Knob and tube wiring was the original wiring used in homes up till the 1930’s – it was replaced by Loomex on the west coast and Romex on the east coast. Extension circuits were allowed up till 1996. Identifying knob and tube wire is reasonably easy to do; typically there is the white porcelain knobs where the wire turns and white porcelain tubes where it passes through lumber or walls. Also if you have proximity electrical testers the wiring has quite a large field around it and can be detected even through drywall. The best place to see the knob and tubes are in unfinished basements or attics, with attics being the most likely place to see it.
Should You Keep Your Knob and Tube Wiring?
Problems start to occur when this type of wiring is modified or when the branch circuit terminations (light switches, plugs, etc) are improperly upgraded. The splices need to be done in proper junction boxes. On the east coast, there has been trouble with the protective insulation coming off the older wire (no reported problems in the west), which can be a fire hazard. Knob and tube is still electrical code-approved if electrified, but if renovations are being done then we recommend that it probably be replaced. Whenever knob and tube wiring is identified, it is always best to have a qualified electrician evaluate it and, cost permitting, replace it.