•  Electrical - Knob-and-Tube

Knob-and-tube wiring was used residentially until approximately 1950. While different than the wiring that is used now. This wire gets it's name from the ceramic knobs by which it is secured and the ceramic tubes which are used where the wires pass through wood-framing members, such a joists.

The main difference between this system and modern cable, is that the black wire and the white wire run separately, independent of each other. In modern cables, the black wire, white wire (and ground wire) are all wrapped up in a single cable. With knob-and-tube wiring, it took roughly twice as long to wire.

No Junction Boxes is Another difference between knob-and-tube wiring and modern cable is that with knob-and-tube wiring, electrical junction boxes were not used wherever wires were joined together. Instead, any time two wires were connected they were twisted together, soldered and wrapped in rubber, then in electrical tape. While no longer a common practice, if properly done, these connections would serve indefinitely.

Another distinction between knob and tube wiring and some modern cables is in the insulation. The knob-and-tube wiring used rubber and cloth insulation around the wiring. In modern cables, each wire has plastic insulation typically, and the entire cable is wrapped with another layer of plastic. Over the years, these sheathing materials have included cloth, paper, rubber, metal and plastic.

Brittle Insulation Breakdown of the insulation on knob-an-tube wiring is most often the reason it has to be replaced. This is frequently the result of overheating or mechanical abuse.

No Ground Wire One last difference between knob-and-tube wiring and some modern cable is the absence of a ground wire. As mentioned earlier, knob-and-tube wiring was used up until 1950. From the 1950’s to 1960, two conductor cable was popular although no ground wire was included. In approximately 1960, ground wires began to be incorporated into the two conductor cable, and electrical receptacles included a third hole (U-ground) thereafter.

Replacement?

While knob-and-tube wiring must be recognized as old, it is not necessary to replace it as matter of course. It should be inspected and evaluated on an individual basis.

Knob-and-Tube Wire Problems

Poor Connections Problems with knob-and-tube wire almost always result from amateurish connections made subsequent to the original installation. Since original connections were made without junction boxes, many homeowners have felt that they could make connections to knob-and-tube wiring without junction boxes, as well. This is an unsafe practice, particularly since the chance of making a splice as good as the original connection is very remote. In any case, this practice violates modern electrical codes. Since knob-and-tube wiring is invariably old, it has been subjected to more home handymen, mechanical abuse (such as skis, lumber, etc. stored on top of the wire in the basement), and is more likely to have suffered wear and tear. Pinched wiring and damaged insulation is a problem, particularly in unfinished basements, where the wiring is exposed.

Brittle Insulation is Another problem with knob-and-tube wiring. If the wire has overheated in the past as a result of overfusing, a poor connection or damaged cable, the insulation may become brittle. Flexing the knob-and-tube wire will give some indication as to whether it has become brittle, although this can be a dangerous practice. It is not recommended that this be done by a layman person. Often the wire becomes brittle in areas where heat builds up, such as in panelboards. In exposed areas, where inspection is easier , There is usually good air circulation, and little heat build-up. The wires are least likely to be brittle in these areas.

Circuits Extended

Since older electrical systems had very few circuits by today’s standards, The odds of a knob-and-tube circuits having been extended over the years are MOST LIKELY. The additional loads and the possibilities of poor connections do create an argument for replacing older knob-and-tube wiring.

Replacement?

While it is typically not necessary to pull out all old knob-and-tube wiring and replace it, it is common to remove the sections exposed when renovations are undertaken. For example, when basements are finished, the readily accessible knob-and-tube wiring at the basement ceiling level could be replaced prior to adding a ceiling finish. The wiring running up through the walls to the first and second floors, however, is usually not changed. This is considered acceptable, as long as the wire is in good repair.

Since most difficulties with wiring occur at points of connection, and since all connection points should be accessible, the wiring can be inspected on a regular basis by a qualified electrician. In practice, unfortunately, this is rarely done. Incidentally, the more connections there are, the more potential trouble spots there are.

Aluminum wiring

Aluminum wiring was commonly used from the late 1960’s until about 1978. It was introduced because it was less expensive than copper. It was recognized from the beginning that aluminum wiring is not quite as good a conductor of electricity as its copper. Therefore, for a conventional 15-amp household circuit, for example, #12-gage aluminum was used in place of #14-gage copper. Other wire sizes were also suitably increased. Thermal Expansion Some other properties of aluminum, however, were not recognized and did cause some difficulties. Firstly, aluminum has a higher co-efficient of thermal expansion than copper. This means that when the wire heats up (as all wire does when electricity passes through) the aluminum tends to expand more than copper.

This leads to the wire trying to move out from under the terminal screw. This phenomenon is called “creep” and can lead to poor connections and subsequent overheating.

Aluminum is softer than copper, and electricians used to working with copper would often nick aluminum wiring inadvertently. Nicking the wire, of course, reduces its diameter, and its ability to carry electricity safely. Localized hot spots can develop where the wire has been nicked. Further, if the wire is bent after it has been nicked, it will often break. Insulating Lastly, the oxide of aluminum that forms on the wire is a very poor electrical conductor. All metals rust or oxidized to some degree. The greenish copper oxide that forms on copper wiring does not result in a problem. The oxide that forms as aluminum corrodes, can lead to higher resistance and higher temperatures. Cu-Al CO/ALR and COPALUM As a result of these difficulties, special components, designated Cu-Al, were produced. These included wire connectors (wire nuts), electrical receptacles, circuit breakers, stove blocks, etc. In most cases, these improvements were found to be satisfactory. However, electrical receptacles continued to be a problem. The subsequently designed receptacles, and those that are now required, were designated CO/ALR.

These are Acceptable As long as proper connectors are used, and the connections are made without damaging the wire, aluminum wiring is considered safe. It is permitted for use by many electrical codes, although it is not commonly used in homes due to the adverse publicity it received during its early problem years. It is still used commonly by utilities in street wiring and for service entrance cables.

Service Size

As the power enters the house, it goes into a service box which has two fuses or two circuit breakers (sometimes connected together to look like one big breaker). One fuse is for the black wire and one fuse is for the red. No fuse is necessary (or permitted) for the neutral wire. The fuses are rated at the amperage that the wire can safely carry (60-amps, 100-amps, etc.). Where there are two 100-amp fuses in the service box, the house has a 100-amp service. Please note that one cannot add the two fuse ratings together to get the house service.

In the U.S., the minimum service size on new work is 100-amps. In Canada, 60-amp services are permitted for houses or residential units up to 800 square feet in floor area. On larger homes, 100-amp service is the minimum.

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