How to Test and Repair Aging Knob-and-Tube Wiring

What is Knob-and-Tube Wiring?

Knob-and-tube wiring was commonly installed in homes built before 1930. It consists of individual wires that are run through the wall cavities, with ceramic knobs used to hold the wires in place and air as the insulator.

The wires are either single conductors with a cloth or rubber insulation or two conductors twisted together with fabric insulation. They are often connected to devices like switches and outlets with soldered joints or wrap-around connections.

Key characteristics of knob-and-tube wiring:

• No ground wire
• Individual wires spaced apart from each other
• Wires pass through ceramic knobs and tubes
• Insulated with cloth, rubber, or fabric materials
• Often has soldered joints and wrap-around connections

While knob-and-tube wiring was suitable for the first half of the 20th century, it lacks safety features that are standard in modern electrical systems. Proper testing and repairs are crucial for ensuring this old wiring is still safe to use.

Dangers of Old Knob-and-Tube Wiring

Knob-and-tube wiring that has aged can pose a few key risks:

• Fire hazard - the old insulation may crack and expose bare wires, allowing arcs, sparks, and overheating. This can ignite nearby combustible materials.

• Shocks and electrocution - damaged insulation exposes live wires, which can energize metal components and pose a shock risk.

• Lack of grounding - there is no equipment ground wire, so faults can energize metal components.

• Unsafe alterations - improper modifications over the years may overload circuits and bypass safety features.

• Circuit failures - the wires and connections can corrode over time, leading to intermittent operation or total failure.

I regularly test and inspect older electrical systems to identify any signs of deterioration or unsafe conditions. Protecting my customers from fire and shock hazards is my top priority.

Testing Knob-and-Tube Wiring

I use a few methods to thoroughly test knob-and-tube wiring and check for any problems:

Visual Inspection

• Check for improper splices, open junction boxes, and exposed wires throughout the home.
• Verify that insulation is intact and wires have not worked loose from knobs.
• Look for signs of overheating like discolored or melted insulation.
• Check condition of outlets, switches, and light fixtures.

Continuity Testing

• Shut off power and verify with a contact voltage tester.
• Disconnect wires and use a multimeter to check end-to-end continuity of each conductor.
• Verify no shorts between conductors of different circuits.

Load Testing

• Turn circuits on and systematically check voltage drops under realistic loads.
• Operate lighting, appliances, HVAC equipment simultaneously to simulate real-world demand.
• Monitor for excess voltage drop that can indicate undersized, damaged wiring.

Infrared Scanning

• Use an infrared camera to check for hot spots along wires in walls and ceilings.
• Hot spots often indicate damaged insulation and potential fire hazards.

Testing is essential for finding issues within walls that are not visible. I assess the entire system thoroughly from the main panel to outlets.

Evaluating Problems with Knob-and-Tube Wiring

Based on the testing results, I evaluate any issues found and determine recommended actions:

• Exposed live parts - This is a serious short-term safety hazard and must be addressed immediately by shutting off power and covering/repairing exposed components.

• Deteriorated insulation - Insulation that is damaged or completely missing will need to be replaced or have wires rerouted. Allows arcing risks.

• Splices in unsafe locations - Any splices in open walls or attics should be moved to covered electrical boxes to avoid accidental contact.

• Overloaded circuits - Knob-and-tube wiring is often insufficient for modern electric loads. Additional circuits may be warranted to reduce fire risks.

• Whole home rewiring - In some cases, the extent of damage and alterations may warrant a complete removal and rewiring with modern NM cable and components.

I thoroughly explain any issues found and risks identified so my customers can make informed decisions about repairs. The goal is to keep their family and home safe.

Repairing or Replacing Knob-and-Tube Wiring

For minor issues, I may recommend targeted repairs:

• Reinforce loose wire attachments at ceramic knobs.
• Reroute damaged sections through non-combustible materials.
• Replace cracked outlet, switch, and light fixture components.

However, many homes with knob-and-tube wiring require:

Full Rewiring

This involves:

• Installing new NM cables from the panel to each outlet and light.
• Removing old knob-and-tube wiring.
• Installing new outlets, switches, lighting fixtures.
• Providing proper grounding.

Rewiring maintains the existing outlet locations but provides a modern, safer electrical system.

Circuit Breaker Panel Upgrade

An updated panel provides:

• Adequate circuit capacity for modern loads.
• Safe circuit breakers instead of old fuses.
• Grounding system.

I follow best practices and local codes when repairing or replacing old wiring. Safety is always the top priority.

Maintaining Old Electrical Systems

For homes still using knob-and-tube wiring, I recommend:

• Avoid overloading circuits or adding high-draw appliances.
• Do not make modifications or additions yourself.
• Have the system periodically inspected by a qualified electrician.
• Check for hot spots with an infrared camera.
• Keep combustible materials away from wiring.

With routine inspections and proper maintenance, knob-and-tube wiring can still be used safely in many homes. But upgrades provide much greater safety margins.

Summary

As homes age, thorough inspection and testing helps identify risks with old knob-and-tube wiring. Targeted repairs can improve safety, but rewiring may be required in many cases. With the right approach, I can keep my customers' homes and families protected, whether repairing or replacing their aging electrical systems.