Knob and Tube Wiring - Disadvantages

Disadvantages

Historically, wiring installation requirements were less demanding in the age of knob-and-tube wiring than today. Compared to modern electrical wiring standards, these are the main technical shortcomings of knob-and-tube wiring methods:

• never included a safety grounding conductor
• did not confine switching to the hot conductor (the so-called Carter system places electrical loads across the common terminals of a three-way switch pair)
• permitted the use of in-line splices in walls without a junction box (thus exposing the potential fire hazard of an uncontained spark caused by arcing following mechanical failure of the splice).

Over time, the price of electrician labor grew faster than the cost of materials. This removed the price advantage of K&T methods, especially since they required time-consuming skillful soldering of in-line splices and junctions, and careful hand-wrapping of connections in layers of insulating tape.

Knob-and-tube wiring can be made with high current carrying capacity. However, most existing residential knob and tube installations, dating to before 1940, have fewer branch circuits than is desired today. While these installations were adequate for the electrical loads at the time of installation, modern households use a range and intensity of electrical equipment unforeseen at the time. Household power use increased dramatically following World War II due to wide availability of electrical appliances.

Modern home buyers often find that existing K&T systems lack the capacity for today's levels of power use. First-generation wiring systems became susceptible to abuse by homeowners who would replace blown fuses with fuses rated for higher current. This overfusing of the circuits subjects wiring to higher levels of current and risks heat damage.

Knob-and-tube wiring may also be damaged by building renovations. Its cloth and rubber insulation can dry out and turn brittle. It may also be damaged by rodents and careless activities such as hanging objects from wiring running in accessible areas like basements.

For those concerned about stray magnetic fields, K&T wiring produces a much stronger effect at a given level of current, since the conductors are separated by a greater distance and their fields do not cancel as well as more closely spaced conductors. According to the theory of magnetic fields, two parallel conductors carrying equal currents in opposite directions form a balanced line, partially cancelling each other's magnetic field at a sufficiently large distance from the pair. As a rule of thumb, if two parallel conductors carrying opposite currents are then separated by 10 times the distance, the stray magnetic field will then extend 10 times further than before.

Currently, the United States National Electrical Code forbids the use of loose, blown-in, or expanding foam insulation over K&T wiring. This is because K&T is designed to let heat dissipate to the surrounding air. As a result, energy efficiency upgrades that involve insulating previously uninsulated walls usually also require replacement of the wiring in affected homes. However, California, Washington, Nebraska, and Oregon have modified the NEC to conditionally allow insulation around K&T. They did not find a single fire that was attributed to K&T, and permit insulation provided the home first passes inspection by an electrician.

As existing K&T wiring gets older, insurance companies may deny coverage due to a perception of increased risk. Several companies will not write new homeowners policies at all unless all K&T wiring is replaced, or an electrician certifies that the wiring is in good condition. Also, many institutional lenders are unwilling to finance a home with limited ampacity (current carrying capacity) service (which, as noted above, often goes hand-in-hand with K&T wiring), unless the electrical service is upgraded.