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Knob-And-Tube Wiring

Knob-and-tube wiring was used in homes throughout North America from about the 1880s to the 1950s.  It consisted of ceramic knobs nailed to the wall studs and floor joists.  Knob-and-tube is two-stranded, one being the hot wire, and one being the neutral wire.  The knobs helped secure the wires in place to prevent them from coming in contact with combustible framework.  Ceramic tubes were inserted in to any holes that were drilled through the wood framing, to allow for feeding the wire through.  This aided in keeping the wires from coming in contact with the wood framing.  Ceramic bushings were used to protect the wires where they entered a wiring device enclosure.  Looms (woven insulating sleeves) were also slipped over the insulated wiring for extra protection when wiring passed over each other, or entered a wiring device enclosure.

What are some of the problems associated with knob-and-tube wiring? 

Knob-and-tube wire covering tended to deteriorate over time, get brittle, crack and even fall off, leaving the wire exposed.  The wires were connected and soldered together, and usually wrapped with electrical tape.  Over time, the electrical tape had a tendency to fall off as well.  A lot of the wiring is usually concealed in walls and ceilings, and even covered with the home’s insulation, and therefore cannot be fully evaluated.  
Poor connections and unsafe modifications were also a very common problem with the wiring.  The older systems used a 60 amp service, but the wire is only fused with 15 amps.  With more current flowing through the wire than it could handle, this lead to overheating and possibly even fire.  Additionally, not having a ground wire for protection also increased the risk of shock or fire.

Should knob-and tube wiring be replaced, if found in a home?

While a knob-and-tube wiring system in good condition doesn’t present much of a risk, not knowing its conditions in the hidden areas is always a concern.  Most homes today require a larger electrical capacity than a knob-and-tube system can provide.  Additionally, a lot of insurance companies refuse to insure a home with knob-and-tube wiring.  Having it replaced with an updated electrical system that can handle what most of today’s homes require is always recommended.  With an updated system, also comes peace of mind, knowing that your homes electrical system is protected in the best way possible.
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