What Is Knob and Tube Wiring? Should You Replace It?

featured image showing a knob and tube wiring system.

Knob and tube wiring (k & t wiring) is a type of electrical wiring common in older houses built before the 1950s. It is generally safe to use. However, its wiring performance is affected by age, wrong medications, and poor system maintenance. It also doesn’t have ground wiring and lacks the safety benefits offered by modern wiring.

What Is Knob and Tube Wiring?

A picture showing the parts of knob and tube wiring

K&T wiring consists of parallel hot and neutral copper wires suspended in holes in ceilings and walls. These wires run through porcelain insulated tubes in drill holes in wooden floor joists. It also consists of ceramic knobs and cylinders that act as insulators of the electrical system. So, if you see white porcelain knobs, your home has K&T wiring.

Knob and tube wiring was dominant in North America between the 18th and mid-20th centuries. It has an outstanding performance and meets the energy needs of most households. However, it has several drawbacks.

The wires connecting the lamps or switches are insulated using a rubber cloth fabric. Unfortunately, this rubber installation can break down and cause overheating of cables over time. That’s why you face challenges when you need home insurance if your home has K & T wiring. But, again, these issues are why most homeowners are shifting to modern wiring, which is safer. 

How Safe Is K & T Wiring?

Most homeowners wonder, is knob and tube wiring safe? The answer is that K&T is safe if the installation is correct and done by a professional. As a matter of fact, this obsolete wiring has excellent performance and provides a high current enough to power most homes. However, replacement is of utmost priority once you see signs of deterioration like cracks. More so, if you use three-prong appliances (K&T wires don’t have a ground wire) and if additional home insulation is added on top of the wiring.

What Are the Advantages of K + T Wiring?

Though K+T is an old wiring system, it still offers plenty of advantages, including:

Higher Capacity

Knob and tube wiring systems have a higher carrying capacity because their hot and neutral wires are separated from each other — they are approximately four to six meters apart. The advantage of a high capacity is that it allows the wiring system to remain stable and prevent recurring power losses. Thus, the wires can dissipate heat into the air, enabling the thermal limit of the porcelain conductors. In addition, this wiring system supplies adequate power to the connected appliances without problems if installed correctly.

Less Likely to Be Punctured

Nails can’t pierce the wires because the porcelain knobs keep them in place. Moreover, porcelain is a highly durable material. The entire system also typically remains at the center of the drilled wooden frames in ceilings and walls.

Dissipate Heat Faster

Wire and cable overheating can affect the system’s efficiency, cause damage to electrical appliances, and even start a fire. K&T wiring system addresses this concern by dissipating heat faster. Further, this heat doesn’t affect the hot and neutral wires running in the porcelain cylindrical tubes in the wooden joists.

Expert Tip: Fast heat dissipation is advantageous because it prevents the wires from melting or burning. This also enables the system to carry strong currents without affecting its performance.

What Are the Disadvantages of K & T Wiring?

If there are no visible signs of damage on your home’s K&T wiring, you should still consider replacing it. More so, if loose or expanded foam insulation covers the knob and tube wiring. The disadvantages of K + T wiring include:

Incorrect Modifications

Most homeowners prefer DIY modifications and repairs because it is relatively cheaper than hiring a professional.

For example, some will use masking tape instead of electrical tape. This can be a problem because masking tapes are flammable and are not rated to conduct electricity. Another mistake is installing fuses with a higher amperage which can overload the system.

Basically, any incorrect modifications of a K&T wiring system can put you and your family at risk.

Poor Wiring Insulation

The wiring in this system is coated with a rubberized fabric that insulates the wires. Unfortunately, this insulation can cause the wires to get hotter than required. Thus, it needs a lot of space to dissipate heat into the air to prevent overheating of wires.

If the wiring system lacks enough space, it could affect the electrical current flowing through the wires. As a result, there’s a chance that the cables could allow higher current flow, and fire can break out. Thus, it would be best to have frequent inspections to ensure the fabric insulation is in good shape for safety.

No Ground Wire

This wiring system lacks a ground wire, unlike the modern wiring system. A ground wire is vital because it allows the secure flow of electrical current. Since the wires in this system run separately, it’s incompatible with modern three-plug appliances. The danger of lack of ground wiring is that the electrical system can easily get damaged due to electrical voltage surges. Again, it exposes your home to fire hazards and electrical shocks to your family.

K & T Wiring FAQ

Does K & T Wiring Have to Be Replaced?

No, you don’t have to replace a knob and tube wiring. However, you should consider replacing this wiring system if you need to use appliances such as air conditioners and microwaves. Again, the wires of this system run parallel to each other, so it isn’t designed for modern home appliances.

When Should the K & T Be Replaced?

You should replace K & T wiring whenever there is a sign of wear and tear on its wires, tubes, and knobs. If you need to re-insulate your home, consider replacing the wiring since covering them with additional insulation can overheat the wires.

Further, you can consider replacing this system if you use modern appliances. This system lacks a three-prong knob and tube wiring outlet. However, ensure to hire a qualified technician when the need to replace the wiring system comes.

How Much Does It Cost to Rewire a House From Knob and Tube?

Knob and tube wiring replacement cost ranges between $3000 and $8000. However, the price is about $5000 and $9000 for a two-story building. Again, you need to add $2000 to rewire each additional story building in your home. The cost is affected by the number of wires, complexity of the system, copper price, and labor cost.

Do Insurance Companies Cover K&T Wiring?

Most insurance companies refuse to cover K&T wiring because it lacks a ground wire. Home inspectors also have difficulty assessing the wiring because of the system’s location, which is often hidden in the ceiling and walls. Other concerns faced by insurance companies include dangerous modifications and cracked ceramic knobs. However, you may get insurance coverage if your home has an electrical conductor which protects it from fire hazards. 

Have Your Knob and Tube Wiring Checked

Knob and tube wiring is an outdated system common in older homes but unsuitable for modern homes. It has no ground wire, and wires may wear and tear, and they are a fire hazard, especially when installed in attics, laundry rooms, and other areas surrounded by insulating materials. 

If you suspect you have a K & T system, the International Association of Certified Home Inspectors (InterNACHI) suggests calling a licensed electrician for a complete electrical inspection. And, if you do have this type of wiring, determine if the installation is correct.

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