The Dangers of Knob and Tube Wiring

Knob-and-tube wiring was an early standardized method of electrical wiring in buildings, in common use in North America from about 1880 to the 1940s. Today the system is considered obsolete and a significant safety hazard. Surprisingly, many homes in the Indianapolis area still utilize live knob and tube wiring to this day.


Facts About Knob and Tube Wiring:

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

·While it is considered obsolete and a safety hazard, there is still 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.

·It is not permitted in any new construction.

How Knob-and-Tube Wiring Works:

Knob and 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 a “loom.”

Problems Associated with Knob and Tube Wiring:

·Unsafe modifications are far more common with knob and tube wiring than they are with Romex and other modern wiring systems. Part of the reason for this is that knob and 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, the wiring is insulated with varnish and fiber materials that are susceptible to cracking and deterioration.

Knob and tube wiring was insulated with cambric and asbestos and is not rated for moisture exposure. Older systems contained insulation with additives that may oxidize the copper wire. Bending the wires in any way may cause insulation to crack and peel away.

Today, knob and tube wiring is most often found spliced with modern wiring incorrectly by amateurs. This is perhaps due to the ease by which knob and tube wiring is accessed.

Building Insulation:

Knob and tube wiring was designed to dissipate heat into free air, and insulation will disturb this process. Insulation around knob and tube wires will cause heat to build up, and this creates a significant 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.


When knob and tube wiring was first introduced, common household electrical appliances were limited to little more than toasters, tea kettles, coffee percolators and clothes irons. The electrical requirements of the mid to late 20th century homes could not have been foreseen during the late 18th century, a time during which electricity, to many, was seen as a passing fad. Existing knob and tube systems are notorious for modifications made in an attempt to match the increasing amperage loads required by todays televisions, refrigerators, and a plethora of other electric appliances. Many of these attempts were made by insufficiently trained handymen or DIY’ers, rather than experienced licensed electricians.

Many homeowners adapted to the inadequate amperage of knob and tube wiring by installing fuses with resistances that were too high for the wiring. The result of this particular 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 home inspectors to find connections wrapped with masking tape or Scotch tape instead of electrical tape.

Knob and Tube Wiring and Insurance:

Many insurance companies today will refuse to insure houses that have knob and tube wiring due to the significant risk of fire. Exceptions are sometimes, however seldom made for houses where an electrical contractor has deemed the system to be safe.

Simple advice for those either living with knob and tube wiring or those about to purchase a home where knob and tube is present:

Replace it.

Yes, rewiring a house can take weeks and cost thousands of dollars, but unsafe wiring such as knob and tube can cause fires and death. Prospective home buyers should always get an estimate of the cost of replacing knob and tube wiring. They can use this estimate to negotiate a less expensive price for the house.

In summary, knob and tube wiring is a significant safety hazard in and of itself and also due to improper modifications made to it throughout the years and the addition of needed building insulation.

Good home inspectors are wary of this old, outdated and unsafe system and should always inform their clients about its potential dangers and recommend its removal.