What is "Knob and Tube" Wiring?

I have seen this mentioned in reference to blowing in cellulose insulation, and how it is a fire hazard. What exactly is this type of wiring? I have a house built in 1961, and as far as I know, it just has the cable with fabric sheathing.
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Buck Turgidson wrote:

K&T was used in the Midwest until about 1965.
This link tries to explain the insurance companies' concerns: http://www.insure.com/home/knobtube.html
Excellent dwgs here of its application just after turn of the century: http://www.codecheck.com/KNOB_TUBE/knob_tube.htm
Jim
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http://search.yahoo.com/search?p=what+is+knob+and+tube+wiring
randy

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The first wiring system used in houses. Insulated wires run on insulators (the knob) and through short insulated tubes when the wire has to be run through a wall stud or joist.
Houses were wired up with 2 circuits, usually 30 amps, 3 wire 220v, and #10 wire, 2 screw in fuses.
K&T wiring is very safe, the workmanship required is very high to do right, and it was done right decades ago.
Where it is unsafe, is when people expand the wiring buy just adding on ad hoc, for example, BX that is just connected to the hot and neutral, since there is no ground in a K&T system.
And way, way past the original limits.
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I've seen this exactly once. It's when the old guys used to use porcelean insulators, nailed in. They would string the wire from porcelean to porcelean.
Sometimes seen in very old barns.
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Christopher A. Young
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You almost certainly wouldn't find knob & tube wiring in a 1961 house. There is nothing inherently wrong with it, but it did require real workmanship to install properly and due to the insulation used had a low temperature rating. To meet code it must be installed in free air movement. When covered with thermal insulation, it no longer meets the temperature rating required. In extreme cases, the heat could build up and cause a fire in the wire insulation or surroundig materials. In fact, knob & tube could still be installed in certain cases and meet code.
Dan
On Thu, 07 Oct 2004 23:14:52 GMT, "Stormin Mormon"

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Dan wrote:

As I posted, it was used in the Midwest almost exclusively until right around 1965. As for conductor insulation, later installs all used TW or better, so decaying rubber was no longer a problem. We have millions of homes out here with original work dating back to the 20's which is still in good/serviceable shape. I'm not defending K&T, just saying it was widely used and (left undisturbed) was very reliable.
Jim
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I finally removed the last of my K&T wiring last last month in my 104 yr/old house. The wiring was the first generation installed in the house (sometime in first half of 20th century). My house had gas lamps originally.
The K&T was in relatively good condition. The insulation on the wiring was a little brittle but still fairly strong. My biggest concern was where new wiring was connected in. I often found connections without J-boxes and covered by insulation.
-borninPA
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