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What is knob and tube wiring?

Knob and tube, or open wiring, was installed from the initial electrical installations in the early 1900’s into the 1940’s. It is named for the porcelain fittings that were used to support and run cables through wood structure. If it is left alone and no changes have been made to the original installation, and the insulation is still in good, flexible condition, it may still be a safe installation. However, there are a number of issues you should be aware of. If you have knob and tube wiring in your home, you should have it inspected by a licensed electrician.

  • The insulation used in knob and tube wiring has a tendency to dry out and become stiff and brittle. This can result in insulation actually cracking and falling off the conductor, leaving it exposed and creating both a shock and fire hazard.
  • Since a knob and tube wiring installation has most likely been in place for over 70 years, the chances of the wiring in being untouched and not damaged or revised in some way is pretty small. Once these systems are tampered with in anyway, intentionally or otherwise, that section of wiring should be replace with new up to date wiring.
  • Knob and tube wiring was installed at a time when there was very little demand inside homes for electrical devices, and the circuiting and wiring was designed to that standard. Today we are using many more electrical and electronic devices in our homes than ever before, and the way homes were wired in the early 1900’s does not support this many devices being present. This causes circuits that are 70 years old and probably weak to begin with to now be overloaded, causing fire and shock hazards in some circumstances.
  • Knob and tube wiring was install as insulated individual conductors with a “hot” and a “neutral” line. In that day there was no grounding run alongside them. In many homes people have changed receptacles to match changing times over the years, and may have changed from two prong ungrounded receptacles to two prong grounded receptacles. If you have three prong receptacles installed on an ungrounded circuit you have created a situation where devices that require the ground for safety may be plugged into power that does not provide that safety, creating a potential shock hazard.