Peter McCarthy Electric Co., Inc. - Specializing in Older Buildings in Chicago
Adventures in Old House Wiring....

Rewiring and remodeling; Cloth-covered wire part 2

I originally wrote this as a reply to Kef on his comment. He had raised the point that away from the junction boxes, cloth-covered wire can seem "just fine."Thanks so much for your insight. You make some valid points. I think is most critical one is that at the point of use is where the cloth-covered wire insulation has generally deteriorated to the point of no return. I agree, I have pulled out some runs of 1930s -1950s cloth covered wire where other than being crunchy and non-pliable, it is just about as it was new, except for the last 6-8 inches at each side that has crumbled away to nothing. In general at the fixture locations where they have been subject to excessive heat as transferred through the sheet metal light fixture housing, that is where they are completely shot. That is where the fire hazard is most prominent. The fact that in most vintage fixtures, the bulbs were hanging further away from the wire than in the modern surface mounted fixture makes a big difference. Often that fixture was the only light in the room, so in some cases fixtures have been overlamped with 100 or even 150 watt bulbs for years. If you or your sister ever had an "easy bake oven" as a kid, the similarity is remarkable. Add to that the lack of air circulation/cooling space in the older style "pancake" boxes, which can be as shallow as 1/4" or 3/8", which force the wires to be crammed very tightly against the baking sheet metal housing, and its not a surprise that they decay and insulation crumbles away to nothing once they are disturbed. Unfortunately, they almost always do. If you look closely, most vintage fixtures had a fairly deep cup-shaped canopy. They assumed the building premises wiring would be contained there, and they allowed space for it. The 1 1/2" deep 1900 or 8-b (Octagon) boxes we use today, with their ample depth to provide air circulation, lower ambient temperatures, and to contain the wiring within the recessed box of the ceiling did not exist. Thus if you place a hot pan of sheet metal fixture housing and screw it up tight to the ceiling to look flush and proper to our modern eyes, you are asking for trouble in an old cloth-covered wired house. I try to recycle the copper wire so it is not completely lost. In a sense, its a shame that just the 6-8" of wire at each end can render the whole 25 or 30 foot piece useless. But I also find slight oxidation has occurred on the copper that has been exposed for generations, which is best to be rid of as well. With labor costs much higher than wire cost, it always makes economic sense to replace it. Plus, the gain in available space in the conduit when the thick cloth and rubber insulation has been extracted, which allows for higher conduit fill and more circuits to be pulled into the same size conduit more than offsets the loss at the other end. I like your long term perspective, Kef. Are you an engineer? Thanks, too, for your comment. It's nice to know this "pontificating" of mine is not just floating off into cyber-space and not doing the world any good! 
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