Published on

Building Science by Norman Aceron Garcia

Knob-and-tube wiring

by Norman Aceron Garcia

TNKKnob-and-tube (K&T) wiring was a standard electrical wiring method in North America from 1880 to the 1940s. Back in those days when electricity was perceived to be just a passing fad, the common household electrical appliances were basic clothes irons, coffee percolators, teakettles and toasters. Fast forward to 2016, we now use clothes dryers, washing machines, ranges, refrigerators, dishwashers, microwave ovens, LED televisions, computers, electrical water heaters, furnaces, baseboard heaters, and air conditioners.

K&T wiring is comprised of insulated copper conductors running through lumber framing drill-holes with protective porcelain insulating tubes. Nailed-down porcelain knobs support the wiring along its length. Rubber or flexible cloth insulation called “loom” protects wires where it enters a lamp or switch, or where pulled into a wall.

Existing K&T systems are usually questionable for alterations meant to match the higher amperage loads required by 21st century electric appliances. Most of these modifications were done by insufficiently trained Do-It-Yourselfers, whose workmanship made the wiring system susceptible to excessive amperage loads. To make matters worse, many homeowners installed fuses with resistances that were too high for the wiring to somehow adapt to the inadequate amperage of K&T wiring. The result of this kind of alteration is that the fuses would not blow and the conductor would be subjected to heat damage due to electrical overloading.

The hazards from this system are due to its age, substandard workmanship, and locations where wires are surrounded by building insulation. The homeowner or an electrician must carefully get rid of any insulation that is found covering K&T wires. K&T wiring system has no ground wire and thus cannot service any three-pronged modern appliances. For this reason, it is not permitted in any new construction anymore. Most insurance companies will refuse to insure houses that have knob-and-tube wiring except for houses where an electrical contractor has deemed the system safe.

If you find K&T wiring in your home, have the electrical system evaluated by a certified electrician immediately to verify if the system was installed and modified properly. Homeowners must carefully consider all their options before deciding to rewire their house, as it can cost thousands of dollars to job this job. However, unsafe wiring can cause fires, obscure estate transactions, and make insurance companies anxious.

K&T wiring must not be installed in equipment rooms, laundry rooms, washrooms, kitchens, or outdoors because the wiring must be grounded in these locations. Immediately replace brittle or cracked wiring. Do not operate an excessive number of appliances in the home to minimize the risk of fire. Potential home buyers must get a quote on replacing K&T wiring to upgrade it to an up-to-code electrical system, as they can use this price to negotiate a lower price on the house.

Norman Aceron Garcia is a Professional Engineer of Mr. Peg Property Inspections Inc. Please email for free technical consultation.

Have a comment on this article? Send us your feedback