What the heck is "Knob and tube?"

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​Knob and tube, aka “open wiring” or “K&T”, was installed in most homes up to 1950.

This vintage wiring was comprised of single conductors supported on wood framing with ceramic knobs, insulated from contact by passing through ceramic tubes embedded in the framing. Wire connections were completed in open air, soldered and covered with insulating tape (looks like hockey tape.) The conductors were covered with rubberized, flame-retardant cloth. The conductor quality was good, consisting of heavy gauge copper wire with a limited number of soldered connections leading to receptacles and light fixtures. However with no grounding conductors, the receptacles (plug-ins) had no third prong and were not grounded; neither were the light fixtures.

Aging of these systems, many of which are now over 100 years old, along with inappropriate additions and modifications over time, has led to concerns about the safety of these vintage wiring systems.

• Ungrounded receptacles:  Original 2-prong ungrounded receptacles have often been exchanged for modern 3-prong receptacles, giving the false impression of ground protection.
• Poor connections:  To satisfy modern electrical requirements, new circuits are often found connected to the knob-and-tube, often installed by the homeowner or persons other than licensed electricians. These add-on circuits can be most dangerous, sometimes resulting in hot spots at the added connections.
• Insulation breakdown:  If there has been "over-fusing" (overrated fuses or breakers installed on the circuits) there can be insulation breakdown, as oversized fuses combined with overloading the circuits significantly raises the temperature of the conductors beyond their designed temperature limits, resulting in a fire hazard.
• Insurance coverage:  Due to concerns with modifications and age-related deterioration insurance can be difficult, often impossible to obtain.

 
INSPECTION PROCEDURE

If a home inspector or anyone else discovers knob and tube wiring in a home, a thorough inspection should be completed by a licensed electrician.

Receptacles are inspected to assure they are the correct type. The quality of the connections is determined by "load testing" (to determine if there are any poor connections between the electrical panel and the receptacles, or the light fixtures). The electrical panel is checked for any signs of over-fusing and the wiring insulation is checked. If any of the above are found to be deficient the knob-and-tube circuit is unacceptable, and repairs or replacement should be performed.

Although the Ontario Electrical Safety Code currently includes knob and tube wiring, provided it’s in good condition and protected by ground fault circuit interrupters (GFCIs) homeowner insurance may nonetheless be unavailable.