Knob-and-tube wiring

An early wiring method using ceramic knobs and tubes to support single insulated conductors, common in homes built before about 1950.

Glossary

What is Knob-and-tube wiring?

Knob-and-tube wiring runs the hot and neutral as separate conductors supported on ceramic knobs and passing through ceramic tubes at framing members. It has no equipment grounding conductor, its original rubber or cloth insulation becomes brittle with age, and it was designed to shed heat into open air — which means burying it in later-added insulation is a recognised hazard. Functioning knob-and-tube is not automatically unsafe, but modifications and splices made over the decades usually are, and many insurers decline or surcharge homes that still have active circuits. Inspectors should report where it was observed, whether it appears active, and whether it is in contact with insulation.

Related terms

AFCIAn arc-fault circuit interrupter is a breaker that opens the circuit when it detects the electrical signature of arcing, which is a common ignition source in residential fires.Aluminum branch circuit wiringSolid aluminum wiring used for 15- and 20-amp branch circuits in many homes built between roughly 1965 and 1973, associated with an elevated risk of overheating at connections.BondingConnecting metallic components of a building's systems together so they remain at the same electrical potential, reducing shock hazard.Double-tapped breakerTwo conductors terminated under a single breaker lug that is listed for only one conductor.

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