Knob and tube wiring

Reviewed by Arthur Kavanagh

Updated February 6, 2024 | Published July 17, 2014

Buying an older, heritage-style home can be thrilling, but also incredibly challenging. If your house was built prior to 1950, you’ll need to consider the type of wiring the home has— for instance copper, aluminum, or knob and tube. Older homes that were never renovated could have knob and tube wiring. Knob and tube wiring has a reputation for being problematic, but is it really?

Just because the home has K&T wiring, it doesn’t mean the house is in imminent danger of burning down. If the wiring has been well maintained and hasn’t been used as a clothesline in the basement, it may be just fine.

In this article, we’ll explain what knob and tube wiring is, how to maintain it, and the home insurance implications of having it in your home.

Knob and tube wiring

What is knob and tube wiring?

Knob and tube wiring, usually abbreviated to K&T, is a form of electrical system that was in common use prior to about 1950.

K&T gets its name from the knobs and tubes used to run electical cables through a house. In a K&T system, the wires run through a series of porcelain cylinders (the tubes) in the floor joists. Porcelain knobs help keep the wiring secure (and form the second half of the name). K&T wires are typically insulated with a rubberized cloth fabric.

Example image showing knob and tube wiring in a basement

Source, under license CC BY-SA 3.0

One of the main differences between modern wiring and K&T is that there is no ground wire. Therefore, K&T wiring cannot accommodate any electrical items with three pronged plugs, and the risk of shocks and fires is much greater. Also, the black and the white wires run separately. In more modern wiring, the black wire, the white wire, and the ground wire are all enclosed in a single cable.

Another difference is the wire insulation. Modern wiring is insulated with plastic, while K&T uses rubber. The breakdown of the insulation over time on K&T wiring is often the reason it needs to be replaced. It’s important to note that this is frequently the result of overheating or mechanical abuse.

You may have found it difficult to obtain home insurance if your home has K&T wiring. In fact, K&T wiring is not inherently dangerous. Problems arise when the insulation around the wires begins to deteriorate with age, or when home handymen have made alterations to the wiring. K&T wiring should never run through insulation, especially blown-in insulation, as the wires need open space to keep cool. Any insulation surrounding the wiring can cause serious problems. In fact, K&T is sometimes referred to as “Open Wiring,” which helps to emphasize the importance of maintaining open airspace around the wire at all times to prevent any overheating.

There is nothing in the building code in Canada which states K&T wiring must be removed from existing homes. It is, however, obsolete, and can’t be used in any new construction.

What can go wrong with knob and tube wiring?

Any of the problems listed below can cause short circuits or overheating. To avoid these problems, you may need to replace your house’s wiring. If in doubt, have an electrical inspection done.

  • Insulation over the wiring: If household insulation is installed over K&T wiring, a fire is just waiting to break out. The wiring is coated with a rubber/cloth insulation. It needs lots of space to dissipate the heat that builds up when an electrical current is flowing through. If there’s no room because it’s covered with insulation, it creates a very dangerous situation.

  • Excess use: K&T wiring was installed when there were very few electrical appliances in the average home. Nowadays, with TVs, sound systems, computers, washers, and dryers, the system can easily overheat. Often, there is overuse of extension cords and power bars as well. Old systems aren’t designed to handle the modern electricity demands. The ground pin (or third prong) on power bars or other electrical items should never be removed to accommodate the two pin outlets used in K&T wiring.

  • Alterations: Most problems occur with K&T wiring as a result of improper alterations being made to the existing wiring. As it’s such an old system, proper replacement parts are not always available, which could be the reason a lot of makeshift handyman fixes are so dangerous. K&T wiring is easily to access from the basement, which is perhaps the reason why this wiring is often spliced unsafely with modern wiring by home handymen, as opposed to certified electricians.

  • Damage: Serious problems can occur when K&T wiring is damaged, either due to wear and tear, DIY fixes, or other types of damage. Porcelain knobs and tubes can crack, and the wires tend to sag and fray over time exposing live wires.

  • Brittle insulation: The rubberized cloth insulation on K&T wiring becomes brittle over time and can flake off.

Plugging in a lamp, or even a TV, in your living room or bedroom really doesn’t pose much of a risk when you have K&T wiring. However, in places where there’s a possibility of contact with water, for instance the bathroom or kitchen, an ungrounded system can be extremely dangerous.

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What can you do to maintain your knob and tube wiring?

If you currently have K&T wiring in your home, here is some important maintenance advice to be aware of:

  • Bring in a qualified electrician to evaluate the system.

  • Don’t run too many applicances simultaneously, as this increases the risk of a fire.

  • Replace all you outlets with ground fault circuit interrupter (GFCI) outlets.

  • Replace any wiring that appears brittle or cracked.

  • Replace any K&T wiring servicing kitchens, bathrooms, laundry rooms, or outdoors. Wiring for these areas must be grounded.

  • Carefully remove any insulation surrounding K&T wires.

What is the life expectancy of knob and tube wiring?

Knob and tube can last a long time, theoretically. The functional part of K&T is copper wire, which can last upwards of 100 years. But, one of the main problems with K&T wiring is the insulation. The standard K&T insulation tends to become brittle and can easily break off, leaving bare wires exposed. Experts recommend reaplcing the wiring as soon as the insulation becomes brittle.

The replacement cost of a home’s entire system with copper wiring really varies depending on the size of the home and the amount of K&T wiring in place. But it is not outrageously expensive, and can be done fairly quickly, and with very minimal damage. The end result will be a more efficient, safer home, and will also increase the resale value should the time come when you decide to sell.

What will your home insurance company want to know?

Your insurance company will always want to know what type of wiring your home has. They’ll want to know if the entire house is on K&T wiring, or if some of the home has been updated. They may require you to have an electrician inspect the wiring before they can offer you home insurance. A number of problems can occur when only part of the house has been rewired, and the rest of the home still has K&T. The connections between the two systems may expand and contract at different rates, causing connections to loosen.

Also, K&T wiring runs on a 60-amp service, but most insurance companies will require your home to have 100-amp service.

There are companies that will refuse to insure a home with K&T wiring, as they consider the risk to be too high. However, there are others that will offer coverage with stipulations, perhaps with a higher premium or higher deductibles. There may also be a requirement to have an electrical inspection done by a professional before coverage can be offered.

Buying an older home has its benefits as well as its challenges. Having to cope with K&T wiring is definitely one of the biggest challenges you’ll face. However, if the wiring in your home is well-maintained and in good shape, even though it has been around for a while, you shouldn’t have a problem obtaining home insurance coverage.

Commonly asked questions

How can you tell if a house has knob and tube wiring?

If you’re not sure whether or not your house uses K&T wiring, take some time to do a brief inspection to find out. The easiest method is to visually check a few key areas, such as exposed joints in the basement or attic. Look for any white ceramic knobs nailed to the joists with electrical wiring twisting through them. Look for ceramic tubes wherever a wire passes through lumber. These are the main telltale signs that your house was wired using the K&T method.

You may still have K&T wiring in your home even if you don’t find any of these indicators. In these cases, plan a complete diagnostic to be absolutely certain of what wiring your home uses. In the end, you’ll feel safe, secure and more knowledgeable of your home overall — and both you and the property will be better off for it.

Can knob and tube wiring be grounded?

If the electrical outlets in your home have three prongs, it’s safe to say that the wiring connected to it has been appropriately grounded. Knob and tube wiring does not feature a third plug-in located just below the other two, and thus does not adequately protect from electrical shock if a wire comes loose.

If you’re trying to figure out what type of wiring your property has overall, however, simply checking your outlets for two or three plugs will not be enough. Since other electrical systems, such as sheathed duplex electrical wiring, also use two-pronged outlets, you’ll need to be more in-depth with your research before coming to a final determination.

Unfortunately, if you’re hoping to ground an existing K&T system you will be sorely disappointed. In the interest of staying up to code — not to mention ensuring your home is properly and safely wired top-to-bottom — you’ll have to rewire the whole property.

What is the cost of replacing knob and tube wiring?

The exact price of rewiring your house can change dramatically depending on size, overall conditions, age of the property, access to wiring, and other factors that are impossible to predict without a firsthand look. So, it’s best to consult with a licensed electrician to get an accurate estimation of what you can expect by project’s end.

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About the expert: Arthur Kavanagh

Arthur has been a natural leader since the age of 18, having completed a 4-year electrician apprenticeship and overseen crews of up to 60 electricians. He is the founder of his own premiere electrical contracting company: Kato Electrical Inc.

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