08 - Old Wiring Methods: Knob & Tube, Cloth-Covered, Aluminum

Send us Fan MailKnob and Tube Wiring (1900s-1930s)Named for ceramic knobs (support conductors) and ceramic tubes (protect conductors through framing). Most popular early 1900s to 1930s. Still may be installed as extension of existing systems.Correct Installation Standards:≥1" distance between wires and objects (walls, floors, framing)≥3" distance between wires≤6" between knobs and wire splice≤4½ feet between knobs in wire runsWires on sides of joists/rafters/studs (not on top)Tubes where wires penetrate framingTaps between knob-tube wires: May occur outside boxes (soldered/taped)Taps to other wiring: Must be in covered box with appropriate protectionMajor Problems:Covering with thermal insulation NOT allowed (NEC Article 394.12)Contact with metal (pipes, ducts, foil) is major deficiencyAt end of service lifeUsually no EGC (safety issue)15-amp circuits (may be overloaded)Potential asbestos in insulationInsurance/mortgage underwriting issuesCloth-Covered NM (1940s-1950s)Cotton or rayon sheathing (derogatory nickname: "rag wire"). Found late 1920s through 1960s. PVC sheathing replaced cloth in 1960s.Problems:Cloth sheathing and insulation deteriorate/become brittle with ageVermin chew on clothNo EGC (most installations)Insurance underwriting issues (some companies)Tin-Coated Copper Wire (1940s-1950s)Most popular 1940s-1950s. Looks like solid-conductor aluminum but check: cut wire shows copper, cloth insulation (not plastic).Solid-Conductor Aluminum Wiring (1964-1980)Manufactured 1964-1980 in #8-12 AWG NM cable for residential use. Copper became expensive; aluminum seemed like good alternative. Problems emerged quickly.Why Aluminum Wire Failed:Galvanic reaction: Different metals in contact (with moisture) cause corrosionOxidation: Aluminum "rusts," increasing resistance at connectionsExpansion/contraction: Different rates than copper and steel terminalsCreep (cold flow): Wire becomes permanently distorted, smaller, brittle after expansion/contraction cyclesResult: Loose connections, high resistance, arcing, heat, firesNew Aluminum Alloy (1972): More stable alloy introduced 1972, required 1981. By 1980, solid-conductor aluminum had bad reputation; manufacturers stopped producing it.Distinguishing Aluminum Wire Types:Stranded aluminum (#8+ AWG): No problems with CO/ALR devices and CU/AL breakers. Used for large appliances, service entrance, feeders. Anti-oxidant paste highly recommended.Copper-clad aluminum (1970s): Uncommon. Looks like copper except aluminum visible at cuts. No reported problems. Rated as aluminum for ampacity.Solid-conductor aluminum (#8-12 AWG, 1964-1973): PROBLEMATIC. Especially old technology (pre-1972 alloy).Typical Repair Solutions:Rewire entire house with copper (safest, expensive, disruptive)COPALUM connectors (cold-weld copper to aluminum, expensive ~$60/outlet, requires special tools/training, CPSC-recommended)AlumiConn connectors (less expensive, sold to public, requires proper installation)Replace devices with CO/ALR (helps post-1972 wire, doesn't address creep in old wire, doesn't address light outlets)IRC/NEC ReferencesNEC 2017: Article 394 (knob and tube)Intro This episode is brought to you by GetSync.pro This episode is hosted by Charlie Bellefontaine of Chicagoland Home InspectorsFor complete training with visual materials, practice exams, and certification support, visit nhiexamprep.com© 2025 National Home Inspector Exam Prep Podcast. All rights reserved.