Knob & Tube Wiring

Total Care Home Inspections will advise you if there is knob-and-tube wiring in the home. It was an early standardized method of electrical wiring in homes and buildings, in common use in North America from about 1880 to the 1940s. The system is considered obsolete and can be a safety hazard, although some of the fear associated with it is undeserved.

Facts About Knob-and-Tube Wiring:

• It is not inherently dangerous. The dangers from this system arise from its age, improper modifications, and situations where building insulation envelops the wires.

• It has no ground wire and thus cannot service any three-pronged appliances.

• While it is considered obsolete, there is no code that requires its complete removal.

• It is treated differently in different jurisdictions. In some areas, it must be removed at all accessible locations, while others merely require that it not be installed in new construction. Inspectors should be sure to know the codes in the jurisdictions in which they operate.

• It is not permitted in any new construction.

How Knob-and-Tube Wiring Works:

Knob & Tube wiring consists of insulated copper conductors passing through lumber framing drill-holes via protective porcelain insulating tubes. They are supported along their length by nailed-down porcelain knobs. Where wires enter a wiring device, such as a lamp or switch, or were pulled into a wall, they are protected by flexible cloth or rubber insulation called "loom."

Advantages of Knob-and-Tube Wiring:

• Knob & Tube wiring has a higher ampacity than wiring systems of the same gauge. The reason for this is that the hot and neutral wires are separated from one another, usually by 4 to 6 inches, which allows the wires to readily dissipate heat into free air.

• Knob & Tube wires are less likely than Romex cables to be punctured by nails because Knob & Tube wires are held away from the framing.

• The porcelain components have an almost unlimited lifespan.

• The original installation of knob-and-tube wiring is often superior to that of modern Romex wiring. Knob & Tube wiring installation requires more skill to install than Romex and, for this reason, unskilled people rarely ever installed it.

Problems Associated with Knob & Tube Wiring:

• Unsafe modifications are far more common with Knob & Tube wiring than they are with Romex and other modern wiring systems. Part of the reason for this is that Knob & Tube is so old that more opportunity has existed for improper modifications.

• The insulation that envelopes the wiring is a fire hazard.

• It tends to stretch and sag over time.

• It lacks a grounding conductor. Grounding conductors reduce the chance of electrical fire and damage to sensitive equipment.

• In older systems, wiring is insulated with varnish and fiber materials that are susceptible to deterioration.

Compared with modern wiring insulation, Knob & Tube wiring is less resistant to damage. Knob & Tube wiring insulated with cambric and asbestos is not rated for moisture exposure. Older systems contained insulation with additives that may oxidize copper wire. Bending the wires may cause insulation to crack and peel away.

Knob & Tube wiring is often spliced with modern wiring incorrectly by amateurs. This is perhaps due to the ease by which Knob & Tube wiring is accessed.

Building Insulation:

Knob & Tube wiring is designed to dissipate heat into free air, and insulation will disturb this process. Insulation around Knob & Tube wires will cause heat to build up, and this creates a fire hazard. The 2008 National Electrical Code (NEC) requires that this wiring system not be covered by insulation. Specifically, it states that this wiring system should not be in…

hollow spaces of walls, ceilings and attics where such spaces are insulated by loose, rolled or foamed-in-place insulating material that envelops the conductors.

Local jurisdictions may or may not adopt the NEC’s requirement. The California Electrical Code, for instance, allows insulation to be in contact with knob-and-tube wiring, provided that certain conditions are met, such as, but not limited to, the following:

• A licensed electrical contractor must certify that the system is safe.

• The certification must be filed with the local building department.

• Accessible areas where insulation covers the wiring must be posted with a warning sign. In some areas, this sign must be in Spanish and English.

• The insulation must be non-combustible and non-conductive.

• Normal requirements for insulation must be met.

Modifications: