What is Knob-and-Tube Wiring?

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 consisted of single insulated copper conductors run within wall cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported along their length on nailed-down porcelain knob insulators.

While no longer used in modern construction, knob-and-tube wiring is still present in many older homes and buildings. This antiquated wiring system is often seen as a safety hazard due to its lack of grounding and susceptibility to insulation breakdown. However, with proper maintenance and overload protection, knob-and-tube can often continue to function safely and reliably.

Why Knob-and-Tube Causes Overload Issues

The main problem with knob-and-tube wiring is that it was never designed to handle the amount of electricity consumed by modern devices and appliances. An average home circa 1950 might have a few simple lighting circuits, a radio, refrigerator, and some small kitchen appliances.

Modern homes by comparison often contain dozens of high-wattage electronics like big screen TVs, computers, game consoles, and other appliances that consume substantial amounts of power. This can overload the limited current capacity of old knob-and-tube circuits.

Additionally, early wiring insulation tended to dry out and crack over time, further reducing the current capacity of knob-and-tube wires. Failed insulation can also create fire and shock hazards.

Strategies for Using Knob-and-Tube Safely

While upgrading wiring is the ultimate solution, there are ways to safely use knob-and-tube circuits without overloading them:

Carefully Limit Circuit Loads

Maintain Insulation Integrity

Add GFCI Protection

Whole House Surge Suppression

When to Consider Upgrading

While the above strategies can help compensate for knob-and-tube wiring, upgrading is the best long-term solution in many cases. Some signs it's time to consider wiring upgrades:

Though sometimes expensive, rewiring with modern wiring and circuit breakers provides the highest level of safety and capacity for electricity-hungry modern homes.


Knob-and-tube wiring creates risks of overloading and fire hazards in modern homes. However, with diligent load monitoring and protection methods, this antiquated wiring can often continue functioning safely for normal household needs. Upgrading wiring is still the best option when feasible, but the techniques outlined here can buy extra time before such a major upgrade is warranted. By understanding knob-and-tube's limitations and how to compensate for them, you can successfully leverage this old wiring system to meet your home's electrical needs.