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 joists and studs.
The wiring was supported along its length by knob insulators. Where wires entered a wiring device such as a lamp or switch, or were pulled into a wall, they were protected by flexible tubing insulators. This system was an improvement over previous wiring methods as it included some safety features such as separation of conductors and placement away from combustible materials.
Key Features of Knob and Tube Wiring
- Separate hot and neutral wires - The hot and neutral conductors were run through separate knob and tube insulators, reducing risk of shorts.
- Open air installation - No insulation surrounded the conductors within wall cavities, allowing heat dissipation.
- Solid conductor wires - The wiring was made from solid copper conductors rather than stranded wires, allowing for neat and secure connections.
- Insulating supports - Knobs, tubes, and sleeves prevented conductors from contacting combustible materials.
Why Did Knob and Tube Wiring Fall Out of Favor?
Knob and tube wiring began to decline in popularity in the 1930s and was essentially obsolete by the 1950s. Several factors led to the shift away from knob and tube:
Limitations of Knob and Tube
- Insufficient for high loads - The wiring was only designed for basic lighting and receptacle circuits, not large appliances.
- Difficult to modify - Extending or altering knob and tube circuits within walls was labor intensive.
- No grounding conductor - The system lacked a ground path to prevent shocks.
Safer New Alternatives
- Nonmetallic sheathed cable - NM, with insulated wires jacketed in bendable armor, become available in the 1930s.
- Metal clad cable - MC cable provided a grounded metal covering around conductors.
- Conduit wiring - Rigid conduit and EMT (electrical metallic tubing) became popular for commercial wiring.
Obsolescence of Old Construction Methods
- Knob and tube relied on open cavities for ventilation, but insulation and drywall became standard, filling wall voids.
- Metal electrical boxes with Romex connectors replaced simple hold cutouts, preventing contact with combustibles.
Why is Knob and Tube Making a Comeback?
While knob and tube wiring is obsolete for new construction, interest in maintaining or replicating it in historic restorations has risen. Some of the reasons it's gaining renewed popularity include:
Historic Preservation
- Many residential and commercial buildings from the 1880-1930 period contain original knob and tube wiring.
- Historic building codes may require retaining and even matching original knob and tube aesthetically in renovations.
- Vintage styles like industrial, steampunk, and Victorian utilize exposed knob and tube.
sustainability and Reduced Waste
- Reusing existing knob and tube rather than replacing it avoids landfill waste.
- Repairing only damaged sections instead of a complete rewiring is less wasteful.
- The copper conductors have high scrap value for recycling if removed.
Lower Cost
- In sound condition, old wiring can be left in place saving on installation costs.
- Licensed electricians can perform targeted repairs at a fraction of rewiring cost.
- Salvaged period insulation parts support repairs and matches to original styles.
Is Restored Knob and Tube Wiring Safe?
While knob and tube has its limitations, it can be restored and adapted to modern safety standards:
- Overcurrent protection - New circuit breakers can be added to protect conductor ampacity.
- Grounding - Grounded metal conduit or a separate ground wire can provide a ground path.
- Insulation - Open cavities can be backfilled with non-combustible insulation for fire safety.
- Capacity increase - Higher loads can be managed by running new wiring alongside old.
- Visual inspection - An electrician should evaluate condition and repair or replace damaged wiring.
Properly maintained knob and tube can provide continued safe and reliable electrical service while preserving historic charm. Reuse and repair allow reducing demolition waste while maintaining or matching historic aesthetics. With proper precautions and upgrades, adapted knob and tube wiring can meet modern electrical safety codes.