, and tripping "OFF" in a fast enough time to keep from killing you.

  8. agurkas | | #8

    Having bought a house where I am almost done rewiring entire house, out of 30 circuits I found that at least 1/3 was improperly fused. 20A breakers on 14AWG wire (white Romex most often is 14AWG, yellow is 12AWG). I also found 12AWG wire with 20A breaker, but mid-stream I would come across a j-box with 14AWG wire spliced in. So add over-amping to heat already under insulation in summer - not good.
    My rule of thumb now: everything is 15AMP circuit, unless I traced every wire in that circuit and I knew 100% for sure it is all 12AWG Romex all the way to outlets, switches, and light sockets (like code requires).

  9. Jon Garbarino | | #9

    I see that this is an old post, but I think the marks are reaction between the bitumen in the kraft facing and the plastic of the romex. I'm not an expert, but have seen this myself in an addition that is taking me forever to finish. The wires that left the marks where not even electrified. Also, the air barrier hadn't been finished so I know there was a lot of moisture movement in that area.

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