Knob and tube was the standard residential wiring method in North America from roughly the eighteen eighties through the nineteen forties. It is easy to recognise once you know what to look for. Individual conductors, not bundled in a cable, running through ceramic tubes where they pass through framing and supported on ceramic knobs where they run along it.
The design made sense for its time. Separating the hot and neutral conductors by several inches let heat dissipate into open air. The ceramic supports kept the conductors off the wood. Within its own logic, it was a reasonable system.
Three things make it a problem in a modern house.
The first is insulation. The system relies on air circulation around the conductors. When a house is retrofitted with blown-in or batt insulation in the walls and attic, the conductors lose that cooling. Most current requirements prohibit burying knob and tube in insulation for exactly this reason, and a great many older houses have had insulation added over wiring that was never intended to be covered.
The second is the absence of a ground. These are two-wire circuits. There is no equipment grounding conductor, which means no path for fault current and no grounding for the three-prong appliances that fill a modern home.
The third is age. The original insulation on the conductors was rubber or cloth. After eighty or a hundred years it is brittle and often cracked, particularly near junction boxes and where the conductors were flexed during installation.
Compounding all of this is what was done to it later. Very few homes still have original knob and tube untouched. Most have had extensions spliced in over the decades, often with modern cable joined to old conductors using methods that were never acceptable. Those splice points are usually the actual hazard rather than the original work.
Assessment is genuinely case by case. Some knob and tube, in dry conditions, uncovered, unmodified, and lightly loaded, is in reasonable condition. Some is dangerous. The difference is not visible from the panel and requires looking at the runs themselves.
Insurance and lending both come into play here, and in many cases those pressures drive the replacement decision more than the technical condition does.
More on this topic: worldaid.eu.org
Leave a Reply