Older Wiring: Knob and Tube, Cloth NM and Aluminum

Knob and tube

Description

Knob and tube wiring is named for two ceramic parts. Knobs are insulators that support the conductors. Tubes protect a conductor where it passes through a framing member. Its peak years were the early 1900s through the 1930s. The method is considered obsolete. It may still be installed where it extends existing knob and tube, or where the local building department permits it.

Thermal insulation over knob and tube

Houses from the knob and tube era had little or no thermal insulation. In normal operation, knob and tube under thermal insulation should run no hotter than other wiring methods. Covering it is still a bad idea. Attic traffic may damage it, and insulation installed carelessly may push the wires too near framing and other surfaces.

NEC Article 394.12 prohibits thermal insulation over knob and tube in the hollow spaces of attics, walls, and ceilings. In most areas, covered knob and tube is therefore a reportable deficiency. Some jurisdictions allow it.

Contact with metal is a major reportable deficiency. Examples are foil radiant barriers, foil vapor retarders, ducts, and pipes.

Correct installation

Knob and tube that lacks any of these nine characteristics is a reportable deficiency.

1. A clearance of 1 inch or more from walls, floors, framing, and similar objects.
2. A clearance of 3 inches or more between the wires.
3. A knob within 6 inches of every wire splice, a splice in a box included.
4. Knobs spaced 4-1/2 feet or less apart along wire runs.
5. Wires along the sides of studs, joists, and rafters, not across the top.
6. A tube wherever a wire passes through framing.
7. A tap joining one knob and tube wire to another may be made outside a covered junction box. It should be soldered and wrapped in friction tape.
8. A tap from knob and tube to another wiring method should be enclosed in a conduit body or a covered junction box.
9. Each wiring method should have suitable protection where it enters a box, and should be fastened near the box where required. A knob and tube wire should pass through an opening of its own, with a bushing. NM cable should come in through a clamped opening, fastened within 12 inches of the box.

Problems

Knob and tube has nothing wrong with it if three conditions hold: it is installed correctly, the circuit is not overloaded, and the knobs, tubes, and insulation are intact and safe. Some homeowner insurance companies find those conditions too many.

A home inspector should consider telling the client to contact the homeowner insurance company and the mortgage lender to learn their underwriting standards for a house with active knob and tube. Potentially costly rewiring may be required before a policy is issued or a loan is made.

1. End of service life: knob and tube has reached it. Some standards of practice make this a reportable condition on its own.
2. No EGC: knob and tube usually has no equipment grounding conductor (EGC), a safety issue.
3. 15 amp circuit: more lights and receptacles than is safe may be connected to it.
4. Asbestos: the conductor insulation is considered potential asbestos containing material.

Many home inspectors report knob and tube whether it is active or inactive. They recommend that an electrician experienced with knob and tube evaluate the active wiring. Wiring that is inactive where accessible may be active where inaccessible, for example inside walls.

Cloth-covered NM

Cloth-covered NM may be found in houses from the late 1920s. It is more likely in houses built in the 1940s and 1950s. Its cotton or rayon sheathing gives it the derogatory nickname rag wire. PVC sheathing replaced the cloth during the 1960s.

With age, both the wire insulation and the cloth sheathing deteriorate and turn brittle. Vermin also chew the cloth. Like most old wiring methods, cloth-covered NM has no EGC.

Some home inspectors report cloth-covered NM, especially where its visible sheathing looks deteriorated. Some homeowner insurance companies also have underwriting issues with it. Tape over damaged cable is not an acceptable repair. The junction box beside the damaged cloth NM is not covered.

Solid-conductor aluminum wiring

Description

By around 1964, copper had become expensive. For years, aluminum wire had transmitted and distributed electricity, and some building wiring had used it. In 1964, solid-conductor aluminum wiring went into production in #8 to #12 AWG nonmetallic sheathed cable made for residential use.

Buildings differ from the transmission and distribution grid. They have many devices, such as switches, receptacles, and circuit breakers. Copper wires in light fixtures and appliances have to join the aluminum wires. Buildings were designed around copper.

Many metals in everyday use are not pure. Each is an alloy, a combination of materials, and those materials can change the metal's behavior significantly. The original alloy in solid-conductor aluminum wire caused creep and made the wire brittle.

Problems

Under Joule's law, the heat in a circuit depends on three things: current, resistance, and time. More resistance means more heat, and heat can cause electrical fires.

When different metals touch in the presence of moisture, a galvanic reaction (electrolysis) occurs. It may cause electricity to flow, and batteries depend on it. Where the flow is not intended, corrosion can occur. Examples are between copper and aluminum wires, and between aluminum wires and the steel terminals made for copper.

Aluminum, like other metals, tends to oxidize, and oxidation is similar to rust. On any wire, and especially on aluminum, oxidation or corrosion raises the resistance at connections such as switches and receptacles.

When heated, the original alloy expanded more than copper did. Its expansion rate also differed from that of steel device terminals made for copper. Two problems followed.

Expansion and contraction worked the wire loose at connections, especially where receptacles and switches were wired from the back rather than at the side screws. Loose connections mean higher resistance and arcing.

Over many expansion and contraction cycles, aluminum at the connections became distorted, smaller, and brittle for good. This is called creep, and some call it cold flow. Crept aluminum acts as a smaller gauge wire, and a smaller wire carrying the same current runs hotter.

A new, more stable alloy replaced the original in 1972, and by 1981 all aluminum wire had to use it. By around 1980, the reputation of solid-conductor aluminum was so bad that nobody wanted the wire. Manufacturers stopped production. Apart from some solid conductor #8, no United States source supplies it now.

Aluminum wire performed poorly on device terminals designed for copper. Devices labeled CU/AL were the first fix, and they did not work well either. Devices labeled CO/ALR came next. CO/ALR devices work, and the NEC has required them since 1981.

Typical repairs

Rewiring the house with copper wire is the safest repair, and possibly the best. The work is expensive and disruptive, and still worth considering.

A COPALUM connector repair cold welds copper wire onto the aluminum wire with a patented connector and a crimping tool. The process has been in use for years, and the Consumer Product Safety Commission recommends it. Its cost is high: the tools and materials are proprietary, and an electrician must be trained before using them. Every outlet served by solid-conductor aluminum must be repaired, meaning each switch, receptacle, light, and appliance. Each outlet costs up to $60 or more. An electrician who can do the work may be hard to find at any price.

AlumiConn connectors have been available since the early 2000s. They cost much less than COPALUM connectors and are sold to the public. Public sale has a drawback: an inexperienced person installing them could cause more harm than good.

Replacing every switch and receptacle with CO/ALR devices might help where the aluminum wire was made after 1972. It does not address creep in old technology aluminum wire, or other aluminum terminations such as light outlets.

Distinguishing between different aluminum wiring

Stranded aluminum wire

Stranded aluminum wire comes in #8 AWG and larger sizes. Feeder wires, service entrance wires, and large appliance branch circuits use it. On circuit breakers labeled CU/AL and devices labeled CO/ALR, it has no problems.

Aluminum tends to oxidize, so anti-oxidant paste is highly recommended at terminals. The paste is also highly recommended at copper to aluminum wire connections. The NEC does not specifically require it. Some wire manufacturers recommend it, and some jurisdictions require it.

Copper-clad aluminum

Copper-clad aluminum was made in the 1970s. Its cut end shows aluminum; otherwise it looks like copper. It does not share the problems of solid-conductor aluminum wire. It is also uncommon. That may explain why no problems have been reported. For ampacity, it is rated as aluminum wire.

Tin-coated copper

Tin-coated copper had its peak use in the 1940s and 1950s. It also appears in older houses and in houses as recent as the 1960s. It resembles solid-conductor aluminum wire. Three checks tell them apart:

1. Age of the house: solid-conductor aluminum is unlikely, though possible, in a house built before 1964.
2. Cut end: copper shows where the wire is cut.
3. Insulation: plastic insulation means the wire is not tin-coated copper.

All tin-coated copper wire has nearly used up its expected service life. Its old rubber insulation can fail by cracking, especially if the wire is disturbed.

Numbers and Rules to Memorize