K&T wiring and insulation

[Jim Katen]

Originally posted by kurt

. . . One of the experiments was testing #14awg wire. We took a single strand of #14 bare copper wire, wrapped it in tissue paper, wrapped that in newspaper, and then enclosed the entire thing in a fireproof box w/an interior temperature of 120degF. We hooked up the ends of the test wire to an amperage/voltage generator, and cranked a continuous 30 amps through it (fully twice it's rated ampacity) and left for the evening. When we came back in the next day, we shut the system down, opened up the box, unfolded all the paper, and looked it all over.

The tissue was not even slightly discolored, and honestly, the wire didn't even feel warm; at least, it didn't feel any warmer than the 120 temp in the box.

What all this tells me is the NEC is extremely conservative.

That said, I still tell folks to tear it all out, . . .

That was an interesting experiment, but it tells us little about old K&T wiring.

First of all, the ampacity tables in the NEC really apply to the insulation around the wires, not the wire itself. So it's misleading to say that 30 amps is twice the ampacity of #14 wire. For instance, if it were covered with asbestos insulation, a #14 wire would have had an ampacity of 45 amps in free air. The wire can carry all kinds of current. It's the insulation that's the limiting factor.

Second. Tissue paper has very different properties than rubber. You probably could have repeated that experiment using an ambient temperature of 350 degrees and 50 amps and still found the tissue undamaged while rubber insulation might have shrunken or become brittle.

Third. That was an extremely short-lived test. Let it run for 30 years under varying ambient temperature & humidity conditions and varying loads and see what happens. Be sure to include some splices that are made up with lead solder and covered with friction tape.

- Jim Katen, Oregon

[Bain]

Why light fixtures, Jim? Why not outlets, where someone may have had microwaves, toasters, coffee pots and other stuff all crackling at the same time?

[Jim Katen]

Originally posted by Bain

Why light fixtures, Jim? Why not outlets, where someone may have had microwaves, toasters, coffee pots and other stuff all crackling at the same time?

I believe it's the heat produced by the light bulbs. It rises up through the ceiling and cooks things pretty well. Also, the K&T wires just above the light fixtures have loom around them. It acts as insulation and, I believe, contributes to the decay of the rubber.

Of course, the kitchen receptacles might also have toasty wiring behind them. I just rarely get to see those areas.

- Jim Katen, Oregon