What is Knob and Tube Wiring?
Knob and tube (KT) wiring was an early standardized method of electrical wiring used in buildings in the United States from about 1880 to the 1940s. It consists of single insulated copper conductors run within wall cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported on nylon knobs.
KT wiring has the following key features:
- Two separate wires, a hot and a neutral, providing a single circuit - there is no ground wire
- Wires are separated by 4-6 inches to prevent overheating
- Insulated by rubber, cloth, or asbestos - no plastic
- Supported by porcelain knobs attached to framing
- Routed through porcelain tubes where wires enter boxes
This old wiring system has largely been replaced by modern wiring methods like NM cable or conduit wiring. However, you can still find KT wiring in older homes built before 1950.
Why KT Wiring Still Exists in Older Homes
There are a few reasons why knob and tube wiring persists in some older homes today:
1. It was extremely reliable and well-built
- KT wiring was designed conservatively and uses quality materials like thick copper and porcelain.
- It was installed correctly to code standards, providing ample wire separation.
- This makes it resistant to overheating or arc faults.
- If undisturbed, it can continue working for 100+ years.
2. Upgrading can be difficult and expensive
- Running new wires means extensive structural modifications.
- Walls, ceilings, and floors may need to be opened up.
- Cost can easily run tens of thousands of dollars for a whole house rewire.
3. Safety concerns are often exaggerated
- KT wiring is actually safer than some poorly-done modern wiring jobs.
- As long as it is undisturbed and unaltered, it poses minimal fire risk.
- Many insurance companies don't require upgrading.
Why KT Wiring is Still a Concern
However, there are also some valid reasons why knob and tube wiring raises safety concerns today:
1. Lack of grounding
- KT wiring has only a hot and neutral wire, no ground.
- This means no path for faults to trip breakers.
- Greater risk of shocks and fire without grounded outlets.
2. Insulation breakdown
- The old rubber/cloth insulation can become brittle and crack over time.
- Exposed conductors could arc and ignite surrounding wood framing.
- Damage is often hidden within walls.
3. Improper modifications
- Hazards arise when homeowners try to improperly tap into KT circuits.
- Overloading circuits risks overheating wires.
- Adding grounds doesn't make outlets safe without rewiring.
4. Difficulty obtaining insurance
- Many insurance companies are reluctant to underwrite homes with knob and tube wiring due to fire risk.
- May need to replace KT wiring to get coverage.
Weighing the Options for Homes with KT Wiring
If you have knob and tube wiring in your older home, you have a few options:
- Leave it alone if it's undisturbed - maintain with care
- Rewire some circuits where necessary only
- Complete rewire of entire home
I would get an assessment from an electrician to identify any problem areas or urgent upgrades needed. They can also give you a rewiring cost estimate.
Consider your budget, plans for renovations, and insurance requirements when deciding your approach. With proper maintenance and limited upgrades, KT wiring can often be left safely in place. But be aware of risks like insulation breakdown and lack of grounding.
Conclusion
While knob and tube wiring is antiquated, it remains present in many older homes due to its durability and the high cost of replacing it. Safety issues arise when it is disturbed or improperly modified. With informed decision making, you can address KT wiring risks while also retaining some of its original circuits. Understanding the pros and cons allows homeowners to make smart choices about maintaining or upgrading this vestige of old electrical systems.