Concealed Knob-and-Tube Wiring

Knob-and-tube was how America's early electrified homes were wired: individual conductors held away from framing on porcelain knobs and passed through porcelain tubes where they crossed wood. The generous air space around each wire was the whole cooling strategy, and there was no equipment-grounding conductor.

The article now mostly governs existing K&T and its limited extension. The great danger is thermal insulation: when homeowners later blow insulation into walls and attics, it smothers conductors designed to breathe, so the Code prohibits covering K&T with insulation. It's rarely installed new and any work on it demands care and, often, replacement.

When it applies

You mostly meet knob-and-tube on old work, not new. Single conductors ride on ceramic knobs and pass through ceramic tubes, kept spaced apart and away from framing. New installations are extremely limited, so in practice this article guides how you evaluate, extend or retire what is already in a house.

Common mistakes

• Burying old knob-and-tube in blown-in insulation, which traps heat around conductors that were designed to shed it into open air.
• Tapping modern circuits onto brittle, cloth-insulated legacy wiring instead of replacing the run.
• Assuming it is grounded — these systems have no equipment grounding conductor.

In the field

When I open a wall and find knob-and-tube packed in insulation, that is a red flag I document immediately. The wiring may have been fine in free air a century ago, but smothered in insulation it runs hot, and hot old insulation is how these fires start.