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Joined: Mar 2007
Posts: 5
New Member
Hey everyone. Stopped by a house to look at some remodel work today. The drywall is all off, and I noticed that every place there was an electric line run, there was black tar seeping through the insulation exactly where the wires were. Looked like someone drew a line where each wire ran. All the insulation was tucked behind the wiring, so the wires where all on the facing of the insulation. Wondering if for some reason the wiring had at some point got hot and somehow heated up the tar that old insulation used to use to adhere the fiberglass to the paper and for some reason drew the tar through the paper. This house was hit by lighting years ago, so wondering if that may be the reason. Hopefully one of you had run across this. Thanks for any input.

Joined: Oct 2000
Posts: 2,722
Broom Pusher and

Welcome to ECN!!! cheers thumbs

I have run across similar scenarios, but not to the extents found on your Project.

Most I saw was maybe Three concurrent Studs, with the "Older" NM Cables running through them, having the "Tar Seepage" issue.

You may be on to something, regarding the Lightning discharge, but the overheated Conductor / Cable Insulation sounds more accurate - especially since the Wall Cavity contained Building Insulation.

If the "Tar" was found in well Insulated Southward facing Exterior Walls, the Ambient Temperature in that Wall Cavity could easily reach or exceed +140F daily; possibly sustaining that Temperature for as much as 6 Hours each day.

That would certainly let the Tar out of me, if I was stuck in that Wall Cavity all day, every day!
wink eek

Is the Wiring Knob & Tube, or the "Older" NM Cable?
Also, do all Walls have the same issues (Interior Partitions and Exterior Partitions).


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Oct 2000
Posts: 5,392
Hey there Juiceman.....

I'm back to being an 'old work' sparky lately

imho, one can see if it's been heated, or just deteriorated. The deteriorated tar is almost powdery, and can be brushed off easily, the heated tar has that melted on effect....


Joined: Mar 2007
Posts: 5
New Member
Hey Scott,
This is all somewhat "newer" NM. Plastic sheathing like you see nowadays, but probably 25 years old.
The marking were on every wire, every wall, even the ceiling! So, though your idea of maybe a south facing wall getting hot seems to make sense, unfortunately not in this case due to it being everywhere. All the wiring seems to be fine. Put an arc fault breaker on each of the circuits just to see if there where any problems, but the breaker holds.
So, I am guessing this one probably has something to do with the extreme heat the lighting strike caused. Would have thought a strike like this would burn the wire like a firecracker fuse, but I guess the lighting is over so fast that the split second of heat just overheated the wires. No big loads on any of the circuits, so not thinking this is a case of bad breakers letting the wire get way overheated all the time.
Thanks for your input!

Joined: Mar 2007
Posts: 5
New Member
Hey Sparky,
This is definately a case of over heating! All bleed-through, sticky tar! Really am guessing this is a lightning strike issue. Four circuits in/passing through the exposed remodel area of the house, all with the same markings. Tried an arc fault breaker to see if it picked up any type of damage on the circuits, but all seemed okay. I guess this is going to be one of those "who the hell knows" issues since I have asked every other electrician and inspector in the area and they have all come up blank too!
Thanks for your help though.
Take care,

Joined: Mar 2005
Posts: 1,803
Condensed smoke? grin

Wood work but can't!
Joined: Dec 2001
Posts: 2,498
My school physics book had pictures of a European house where a lightning strike blew the cables out of a solid plastered brick walls... only leaving charred trenches in the walls. Must have been one seriously big explosion!

Joined: Oct 2000
Posts: 5,392
Tried an arc fault breaker to see if it picked up any type of damage on the circuits, but all seemed okay

heh, yeah i do that sort of 'on site analysis' with arcy's too Juice, and hey, if they were all they're cracked up to be, that's all we'd need to do.....


Joined: Jul 2002
Posts: 8,432
Likes: 3
Welcome to ECN, mate.
If you are unsure of the electrical integrity of a cable, I'd move to throw a Megger on it, just to make sure the insulation resistance hasn't been compromised.

Having said that, over here, in the late 90's, we started striking PVC cables that were leaking a green, slimey type substance from the ends of the cables.
(This was often found at fittings or at the panel end)
I asked a mate of mine at the time, who worked for Olex cables, he said that this is the result of cables being in elevated temperatures during the day and then cooling at night.

Fact is, the plasticiser (the substance that makes PVC flexible) in the PVC starts to break down with the heating/cooling cycles.

Let's get one thing straight here, this only affects the first lot of NM or TPS cable that was manufactured back in the 60's, PVC manufacturing has evolved exponentially since then.

Just my $0.05 worth

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