The forgotten art of knob and tube wiring

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 or ceiling cavities, passing through joist and stud drill-holes via protective porcelain insulators called knobs, and supported along their length by porcelain tubes.

This old system has largely been phased out and replaced with modern sheathed wiring methods, such as non-metallic sheathed cable or Romex. However, some older homes still contain functioning knob and tube circuits. While it can be reliable if maintained properly, knob and tube wiring presents some safety hazards that homeowners should be aware of.

Key characteristics of knob and tube wiring

• Separate hot and neutral wires - The hot and neutral conductors were run separately, with air rather than insulation separating them. This prevents arcing faults between wires.

• Open wiring - The wires were run through open spaces within walls and ceilings, without any casing enclosing them. This facilitates heat dissipation.

• Insulators - Porcelain knobs hold wires away from framing. Tubes protect and guide wires through open spaces.

• Cloth insulation - Early versions used cloth wrapping to insulate wires. Rubber insulation became more common by the 1920s.

• Silver tinned copper - The solid copper conductors were silver tinned to resist corrosion.

The rise and fall of knob and tube wiring

Knob and tube wiring became popular because it provided an affordable and reliable method of wiring homes with electricity. Some key historical points:

• 1882 - The Phoenix Fire Office in Hartford, CT publishes one of the earliest standards for knob and tube wiring.

• 1898 - National Electrical Code allows knob and tube wiring.

• 1920s - Rubber/asbestos insulated wire replaces the original cloth covering.

• 1930s - Nonmetallic sheathed cables start displacing knob and tube.

• 1940s - Use of knob and tube wiring declines significantly.

• 1965 - National Electrical Code formally deprecates knob and tube.

So why did knob and tube fade away? The main factors were safety concerns, inadequate capacity for increased loads in modern homes, and the difficulty of retrofitting it for new circuits.

Working with knob and tube wiring today

If you have knob and tube wiring in your vintage home, here are some key considerations:

• Inspection - Have a qualified electrician inspect it thoroughly before using. Look for cracked or damaged insulation and improper modifications.

• Limited modifications - It is difficult to splice into or extend knob and tube circuits. Some minor modifications can be done safely by an expert.

• Capacity limits - The wiring is often undersized for modern loads like air conditioners. Exercise caution when adding new appliances.

• Fire hazards - Deteriorated insulation can allow wires to arc to metal boxes or framing. Keep flammable materials away from wiring.

• Insurance issues - Many insurers won't cover homes with old live knob and tube wiring due to fire risks. Consider having it replaced.

• Replacement - Properly removing and replacing knob and tube wiring can be labor-intensive. Hire a licensed electrician rather than doing DIY.

While knob and tube served homes well for decades, modern electrical systems offer safer and more robust wiring for contemporary needs. But with proper precautions, remnants of this vintage wiring can still be maintained safely. The key is having a qualified professional assess it thoroughly.

Preserving knob and tube wiring in historical homes

For historic homes, retaining the original knob and tube wiring helps preserve the integrity of the structure. Here are some approaches:

• Consult with a qualified electrician and historical preservation expert.

• Evaluate condition and only keep wiring that is still in good shape.

• Don't overload circuits - use appropriate vintage loads.

• Add new circuits alongside old wiring rather than removing it.

• Use period-appropriate materials and components where repairs are needed.

• Properly weatherproof where wiring passes through exterior walls to prevent water damage.

• Label circuits clearly and educate occupants on loading limits.

• Consider disguising wiring in keeping with the home's era.

With care and common sense, knob and tube can be maintained safely in properties where the antiquated system contributes to historical ambience and charm. But never compromise safety in favor of nostalgia.

While no longer used in modern buildings, knob and tube wiring remains an intriguing example of early electrical engineering. Respecting its legacy while upgrading wisely allows vintage homes to safely enjoy both old and new.