Knob‑and‑tube is an early standardized wiring method using single insulated conductors run separately, supported on porcelain knobs and routed through framing with ceramic tubes; it was common from about 1880 through the 1940s[2][3][4].
Many older cloth‑ or rubber‑insulated wiring varieties share similar age‑related failure modes (brittle or deteriorated insulation) and may be found alongside K&T in older homes[3][2].
Inspectors commonly start with a visual survey for knobs, tubes, exposed runs, damaged insulation, open splices, signs of overheating, improper overcurrent protection, and unprotected conductors[2][3][1].
After the visual inspection, licensed electricians generally perform electrical testing such as voltage checks, continuity tests, insulation‑resistance (megger) testing, and—if overheating is suspected—infrared thermal imaging; these tests are performed by a licensed electrician[3][2].
The National Electrical Code (NEC, NFPA 70) addresses K&T in Article 394; Article 394.12 (Uses Not Permitted) is commonly interpreted to prohibit concealed K&T wiring from being enveloped by loose, rolled, or foamed insulation[5][6][15].
Because of NEC 394.12, many contractors, inspectors, and insulators treat buried K&T as a code violation and a fire risk unless a local amendment or variance applies[6][15][3].
Some jurisdictions provide administrative exceptions or variance procedures that allow insulation over K&T under strict conditions (for example, formal inspection, labeling, and documentation); Washington State and sample Oregon procedures are examples of locales with specific administrative pathways, so homeowners should check their local AHJ for rules that apply to their property[7][8][15].
Where knob‑and‑tube remains in good condition, is exposed (not buried in insulation), is not overloaded, and has been inspected and documented, some jurisdictions or inspectors permit it to remain in place — but acceptance varies by the authority having jurisdiction (AHJ) and by insurers[3][2][7].
Common safeguards when a retained K&T approach is permitted include an electrician inspection, labeling of the circuit and the enclosure location, written documentation for future occupants or contractors, and in some locales formal AHJ approval[8][7][15].
Typical U.S. whole‑house replacement (complete rewiring) cost guides report low‑to‑mid five‑figure ranges, with widely reported spreads roughly $8,000–$36,000 depending on home size, access difficulty, and need for service or panel upgrades; many examples cluster in the $12k–$18k range for 1,500–2,000 ft² homes, and costs increase for plaster walls or difficult access[11][12].
Insurance carriers vary: many standard homeowners insurers are reluctant to insure homes with active K&T and may require replacement or mitigation, while other carriers or specialty markets may insure with conditions — homeowners should check current insurer policies and shop specialty markets if needed[13][14].
Do‑it‑yourself rewiring or informal repairs (undocumented splices, open splices, overfusing) are unsafe; the recommended path is evaluation and work by a licensed electrician who can test, document, and bring circuits into compliance or recommend full replacement[2][3][10].
Knob‑and‑tube and old cloth‑braid wiring are common in older homes and can be safe when exposed, intact, properly protected, and not overloaded — but deterioration, buried runs in insulation, improper splices, or inadequate overcurrent protection create real hazards. Evaluation, testing, documentation, and repairs or replacement should be done by licensed electricians and coordinated with the local AHJ and your insurer before deciding to leave active legacy wiring in place[3][2][7].