How to Install Knob and Tube Wiring

What is Knob and Tube Wiring?

Knob and tube wiring refers to an early method of electrical wiring that was common in buildings constructed in North America from about 1880 to the 1940s. It consists of insulated copper conductors run through porcelain knobs and tubes, which provide protection and structural support.

Some key characteristics of knob and tube wiring:

• Wires are separate rather than bundled together like modern wiring systems. This prevents overheating.
• Wires are surrounded by porcelain knobs or tubes for insulation and protection.
• There are two types of wires - hot wires that carry current and neutral wires that complete the circuit back to the panel.
• Wires are spaced apart from each other and do not touch, reducing fire risk.
• Airspace around the wires provides additional insulation.

While knob and tube wiring was safe when installed to code, it can become hazardous as it ages. It lacks grounding and is unsuitable for higher modern loads.

Planning a Knob and Tube Wiring Installation

Installing new knob and tube wiring is rarely done today due to safer, more efficient modern wiring methods. However, here are some key steps if undertaking this project:

• Consult local building codes and ensure knob and tube wiring is still permitted in your jurisdiction. Most places now prohibit new installations.
• Hire a qualified, licensed electrician with experience in knob and tube installations. This is specialized work.
• Have the electrician inspect the premises and plan the wiring routes. Consider factors like wire length and placement of devices.
• Choose locations for the porcelain knobs and tubes to support the wires. Space them appropriately for the wire gauge.
• Select the proper gauge wires for the electric loads. Thicker wires handle more current.
• Buy authentic porcelain knobs and tubes - avoid plastic replicas. Also purchase junction boxes.
• The electrician should obtain necessary permits before starting work.

Installing Knob and Tube Wiring

Once planning is complete, the installation process generally involves these key steps:

Mounting the Knobs and Tubes

• Mark locations for knob and tube mounting points on ceiling joists, studs and walls. Space them appropriately for the wire gauges used.
• Drill small pilot holes at the mounting points. Take care not to drill all the way through studs or joists.
• Hammer porcelain knobs into the holes. Nail tubes in place along their planned routes.

Running and Securing the Wires

• Run hot and neutral wires through their intended paths, threading them through the knobs and tubes. Maintain proper separation.
• Use tie wires to securely fasten the wires to the knobs and tubes at regular intervals. This prevents sagging.
• When two wires cross, they must pass through a junction box to avoid contact.
• Leave some slack in the wires to allow for expansion and contraction.

Splicing and Terminating the Wires

• Use junction boxes to splice wires together. Join like wires using properly soldered wire nuts or crimp connectors.
• Attach wiring to devices like switches, outlets and light fixtures using the terminal screws.
• Ensure all connections are secure and properly insulated. No bare wire should be exposed.

Grounding and Bonding

• Since knob and tube has no grounding, install GFCI outlets for protection. Or run a separate ground from outlets back to the panel.
• Bond all metal parts like boxes and fixtures to ensure they cannot become energized.

Once the installation is complete, the work should be carefully inspected before turning on the power. The wiring should then be tested for proper function and safety.

Is Knob and Tube Wiring Still Safe to Use?

Knob and tube wiring that was properly installed and well-maintained can still be safe to use in some cases. However, there are good reasons to consider upgrading:

• No grounding - This wiring lacks grounding which protects against shocks and damage. GFCI outlets can help provide protection.
• Insulation breakdown - The insulation on old knob and tube wires becomes brittle and fragile over time. This can lead to shorts and fires.
• Overloading - Heavy modern electrical loads can overheat these older wires, becoming a fire hazard.
• Difficulty with insurance and financing - Many providers won't insure or finance properties with knob and tube wiring due to risks.

Replacing knob and tube wiring with modern wiring eliminates these hazards and gives peace of mind. Many jurisdictions now prohibit knob and tube and may require upgrades during renovations or when selling property.

Conclusion

Knob and tube wiring served homes well for many decades but is now considered outdated and potentially unsafe. While still permitted in some areas, extreme care should be taken if repairing or reusing existing installations. Fully upgrading to modern wiring is the safest and most reliable option for most homes. Consulting local codes and a qualified electrician is highly recommended when dealing with knob and tube systems.