There have been a lot of different types of electrical wiring used in houses over the years (well, centuries), and some of them are very different from modern house wiring types. If you come across knob and tube wiring and you aren’t familiar with it, you’ll probably have a ‘what is this?!’ moment.

What is Knob and Tube Wiring

Knob and tube electrical systems were used at the very dawn of home electrical installations to wiring houses from the late 1800s through to the 1940s. There are 2 parts to it (well, 3 if you count the actual wiring). Rather than being laid in a conduit, casing or piping, individual wires are suspended a few inches from the surface by looping them around a ceramic ‘knob’.

Where electrical wires needed to cross a panel, heavy ceramic cylinders that were installed after boring a hole through the panel. This prevented wires from getting pinched or touching the panels. Tubes were also used to where wires crossed each other.

How to Identify a Knob and Tube System

Knob and tube wiring is fairly easy to identify. Look in your attic or basement, and you should see the ceramic parts. However, just because you don’t see it right away doesn’t mean it’s not there. There’s a chance the previous homeowners might have covered it up. The only way you can know for sure is to drill tiny holes or use a miniature camera to look behind the panels.

Knob and tube wiring does have a few advantages which are listed below, but the disadvantages almost always outweigh them.

Advantages of Knob and Tube Wiring

  • Hot and neutral wires are usually separated by about 4 to 6 inches, so they can dissipate heat faster and handle slightly higher amperage
  • There is also a gap between panel surfaces and the wires, so they’re less likely to be damaged by nails
  • Installing knob and tube wiring requires a fair amount of skill and experience, so the quality of workmanship was usually well above what we have nowadays
  • The ceramic components don’t age, so the installations have a nearly unlimited lifespan (unless they break)

Disadvantages

  • Knob and tube systems use only 2 wires; one for hot or live and the other for neutral, so there’s an enormous safety risk because there’s no electrical grounding
  • Most installations will be at least half a decade old, if not older. While the ceramic parts will last, the rubberized cloth is used to wrap the wires. The sheathing can crack, becomes brittle or even fall off, leaving the wires exposed. This is a huge fire and shock hazard, which gets compounded in case of water exposure.
  • Power demands were much lower in the early to mid-’90s than they are today, and most systems back then just weren’t designed to handle the kind of electrical loads we consider average nowadays.
  • Many homeowners add insulation in wall cavities and attics. Knob and tube wiring systems are designed to remain separated from any surface and insulation will prevent the natural heat ventilation, increasing the risk of an electrical fire.
  • The wiring systems were simpler and quite often easily accessible, so there is a good chance that modifications have been made over the years and improper alterations are quite common.
  • The wires used in knob and tube tend to stretch and sag over time. Additionally, they are not as resistant to wear and damage as modern ones are and often use insulation materials that are not rated for moisture exposure or may cause the copper conductors to oxidize.
  • Perhaps the biggest drawback of having knob and tube wiring is that many insurance companies consider them a fire risk and refuse to insure houses with this system. There are exceptions, but they typically involve a thorough evaluation by an electrical contractor to deem the system as safe.

At D&F Liquidators have a large inventory of electrical supplies, safety switches, and circuit breakers. Contact us here or call us at 800-458-9600 for high-quality, reliable electrical materials from top brands at competitive rates.