Is knob and tube wiring dangerous?

In older systems, wiring is insulated with varnish and fiber materials that are susceptible to deterioration. Compared with modern wiring insulation, K&T wiring is less resistant to damage. K&T 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. K&T wiring is often spliced with modern wiring incorrectly by amateurs.

This is perhaps due to the ease by which K&T wiring is accessed. K&T wiring is designed to dissipate heat into free air, and insulation will disturb this process. Insulation around K&T 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. 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.

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.

Many homeowners adapted to the inadequate amperage of K&T wiring by installing fuses with resistances that were too high for the wiring. The result of this modification is that the fuses would not blow as often and the wiring would suffer heat damage due to excessive amperage loads. It is not uncommon for inspectors to find connections wrapped with masking tape or Scotch tape instead of electrical tape.

Many insurance companies refuse to insure houses that have knob-and-tube wiring due to the risk of fire. Exceptions are sometimes made for houses where an electrical contractor has deemed the system to be safe. Have the system evaluated by a qualified electrician.