Knob and tube wiring can have important implications when buying or selling old homes. This style of wiring was popular from 1880s to 1950s. After World War II, advances in technology required newer systems. Today, knob and tube wiring is inconvenient at best and dangerous at worst. Although outdated, you can still find it in homes today. Luckily, knob and tube wiring can be identified during a home inspection. Once identified, a next step is likely replacing it with modern plastic-sheathed wiring system.

What is Knob and Tube Wiring?

Knob and tube wiring is characterized by insulated copper wire running through either ceramic tubes or knobs. Ceramic tubes pass the wire through walls, studs, or beams. Porcelain knobs suspend wires away from walls and supports. If you look at the top of a telephone pole, the glass insulators between the wire and the pole work the same way.

Before World War II, knob and tube wiring was a great option. Other methods available at the time, such as conduit or armored cable wiring, cost three times as much to install. Knob and tube could supply more than adequate power to period appliances, and it was safe if installed properly. Also ceramic knobs and tubes had an almost indefinite service life if not damaged. Given the benefits, knob and tube wiring was popular across North America for over forty years!

Post World War II, knob and tube wiring became less popular. Knob and tube wiring wasn't regulated like modern wiring systems. This meant that each wiring scheme was one of a kind, and took longer to install. The chaotic nature of installation made repairs time consuming and dangerous. As labor costs increased, it became cheaper to install standardized wiring.

Knob and Tube Wiring Example

Households after 1950 had more appliances, and each appliance drew more power. Knob and tube wiring is generally connected to 60 amp service boxes. Modern service boxes output 100 amps. This means that knob and tube wiring supplies less power. As a result, lesser number of power outlets are available per room. Also, each outlet supplies less power than a modern one. Even worse, knob and tube wiring doesn't have a ground wire, so all the outlets are two prong, making it likely impossible to charge a laptop or other high powered appliances.

The Issues with Knob and Tube Wiring

After initial installation, knob and tube wiring is safe. As years pass and the wiring gets older, the wiring becomes far less safe. Fuse boxes on these old wiring systems tend to be less robust than modern breaker boxes. Knob and tube outlets have two connections, and the lack of a ground wire can pose an electrocution risk.

Knob and Tube Wiring

Mice, squirrels, and other pests may chew on wires. Knob and tube wiring has thin paper or rubber insulation, and as such it is more susceptible to chewing, and thereby creating risks.

Non-sheathed wires need room to dissipate heat inside walls. That means no foam insulation to guard against heat loss. Insulation near knob and tube wiring traps heat and can lead to a fire. To meet code, walls with knob and tube wiring must be empty. According to the Energy Services Group, it costs between $1,200 and $3,000 to heat a home for one year. Heating a house with knob and tube wiring will likely cost more.

Knob and Tube Wiring

Knob and tube wiring does not have a ground wire. Look at any modern outlet and you will see three connections. The slits at the top of an outlet supply power. At the bottom of the outlet a semi round hole connects to a dead wire leading to ground. The grounded wire acts like a lightning rod removing any electricity build up.

Build up of electricity can be dangerous according to . Think static shock, but capable of causing serious injury!

Homeowners modify fuse boxes to support modern power needs. Fuse boxes are safe when used within specified limits, but most old boxes are over drawn. Energy Today states a common form of tampering is replacing fuses. For example, replacing a ten amp fuse with a fifteen amp fuse. This replacement might seem trivial, but knob and tube wiring can get hot under normal operating loads. Adding more power can heat exposed wires to dangerous temperatures. Even worse, some homeowners replace fuses with metal objects such as bolts or coins. These modifications bypass the fuse box altogether! An unlimited supply of power is disastrous when paired with knob and tube wiring.

Insurance/Liability and Wiring

Insuring a home with knob and tube wiring can be expensive. There may be liability associated with older wiring systems, including knob and tube. Insurance companies may ask what wiring a house has, especially older houses. Once insurance companies know a house has knob and tube wiring, they may increase rates or deny coverage.

According to the Electric Safety Foundation International, electrical fires cause over $1.3 billion in property damage every year. In the US alone arcing faults cause $700 million in damages each year. According to The National Fire Protection Agency Home Electrical Fire Report, half of all electrical fires start with home wiring systems. Wiring causes 55 percent of deaths related to electrical fires and 53 percent of injuries. It's no wonder that insurance may not cover knob and tube wiring. The only option may be the replacement of knob and tube with a modern wiring system.


It's not cheap. According to Networx, the average cost of replacing knob and tube wiring in a two story home could be in the range of $7,000. It's important to hire a certified electrician and follow building code. Replacement of knob and tube wiring is costly, but it's a good investment.

Knob and Tube Wiring Replacement

The replacement overhauls the entire electrical system. An electrician starts by removing the old knob and tube wiring. The junction box gets replaced. A breaker box supplying at least 100 amps replaces the old fuse box. Outlets and fixtures are also replaced. Finally new insulated wire is laid and the electrical work is complete.

Access to the old wiring may require the removal of walls, ceilings and floors. If any other renovations need to be performed this can be a great time to do so. Unless you repair the damage yourself (and you shouldn't), a licensed professional has to do it. Putting everything back together can take as long as the replacement itself, adding more cost.

The replacement process is labor intensive and therefore expensive. Cutting corners leads to bigger problems in the future. Knob and tube wiring removal by certified laborers guarantees insurance coverage. When it comes to wiring, doing it the right way can save money in the long run.

Find out what's in your home

A home with knob and tube wiring can be tricky. If you are curious about your home, or a home you plan to buy, a home inspection can help you rest easy. Get in touch with a WIN Home Inspection expert near you by visiting, and one of our fully certified and fully insured home inspector may be able to assist you.