Knob and Tube Wiring was one of the first electrical wiring forms to provide power in the US’s homes and buildings. Knob and tube wiring began to be phased out during the 1940s, so if your home or building was built before 1950, you might still run into this type of wiring.
What exactly is knob and tube wiring?
Is it dangerous?
Can you modify it?
Knob and Tube (K&T) wiring consists of copper wiring supported by porcelain knobs. The remaining K&T systems are usually in poor condition and considered to be dangerous.
Read on to learn about how K&T wiring was originally designed and used, what has happened to these systems over time, and what you should consider if your home or building has K&T wiring.
What is Knob and Tube Wiring?
Knob and Tube (K&T) wiring was an early form of building electrical wiring that consists of copper wiring supported by porcelain knobs. The wiring was encased in rubber tubes and then tied to the porcelain knobs, which were nailed to the building studs and joists.
The wiring had a single hot and a single neutral wire but no ground wire, which is required in modern wiring systems. The hot wire and neutral wire were installed in separate tubes. When the cables needed to pass through a finished wall, electrical device, or stud inside the wall cavity, the holes were protected with porcelain tubes to prevent fires. When the wires made connections with outlets, appliances, or light fixtures, the wires were protected with a “loom,” a flexible cloth or rubber insulation.
As initially designed and installed, K&T wiring was reasonably safe. One primary concern with the K&T system is the lack of a ground wire, which is an important safety feature in modern wiring systems. Ground wires started to be required by the National Electrical Code (NEC) in 1962.
While K&T wiring was mostly phased out during the 1940s, some installations continued throughout the 1950s through the 1960s. An easy way to tell if your system might be K&T is if you have two-prong electrical outlets or three-prong outlets. The third prong in the outlets is the ground prong.
K&T systems have their limitations. The electrical service of these systems is generally smaller than modern systems. Older homes had much lower electrical needs than contemporary homes. For K&T systems, the incoming electrical service is around 60A, while modern homes have a minimum of about 100 A. Many homes have a 200 A, 400 A service or more. The small size of the wires also limits how much current the wire can draw. Most circuits with K&T wiring are limited to 15 A, where modern circuits are around 20 A. Most main panel boards for K&T systems could support up to twelve circuits unless adequately upgraded.
Safety Concerns with K&T Wiring
Even if K&T wiring systems were safe when they were initially installed, over time, the system components could degrade. Modifications could make the system very unsafe if the modifications were done incorrectly. If they haven’t already been replaced, most K&T systems are over eighty years old. Even quality systems would become riskier over time.
Problems with knob and tube wiring include:
- Increased fire risk
- Lack of ground wiring
- Inability to three-pronged service appliances
- Deterioration over time
- Improper modification by previous homeowners
- House may not be insurable
The most considerable concern with K&T wiring is the lack of a ground wire. If a wire were to come loose and touch a metal enclosure, like a mixer or toaster, that metal enclosure would become charged with an electrical current. If you touched the metal parts, you would be shocked. Instead of modern circuit breakers, the K&T systems had fuses, where a filament inside the fuse would burn and break the circuit if there were too much current draw. If the fuse didn’t break the circuit, the wires could be overloaded and catch on fire.
Systems with ground wires typically have circuit breakers. With a ground wire, the earth would draw infinite current through the hot wire. When the current exceeds the rating of the circuit breaker, the breaker will trip and cut power to the circuit.
Electrical wires dissipate heat as current flows through them. K&T wires were designed to dissipate this heat to the air. If the wire touches lumber or insulation inside the wall cavity, there could be a risk that the wire will start a fire. The current National Electrical Code requires that where an existing K&T system is installed, no insulation shall cover the wiring.
The rubber insulation on the wiring is another big concern. The insulation degrades over time, cracking, falling off, and exposing the bare copper conductor underneath. If someone were to brush up against the bare conductor, they would receive a nasty shock. The bare conductors also pose more of a risk for starting fires if they touch combustible or flammable material like insulation or lumber. The wires will sag over time, which could cause them to touch a flammable material. The wire must be supported at regular intervals to prevent the wires from sagging or stretching too much.
Some of the older K&T systems may have even been insulated with varnish, cambric, or asbestos, each of which has its concerns. The varnish is flammable and degrades over time. Cambric is a type of cotton cloth, which, when varnished, provided minor protection around the wire. Asbestos is a good flame retardant but poses dangerous health risks. Cambric and asbestos are not rated for moisture. Some of these materials can even oxidize the copper wire, degrading its ability to carry current.
Unsafe modifications also contribute to the dangers of K&T systems. Many original installations had limitations to the number of circuits, total power draw, or the number of connected fixtures or appliances. As increased electrical needs or renovations drove building or homeowners to modify their systems, many unqualified DIYers or electricians may have rerouted circuits, spliced wires unsafely, or replaced old fuses with ones with higher current ratings.
Rerouted or spliced circuits can be easily overloaded, which can result in a fire. Replacing the old fuses with higher current ratings can also overload the wiring, resulting in a fire. Even if a fire did not start the first time the wiring was overloaded, the wiring could become damaged over time, making it more susceptible to fires later.
Even if three-prong outlets have been installed, there is a chance that the older wiring is still powering parts of the building, or these three-prong outlets may not be grounded.
Do You Have K&T Wiring?
Since the dangers of K&T wiring can be concerning, many home and building owners may worry that their buildings or lives are at risk. Owners should note, though, as long as the K&T system was properly installed and modified by a qualified electrician and the system was properly maintained over time, the system would continue to operate safely. However, age alone can affect even the best installation or quality of materials.
If you aren’t sure if you have K&T wiring, you may try to find exposed knob and tube wiring in an attic or basement. Often, the wiring would be routed under the floor to run up inside the walls or across the attic to drop down into the walls to power outlets or appliances. If you find ceramic knobs with wiring wrapped or tied around them, or if you find tiny wires in old tubing, you may have K&T wiring. Consider hiring an inspector or electrical to conduct a full assessment of your system to ensure it is safe.
If you are considering buying an older property, you should make sure that the building is safe before making the purchase. Many insurance companies will refuse to insure a property that has K&T wiring installed due to all the safety concerns. Before finalizing a purchase on a property, the buyer should make sure that they can get:
- Knob and tube insurance for the property if there is K&T wiring installed
- An inspection to make sure it was installed correctly
- Consider getting a quote for replacing the system
Is Knob and Tube Wiring Legal?
Many buyers have purchased older buildings with illegal electrical installations. Sometimes, even inspectors can miss hidden K&T wiring, since most of the wiring will be hidden inside walls. Without cutting holes in the walls, you may not be able to verify if K&T wiring is installed.
No building codes state that old K&T wiring must be replaced. However, many cities and jurisdictions will require any modifications or renovations to be brought up to current code. For example, most jurisdictions will require an electrical permit if you make any modifications to the existing system.
Understanding Knob and Tube Wiring Replacement
So, if you move a wall and add a new outlet, you may need to add a grounded receptacle, which cannot be done with a K&T wiring system. The “authority having jurisdiction” to make exceptions may be able to approve extending or modifying an existing K&T system if done properly. However, this choice would be at their discretion.
Doing a significant interior remodel would be a great opportunity to upgrade your wiring, especially if you are replacing, recladding, or moving interior walls. If you decide to upgrade your electrical service, you should also consider replacing all your wiring as well to make sure all your circuits are properly grounded. Ungrounded K&T wiring may not allow modern circuit breakers to function properly. A new electrical service, panelboard, and wiring would allow you to safely install more modern appliances, lighting, and electrical outlets.
You should also consider replacing any K&T wiring located in bathrooms, kitchens, laundry rooms, janitor’s closets, or anywhere else that may have water or appliances. Most modern appliances need to be grounded to work properly. Most appliances have a three-prong electrical cord with a ground. While there are adaptors that allow three-prong cords to plug into two-prong outlets, these adaptors can be dangerous.
Another option to improve the safety of a K&T system includes replacing two-prong outlets with GFCI outlets. GFCI stands for ground fault circuit interruption. These types of receptacles detect the imbalance of current between the hot and neutral wire.
There is an induction coil around the hot and neutral wires. If the induction coil ever detects a current in one wire that is not in the other wire, that means that the current has gone to the ground, and it will pop a circuit breaker inside the outlet, cutting the power to the circuit. However, without a grounding wire in the electrical system, the GFCI will only detect if the current found another path to ground, such as your body, or if the current flowing through a metal pipe or water.
The Cost of Knob and Tube Wiring Replacement & Insurance
Knob and tube wiring replacement can be expensive. The walls of the building may need to be opened up, removing the sheetrock or drywall from the finished surface to reveal the wiring hidden within. Many homes or building owners that knew the risks of the system may have tried to conceal the old wiring from the view of inspectors or potential buyers.
Replacing the wiring in your home may cost between $4,000 to $15,000 if your home is between 1,500 and 3,000 square feet.
Many homes and building owners may find it difficult to find an insurer willing to insure a building with K&T wiring. If they do, they will likely pay an increased premium for even basic coverage. Therefore, the savings on the insurance premium may be beneficial throughout the life of the home and increase the resale value of the property in future sales.
The Difference Between K&T Wiring and Modern Electrical Systems
There are several differences between K&T wiring systems and modern electrical systems.
In modern buildings and homes, most electrical wiring is called Romex or NM Cable, which stands for a nonmetallic cable. Romex cable is typically installed indoors in residential buildings. There are three wires, a hot, neutral, and ground, routed in non-metallic sheathings such as plastic or other non-conducting, flame, and moisture resistant materials. NM Cable, or NMC, stands for a non-metallic cable. These types of wires may include multiple hot wires.
The main electrical panelboards for K&T systems were typically small, 60A panels with around twelve circuits. Most residential panelboards are at least 100 A and can have forty-two or more circuit breakers, though many large appliances may require the load from several circuit breakers. In K&T systems, each circuit had a 15 A fuse with a filament that would burn and break if more than 15 A flowed through the circuit. With a modern board, technically, you could have a single circuit breaker at the maximum amperage of your service (though uncommon).
If a fuse serves its purposes of interrupting a circuit, then the entire fuse must be replaced. Replacement parts may be hard to find for older systems. When circuit breakers trip, the circuit breaker can be reset. While over time, the circuit breaker may wear out and trip more often, this usually occurs with very frequent use.
Knob and Tube Wiring Code
The 2014 National Electrical Code provides requirements for the installation of knob and tube wiring. Specifically, Section 394.10 states that K&T wiring:
“… shall be permitted to be installed in the hollow spaces of walls and ceiling, or in unfinished attics and roof spaces as provided by 394.23, only as follows:
(1) For extensions of existing installations
(2) Elsewhere by special permission.”
It is not allowed to be installed in commercial garages, theaters, or similar locations, motion pictures studios, hazard (classified) locations, or hollow spaces of walls, ceilings, and attics that are insulated. In modern buildings, this typically would exclude exterior walls, attics, and basements, as these spaces would typically be insulated.
The code also states that where wires pass through framing members, the holes in the framing members need to be protected with a non-combustible tube. The conductors also need to be no less than three inches apart and more than one inch from any surface. The knobs typically help ensure the gaps are maintained. There also shouldn’t be any more than 4 ½’ between the knobs. If the wiring cannot be supported adequately, then the wires should be routed in flexible nonmetallic tubing.
All splices need to be completely soldered. No inline or strain splices are allowed. If you see bare wires wrapped around each other, or twisted together, or with tape wrapped around the wires, the wires may not be spliced correctly.
If you happen to be able to visually inspect your knob and tube wiring system, check to make sure the system is installed compliant with the current code recommendations. If you notice any deviations from the requirements in the code, your system may not be safely installed.
Now You Know All about Knob and Tube Wiring
Older houses are some of the most beautiful dwellings round and often are full of history. But, many homeowners are concerned about the safety of their old K&T wiring, and for a good reason. We strongly recommend hiring an electrical contractor to replace your old, potentially unsafe wiring with today’s modern systems.
Do you have experience with knob and tube wiring replacement? We’d love to hear your thoughts on it below!