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I assume this is some USA wiring system, am I right?

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3 Answers

up vote 20 down vote accepted

Knob and tube wiring uses two separate copper conductors, each isolated by cotton cloth or soft rubber. The conductors are strung tightly through the cavities of your walls and floors, and depend on being separated by air in order to avoid overheating. When going through joists, the wires pass through porcelain tubes. In the wall and floor cavities, they are attached intermittently by porcelain knobs.

Knob and tube wiring was commonly used in North America from the 1880s through the 1930s.

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See also diy.stackexchange.com/questions/20/… –  Bryce Mar 28 at 22:09
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Wikipedia has a pretty good explanation with pictures.

knob and tube wiring in 1930s home

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Only seen that stuff once. Kinda scary. –  electricsauce Aug 4 '11 at 0:20
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Knob and tube was the standard long ago. Now it's considered verboten by many insurance companies and is often replaced when a home comes under contract (either the seller replaces it or drops the house price by a significant amount and the buyer replaces it).

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As a former owner of a house with knob-and-tube in Texas, I had no trouble with the home inspection when buying the place or selling it, and no issues with insurance. Knob-and-tube is not inherently more unsafe than any other older wiring system. Safety is much more a concern with aluminum wiring. The biggest problem with any older wiring is overloading--older houses simply don't have the appropriate number and load capacity of circuits for modern use. –  richardtallent Jul 26 '10 at 6:33
    
Some safety concerns come about when the wiring cover decays and falls off and you add insullation to a ceiling for instance - it contacts the bare wires and causes short/fire hazards. –  Mark Schultheiss Jun 1 '11 at 14:54
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I had no trouble getting insurance on a knob & tube home. Once I found a firm with K&T experience, the process of getting an inspection certificate and insurance was smooth. See also diy.stackexchange.com/a/20279/5960 . K&T is soldered and (if not hacked to death by renovation) really solid. –  Bryce Jun 24 '13 at 18:27
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