How to Install Knob and Tube Wiring

Introduction

Knob and tube wiring was an early standardized method of electrical wiring used in buildings in North America from about 1880 to the 1940s. It consists of single insulated copper conductors run within wall cavities, passing through joists and studs. It gets its name from the knob or ceramic spacers and tubes used to keep the wires separated and supported.

While knob and tube wiring has largely been replaced with newer and safer wiring methods, you may encounter it in older homes. If properly maintained, it can still be functional. However, there are safety concerns, limitations on modifications, and insurance issues. This article will cover the basics of working with existing knob and tube wiring.

Assessing Existing Knob and Tube Wiring

Before doing any work with knob and tube wiring, you'll want to carefully assess the existing installation. Look for the following:

Condition of the Wiring

• Check for cracked, damaged, or deteriorated insulation on the wiring. This can pose a shock or fire hazard.
• Verify that wiring is still supported properly by knobs and tubes. Sagging wires are dangerous.
• Make sure wires are not resting against combustible materials like wood. This is a fire risk.

Alterations and Modifications

• Watch for improper splicing, amateur additions, or other changes to the original installation. These can be unsafe.
• Confirm that fuses/breakers are properly rated for the wire size. Undersized overcurrent protection is hazardous.

Grounding

• Most old knob and tube wiring has no grounding conductor. This makes it unsafe for many modern appliances.

If you find any major defects, deteriorated conditions, or improperly modified wiring, strongly consider full replacement rather than repairing the existing installation.

Working Safely with Knob and Tube Wiring

If the existing knob and tube wiring appears to be in good condition, you can perform limited work on it safely by following these guidelines:

• Turn off power at the main breaker panel before doing any work. Verify it's off with a tester.
• Do not make any modifications or additions without consulting an electrician.
• Do not overload circuits. Be aware of voltage drop over distance.
• Do not tap in new wire leads or splice new wires into existing knob and tube circuits.
• Do not run modern three-prong grounded cords on ungrounded knob and tube wiring. Use two-prong cords.
• Do not install high-power devices like central AC units on old knob and tube circuits.

Always use extreme care and caution when dealing with exposed knob and tube wiring. Treat all parts as if they are live.

Installing New Knob and Tube Wiring

While it is technically possible to install new knob and tube wiring to code in some areas, it is strongly recommended to use modern wiring methods instead.

If you do wish to install new knob and tube wiring, here are the basic steps:

Planning the Wiring Layout

• Plan conductor runs to minimize length and avoid excessive voltage drop.
• Select appropriate wire gauge for expected load and circuit length.
• Allow for expansion and contraction of conductors through joists and studs.

Running and Securing the Conductors

• Run wires through holes drilled in joists and studs, keeping them separated.
• Use knob or ceramic spacers to keep conductors supported and spaced.
• Use tube protectors where conductors pass through wood or near edges.
• Keep conductors at least one inch from metal piping and surfaces.
• Make sure wiring is not in direct contact with combustible materials.

Making Connections

• Use soldered splices or wire nuts rated for the conductor size.
• Avoid excessive twisting or bending conductors.
• Properly terminate wires at devices and equipment.

Safety and Code Considerations

• Follow all applicable electrical codes for conductor type, size, protection, and installation details.
• Provide overcurrent protection and disconnects as required.
• Post warning signs if wiring is ungrounded.

Alternatives to Knob and Tube Wiring

Instead of using knob and tube wiring for new or upgraded electrical systems, one of these safer and more robust wiring methods is recommended:

• Non-metallic (NM) cable - Insulated wires bundled in a flexible plastic sheath. Common for residential wiring.
• Conduit - Wires pulled through metal or plastic conduit for protection. Allows easy wire changes.
• Armored cable - Insulated wires wrapped in a flexible metal armor. Provides good protection.
• Metal clad cable - Insulated wires with an overall metal covering as armor.

These provide better protection, safety, less voltage drop, and easier upgrades. The small added cost upfront is well worth it compared to continued maintenance of old knob and tube wiring.

When to Call an Electrician

Due to the risks of working with knob and tube wiring, it's advisable to call a licensed electrician for:

• Evaluating the condition and safety of existing installations.
• Any repairs or modifications beyond very minor work.
• Installing any new circuits, grounded wiring, or replacement wiring.
• Ensuring compliance with local codes and standards.

They have the proper training, tools, and experience to handle old wiring systems safely. Don't take risks doing extensive electrical work yourself.

Conclusion

While knob and tube wiring may seem antiquated, it's still found in many older homes. With proper precautions, limited work can be done to maintain existing installations. However, for updated electrical systems meeting modern safety standards, new wiring methods are strongly recommended whenever possible. Call a licensed electrician anytime major work is required.