Knob and tube (K&T) wiring was an early standardized method of electrical wiring in buildings, in common use in North America 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 insulating tubes, and supported on nailed-down porcelain knob insulators.
What is Knob and Tube Wiring?
Knob and tube wiring (also known as K&T wiring) is an early electrical wiring system that was common in North American homes built from about 1880 to the 1940s. It consists of:
- Knobs - porcelain insulating supports that hold the wiring away from framing.
- Tubes - porcelain tubes that protect wires as they pass through wood beams and other structural members.
- Wiring - single copper conductors with rubber or cloth insulation.
Unlike modern wiring systems that use Romex cables, K&T wiring has individual wires that are separated from each other. This separation of wires prevents shorts and overheating.
The knobs and tubes allow the wiring to be protected and separated while passing through open spaces in walls, attics, and ceilings. This gives the wiring some protection and provides ample ventilation to prevent overheating.
Dangers and Risks of Knob and Tube Wiring
While K&T wiring was safe when originally installed, it can become dangerous as it ages. Some of the main risks include:
- Insulation breakdown - the old rubber or cloth insulation can become brittle and crack over time, exposing live wires. This can lead to shorts, arcing faults, and electrical fires.
- Unsupported wiring - knobs and tubes can come loose over decades of vibration and expansion/contraction. This can leave live wires hanging free where their insulation can degrade faster.
- Unsafe modifications - improper DIY upgrades, tapped wires, and junctions added over the years can overload circuits.
- Overloaded circuits - early wiring was designed for lower power consumption. Plugging in too many modern, high wattage appliances can overload circuits.
- Exposed live parts - knob and tube wires were run through open stud and joist cavities. Remodeling or holes in walls/ceilings can leave these wires exposed.
These risks are exacerbated by the fact that K&T wiring lacks modern safety features like grounding. Ungrounded wiring can lead to an increased risk of electric shock.
For these reasons, insurance companies and electrical codes now prohibit or restrict K&T wiring in many areas. Homes with K&T may require expensive upgrades to meet modern safety standards.
Signs Your Home Has Knob and Tube Wiring
Here are some of the common signs that a home likely contains old knob and tube wiring:
- Porcelain knobs and tubes inside unfinished walls, basements, crawlspaces, and attics
- Wiring entering junction boxes through individual holes rather than via Romex cables
- Wiring insulation that appears old, cracked, brittle, or frayed
- Two-prong ungrounded outlets
- Blown fuses or tripped breakers from overloaded circuits
- Dimming or flickering lights when other devices are turned on
- Older home built before 1950
- Lack of ground wires in switch or outlet boxes
Checking for these indicators can help you determine if K&T wiring is present before beginning any work.
Safely Working Around Knob and Tube Wiring
If your home does contain K&T wiring, here are some tips for working safely:
Get a Professional Assessment
- Hire a licensed electrician to inspect the wiring and alert you to any hazards or necessary upgrades.
Don't Conceal Existing Wiring
- Leaving K&T wiring easily accessible allows it to dissipate heat and makes it easier to inspect condition. Don't cover it with insulation or building materials.
Don't Overload Circuits
- Use modern devices sparingly and don't plug in multiple high wattage appliances on an older circuit. Overloading can lead to meltdowns.
Don't Tamper with or Remove Wiring
- Don't attempt DIY repairs or removals. Disturbing intact K&T wires can damage insulation and create new hazards.
Check Wiring Before Drilling or Nailing
- Verify wires aren't present before penetrating walls for mounts, shelves, etc. Drilling or nailing into a live wire can damage insulation.
Use Caution in Remodels
- Have an electrician confirm wires are deactivated before disturbing any wiring during remodeling. Assume all wires are live until proven otherwise.
Update Electrical Panel and Service
- A modern electrical service with sufficient amperage and safety features like arc-fault breakers can compensate for some K&T risks.
Add GFCI Protection
- GFCIs shut off power if abnormal current is detected, providing protection against shocks. Install GFCI outlets or breakers where possible.
Consider Full Replacement
- For optimal safety, replacing K&T wiring with modern wiring removes all risks. Consult an electrician to determine if full rewiring is recommended.
Frequently Asked Questions
Is knob and tube wiring dangerous?
Knob and tube wiring can be dangerous because the insulation degrades over time, leading to fire and shock risks. It lacks modern safety features like grounding and overload protection. However, intact, unmodified K&T wiring is generally safe if not overloaded.
Does home insurance cover knob and tube wiring?
Many insurance companies will not insure a home with knob and tube wiring due to the risk of fire. Some may provide coverage if an electrician inspects the wiring and confirms that it is in good condition. Upgrading the electrical panel may also satisfy some insurers.
Can I just remove the knob and tube wiring myself?
It is not advisable to remove K&T wiring yourself unless you are a licensed electrician. Disturbing intact wiring can damage insulation and create new hazards. Replacement wiring must be properly installed to avoid unsafe conditions.
How much does it cost to replace knob and tube wiring?
The cost to replace knob and tube wiring varies greatly based on the size of the home. On average, complete replacement costs between $8,000-$15,000. Rewiring just some circuits or high priority areas like the kitchen may cost $2,000-$5,000.
Is knob and tube wiring grounded?
No, knob and tube wiring systems do not include grounding. This is another safety hazard, as ungrounded wiring increases the risk of shocks. Upgrading the system to include proper grounding reduces this risk.
Key Takeaways
- Knob and tube wiring can be unsafe due to degradation over time. Signs include cracking insulation and blown fuses.
- Avoid overloading circuits, making modifications, or concealing wiring.
- Hire an electrician to assess any K&T wiring before doing work.
- GFCI protection, panel upgrades, and full rewiring can improve safety.
- Exercise extreme caution when working around old wiring.
Knowing how to identify knob and tube wiring and addressing risks properly allows you to improve the safety of your vintage electrical system. Consult with a qualified electrician to determine the best solution for your home.