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SME Inc of Seattle offers our customers a broad and deep range of electrical services across commercial and industrial industries and residential properties. From tenant improvement, new build or renovations to assembly lines, healthcare facilities or multi- or single-family residences, the team at SME Inc provides experienced engineers and licensed electricians who have the skill and expertise to design, install and complete any project.

Knob and Tube Wiring

Knob and Tube Wiring

Knob and tube wiring is a system that was the best technology had to offer in its day, however, those days have long since gone. Knob and tube wiring is the original wiring method used from the late 1800's until approximately 1947. Knob and tube electrical wiring involved stringing individual insulated electrical wires across porcelain insulators ("knobs") which were nailed to the surface of wooden framing members. When electrical wires needed to actually pass thru wooden framing members, the electrical wires were strung through porcelain insulators called "tubes" which protected the wires.

The first insulation was asphalt-saturated cotton cloth called "loom". Where conductors (electrical wires) entered an electrical wiring device such as a light fixture, receptacle, outlet, switch, or other junction box, they were protected by flexible cloth insulating material ("loom"). Rubber soon followed as an insulating material and became quite common. Some of the concerns and safety hazards associated with knob and tube wiring are listed below:

  • It is very old wiring (at least 62 years old, at best), and in many cases has deteriorated, cracked or is missing insulation, which can lead to fires.
  • Heat directly above ceiling lights and in un-vented attics can degrade the wire insulation. Some types of insulation used on knob and tube wiring seem to be a delicacy for the critters that find their way into old homes. Rats, mice, and other creatures like this can make short work of the insulation covering the wires.
  • It is an ungrounded system, which provides a greater chance of shock or electrocution and of damage to sensitive equipment. Additionally, the whole house electrical wiring systems were very often undersized, which just doesn't hold up to today's electrical loads.
  • It is common to find that newer wiring has been improperly connected to knob and tube wiring by amateurs, resulting in dangerous conditions in the form of shock hazard, reverse polarity problems, switched neutrals, overloaded circuits, and fire hazards, to name just a few.
  • The electrical service panels supplying the knob and tube electrical wiring were often undersized as well, and in addition, they almost never had any type of main electrical disconnect. The National Fire Protection Association requires that all electrical service panels have a main disconnect switch or some other disconnecting means as a safety and fire prevention feature.

In many cases the contacts inside the original switches and outlets are loose, making for a poor electrical connection and increasing the risk of an electrical fire.

Like other components of your home, such as your roof, furnace, air conditioner and plumbing, there comes a time when replacement is necessary. That time has come for knob and tube wiring. Replacing and updating your knob and tube wiring is just one of the many residential electrical services we can provide. With our years of residential electrical installation experience and expertise, we can assist you in bringing your home up to today's NEC (National Electrical Code) requirements and to the standards of the National Fire Protection Association.