Knob and tube wiring

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Knob and Tube Wiring

Many houses constructed pre1950's have what is called knob and tube wiring. One can determine if you have this type of wiring in your home, by closely looking at basement joists or attic rafters.

To determine if your home is wired " knob and tube", look for ceramic knobs or tubes in which the wire gets attached to, or passes through, joists or studs. If the knob and tube wiring is not easily visible, you can usually tell by looking at your electrical outlets and switches. You may only have two prong outlets to plug into. Basically, no ground at each outlet or fixture outlet means knob and tube wiring is present, likewise if you have older pushbutton switches, this is also a good sign you may have knob and tube

Nowadays, Home owners with knob and tube wiring may find it difficult or impossible to obtain insurance on their home because most insurance companies are not likely to insure a house they perceive as high risk. Insurance companies usually require a certificate of inspection  and compliance from a licensed electrician, that all knob and tube has been removed and replaced with modern 3 wire grounded circuits before it will insure a home that previously had knob and tube wiring.  After the electrician rewires your home, they  give you a satisfactory assessment of your home, and the  insurance company will consider giving an insurance policy for your house.

B. Overview of Knob-and-Tube Wiring

Knob-and-Tube wiring was the predominant wiring system through the 1920 s and 1930 s; some installations of knob-and-tube wiring continued in houses up until 1950. There are several distinguishing characteristics of knob-and-tube wiring in comparison to current wiring methods:

When running perpendicular to structural components (such as floor joists), modern wiring runs directly through holes in the components. Knob and tube wiring used protective ceramic tubes placed in the holes to prevent the wire from chafing against the structure.

Modern wiring uses staples to hold the wiring against structural components when the wire is running parallel to the component. Knob-and-tube wiring used ceramic knobs to clamp the wire to the structural member.

Connections between modern wires are completed within enclosed electrical junction boxes. Knob-and-tube wiring had visible connections. The wires were spliced and soldered together and then wrapped with electrical tape. These connections are called pig-tail connections because one wire is wrapped several times around the other wire before the two are soldered together. Ceramic knobs were strategically placed to protect the splice ensuring that inadvertent tugging on the wire would not stress the electrical connection.
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In modern wiring, the hot wire (black) and neutral wire (white), along with a ground wire, are insulated separately and bundled in a single plastic sheathing. In knob-and-tube wiring, the hot and neutral were insulated and run through a house separately, usually several inches apart. (3 inches is the minimum separation prescribed by the NEC). Knob-and-tube wiring did not include a ground wire.

In a modern system, many branch circuits use 14 gauge conductors protected by a 15amp circuit breaker. Larger, 12 gauge conductors are required for 20 amp circuits. Knob-and-tube wiring typically consists of 12 gauge conductors.

While the differences are considerable, there is nothing inherent in knob-and-tube wiring that makes it dangerous. Knob-and-tube wire, properly installed, is not inherently a problem. While opinions regarding the safety of knob-and-tube wiring vary widely, the concerns are not with the original wiring, but rather with what has happened after the fact.

Older homes with knob-and-tube wiring were often supplied with 60-amp service at the main electrical panel. They were also subject to limited distribution in two forms: (1) limited number of circuits, and (2) limited number of electrical outlets per room. Both of these factors opened knob-and-tube wiring to potential abuses of the electrical system after the initial installation.

Over the years, the demand for household electrical capacity has grown dramatically. Most knob-and-tube systems predate television, computers, and dozens of other appliances that are today taken for granted. As the need for electrical capacity grew, older wiring systems were modified for the convenience of the occupants. In some cases, these modifications put undue stress on the wiring system.

In response to the limited number of outlets per room, additional outlets were added on to the existing circuits. In many cases, the quality of the connections was not up to the standards of the original system. For instance, a portion of an existing wire conductor would be stripped of its insulation, and new wire taped on to service a new outlet. The connection may not have been soldered, and the new wire may have been of a lighter gauge. Stress protection for the new connection was rarely considered.

With additional outlets and increased electrical consumption, problems also arose with circuit protection. If circuits became overtaxed and 15 amp fuses were constantly blowing, some ill-informed homeowners would put in 25 or 30 amp fuses to rid themselves of the annoyance. Allowing excessive current to flow through the conductors could lead to overheating, which, in turn, could lead to degradation and embrittlement of the wire insulation and the wire itself. The problem of overfusing can be difficult to determine. A home that has been upgraded to 100-amp service, and is currently properly fused, may have experienced a decade of past overfusing on the knob-and-tube circuitry that is still in use.

Finally, the wiring could suffer from physical abuse over time. Rather than hugging structural components, knob-and-tube wiring was suspended (a minimum of one inch prescribed by the NEC) away from surrounding surfaces. Bumping the wiring could place stresses and cause resultant damage on a portion of the wire. This could be particularly true in accessible attics.

The conditions outlined above can be categorized as an abuse of a home s electrical system. These abuses (improperly added connections, overfusing and wire embrittlement, physical damage) can result in point sources of high resistance. It is at these points that fire potential is greatest  Ultimately, it is wiring that has been abused that is potentially dangerous.

 

 

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