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About the Knob and Tube Wiring in Your Older Home

  • September 2013
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Knob and tube wiring was the accepted standard for home wiring during the early days of electricity, beginning around 1880 and ending in the 1940s. Today, knob and tube electrical systems still exist in some homes built during that period, but these systems are obsolete and may, in many cases, pose a safety hazard. For this reason, having an electrical system inspection done in homes built during this era is essential to ensure that any fire or shock hazards that may exist are identified and remedied before the worst happens.

What Is Knob and Tube Wiring?

Knob and tube wiring was done quite differently than today’s home electrical systems. Copper wire, insulated with rubberized cloth, was used in knob and tube systems, and live and neutral wires were run separately rather than insulated in plastic and bundled together in one cable with the addition of a ground wire, as is done in modern home wiring. The two wires were kept isolated—generally four to six inches apart—by porcelain knobs that supported them along their length and insulated where they passed through wood joists or studs by porcelain tubes.

How Knob and Tube Wiring Can Become a Hazard


Knob and tube systems that have been installed and used properly and remain in excellent condition are not dangerous. However, given the fact that these wiring systems haven’t been installed in homes since the 1940s, knob and tube wiring present in homes today is definitely aging.

Stretching and sagging is a common problem, which can lead to unsafe contact between the wires. Insulation can deteriorate, crumbling away to expose the copper wiring. These systems lack a grounding conductor, which are standard in today’s wiring systems to reduce risk of electrocution and electrical fires. Additionally, when these systems were installed, most of today’s appliances didn’t exist, so they were designed for much lighter electrical loads than is typical today. Due to that increased demand, knob and tube systems are often overloaded, which cause wires to overheat and become brittle, creating hidden hazards inside the walls.

Improper modifications are a very common problem with knob and tube wiring as well, as amateur upgrades have often been made over the years to accommodate increasing electrical needs. One common issue found in these systems are poorly done, unsafe splices that were meant to expand the system. Fuses are another problem area, as many homeowners, in order to reduce the frequency of blown fuses, replaced properly sized fuses with ones with higher resistances. This does prevent fuses from blowing as often, but only by allowing circuits to be overloaded, which causes heat damage to the wiring.

Home insulation and knob and tube wiring can be a dangerous combination, since this form of wiring relies upon open space to dissipate heat. When insulation is placed around these wires, heat can’t escape as it should, which can cause wires to overheat or break, becoming a fire hazard. The National Electric Code (NEC) states that knob and tube wiring should not be in hollow spaces of walls, ceilings and attics that are insulated with loose, rolled or foam materials that envelope the conductors.

Home Insurance Issues


If your home has knob and tube wiring that’s still in use, you may find that home insurance companies are reluctant to sell you a policy. Some insurance companies refuse to insure homes with this type of wiring altogether, while others may insure you after your system has been inspected and declared safe by a qualified electrician.

The bottom line is that knob and tube wiring is very likely to pose a safety hazard in your home. Upgrading the system is an investment, but one that will pay off in greater safety and peace of mind, as well as a more functional and efficient electrical system. For more information on knob and tube systems, inspections and upgrades, please contact Meyer’s Company Inc. We’ve been serving the heating, cooling, plumbing and electrical needs of customers in and around Griffith, Munster, Highland, St. John, Schererville and Gary since 1951.