The Forgotten Knob and Tube Wiring That Could Burn Down Your House
What is Knob and Tube Wiring?
Knob and tube wiring was an early standardized method of electrical wiring used in buildings in the United States from about 1880 to the 1940s. It consisted of single insulated copper conductors run within wall cavities, passing through joists and studs.
The system gets its name from the porcelain knobs used to hold the wiring away from frame members, and the porcelain tubes used to contain wires where they passed through joists and studs.
Key Features of Knob and Tube Wiring
Separate hot and neutral wires - The hot and neutral wires were run through separate knob and tube channels for insulation purposes. This differs from modern NM cable where the hot and neutral wires are bundled together in a single sheath.
Open air wiring - The wires were suspended in open air in wall cavities rather than bundled together in a protective cable sheath. This allowed heat to dissipate easily but left the wiring more exposed.
Porcelain knobs and tubes - The knobs held the wires away from framing members while the tubes allowed wires to pass through wood while maintaining isolation.
Why Knob and Tube Wiring is a Fire Hazard
While knob and tube wiring was an important evolution in electrical safety when first introduced, it poses several fire risks by modern standards:
Deterioration Over Time
The original rubber wiring insulation often deteriorates after 50-60 years, exposing bare copper conductors. This can allow wires to arc to grounded metal junction boxes or framing members.
Porcelain knobs and tubes can crack or break over decades of vibration and expansion/contraction cycles. This can lower the isolation of conductors.
Lack of Grounding
- Knob and tube systems typically lack a grounding conductor. This makes equipment more vulnerable to surges or faults.
Older fuse panels allow overfusing, which can lead to unsafe power loads on outdated wiring. This produces excessive heat which accelerates insulation breakdown.
Lack of grounding coupled with faulty appliances can electrify metal surfaces, shocking anyone who touches them.
Exposure to Combustibles
- Open air wiring in walls can come into contact with insulation or other combustibles added over the years as homes are renovated. This is a major fire hazard.
Signs You May Still Have Knob and Tube Wiring
Many older homes may still contain knob and tube wiring hidden behind walls and ceilings. Here are some signs that your home may need inspection:
- Two-prong ungrounded outlets
- Blown fuses or tripped breakers from minor appliance use
- Uneven power distribution to lights/outlets
- Older electric panel with screw or mercury fuse type
- Notices of insurance coverage changes/cancellations
If your home was built before 1950 and exhibits any of these issues, it is wise to have a qualified electrician inspect for knob and tube wiring.
Dangers and Risks of Old Wiring
Knob and tube wiring poses not just fire risk but other dangers:
Fire - Deteriorated insulation can arc to wood framing or insulation and start electrical fires. This is the #1 risk.
Shocks - Lack of grounding can electrify surfaces like faucets, stoves or metal junction boxes. These can shock or electrocute someone.
Malfunctioning appliances - Grounded outlets provide a safe current return path. Without them appliances can malfunction, overheat or damage sensitive electronics.
Insurance issues - Many insurers cancel or refuse to renew home policies if old wiring is present due to fire risk. This can make your home uninsurable.
Upgrading Old Wiring
If your home does contain old knob and tube wiring, some upgrade options include:
Full rewire - This is the recommended solution but also the most expensive, invasive and time consuming. All old wiring is removed and replaced with modern NM cable wiring.
Pigtailing - The old wiring is left in place but each run is pigtailed or spliced to modern wiring. This improves safety but still leaves deteriorated insulation in place.
AFCIs - Arc-fault circuit interrupter breakers detect dangerous arcing conditions and cut power. This adds protection without rewiring.
I strongly recommend fully upgrading old wiring for maximum safety. Though expensive, it can prevent catastrophic house fires.
Knob and tube wiring was an important evolution in home electrification, but now poses serious fire and shock hazards due to deterioration over 80-100 years of use. If your older home exhibits any signs of old wiring, have it professionally inspected and upgraded. The investment is worthwhile to protect your home and family.