I recently purchased my first home, a charming 1920s bungalow. While inspecting the property, I was surprised to learn that segments of the home still contained an obsolete and potentially hazardous type of electrical wiring known as "knob and tube." This discovery prompted me to research this antiquated wiring system to understand the risks it poses.
What is Knob and Tube Wiring?
Knob and tube (K&T) wiring was commonly installed in American homes built before 1950. It consists of insulated copper conductors that are suspended between ceramic knobs attached to the framing studs. The wires are separated from each other by air space rather than being bundled together like modern NM cable.
Some key facts about knob and tube wiring:
- Used as an early standardized method for electrical wiring from about 1880-1930s
- Insulation materials include rubber, cloth, and paper
- Wires are suspended and separated rather than bundled
- Knobs hold wires in place and away from framing
- Can be prone to deterioration over time
While knob and tube was considered a safe and sturdy wiring method in the early 20th century, it lacks most of the safety features required for modern electrical systems.
Why Knob and Tube Wiring is Dangerous
There are several factors that make aging knob and tube wiring a potential fire and shock hazard:
Lack of grounding - K&T systems do not include a ground wire which is essential for diverting voltage in the event of a short circuit. This makes it more likely for a faulty wire to ignite surrounding material.
Insulation deterioration - The rubber, cloth, or paper insulating material around K&T wires becomes brittle and deteriorated over time. This leads to cracked or exposed conductors.
Unsafe wiring practices - K&T wiring systems often have splices and junctions that do not comply with modern electrical codes. These can arc and overheat.
Incompatible materials - Insulation on knob and tube wires can melt or ignite when in direct contact with modern insulation like that on NM cables.
Overloaded circuits - Homes today require more power. Old 30-amp K&T circuits can be prone to overheating when powering large modern appliances.
These factors contribute to an elevated fire risk. According to the Consumer Product Safety Commission, nearly 40,000 home fires originating from wiring systems are reported each year in the U.S. Knob and tube wiring likely accounts for a disproportionate share of those fires in older homes.
How Prevalent is Knob and Tube Wiring Today?
It may come as a surprise that a wiring system developed in the late 1800s is still present in millions of modern homes.
Exact statistics are difficult to pinpoint, but some key numbers indicate the widespread prevalence:
Approximately 30 million homes in the U.S. were built before 1950 when knob and tube was commonly installed.
An estimated 10 million homes still contain some knob and tube wiring according to the U.S. Fire Administration.
One insurance company survey found K&T wiring in over 75% of homes built before 1930 that they inspected.
My local city permit data shows that over 60% of requested electrical service upgrades involve replacing knob and tube circuits.
So while it has largely faded from use, there is still a very high chance of encountering knob and tube circuits in older homes. Homeowners undertaking renovations often discover previously hidden K&T wiring behind walls and ceilings while doing demolition work.
Dangers and Challenges from Concealed Knob and Tube Wiring
The fact that knob and tube wiring is often concealed and undocumented presents some additional risks and difficulties:
Lack of visibility - Faulty wiring that is hidden from view in walls or ceilings is more likely to degrade unnoticed over time. Problems will only be discovered once a failure has occurred.
Insurance issues - Many insurers refuse to offer coverage for homes containing live knob and tube wiring due to the substantial fire risk it presents. This may only be revealed during the policy application process.
Problems during renovations - If existing K&T wiring is not identified prior to renovations, unsafe interactions with modern wiring materials installed during construction can occur. These create points of failure.
Difficult repairs - Accessing damaged wiring inside enclosed walls is much more invasive and labor-intensive than repairing readily visible wiring. This leads to large repair costs.
Resale obstacles - Homebuyers are often reluctant to purchase a home containing obsolete electrical systems due to safety concerns and insurance difficulties. K&T wiring can threaten real estate transactions.
For these reasons, it is advisable for homeowners in older homes to take proactive steps to identify and replace any existing knob and tube wiring during renovations or a pre-sale home inspection.
Is It Possible to Repair or Work Around Knob and Tube Wiring?
Some homeowners attempt limited repairs or develop makeshift solutions to work around knob and tube wiring issues, however this is inadvisable. Some approaches that may seem convenient but are unsafe include:
- Splicing new wires into existing K&T circuits
- Capping off unused wires and leaving them in place
- Insulating around knob and tube wires to bundle them with newer wiring
- Using electrical tape on cracked wire insulation instead of replacing
The only reliable and code-compliant option is fully replacing K&T wiring with new NM electrical cable and adding grounding. Incremental repairs on old wiring simply mask underlying safety issues that will persist.
Is Replacement Necessary Even if Knob and Tube Wiring Appears in Good Condition?
Even if existing knob and tube wiring appears intact and undamaged to the naked eye, replacement is still recommended. There are a few reasons why:
Deceptive looks - Wiring insulation often looks deceptively intact but is brittle and prone to cracking under pressure or movement. Electrical failure can happen suddenly.
Modern demands - Homes use much more power today through added lighting, appliances, electronics. Old K&T circuits cannot handle the excess load. This leads to overheating.
Peace of mind - With safety risks like electric shock and fire, the consequences of a wiring failure are too severe. Owners feel more secure eliminating the risk entirely through wholesale replacement.
Inadequate grounding - Proper grounding is essential for modern electrical safety no matter the condition of the wires. K&T systems simply cannot provide adequate grounding.
Ideally, knob and tube wiring replacement happens preemptively before any functional failure occurs. This negates the safety issues and provides homeowners with reliable and efficient modern electrical systems.
Does Homeowner's Insurance Cover Replacement Costs?
Since the presence of knob and tube wiring puts the entire property at risk, some insurers will provide funds specifically towards wiring replacement costs as an incentive for the homeowner to upgrade the electrical system.
However, policies and coverage vary widely between providers. Some key considerations are:
Review policy terms - The policy contract will stipulate what types of wiring upgrades are covered and what limits/deductibles apply. Replacement of the full K&T system may or may not be required.
Inquire about discounts - Some insurers offer premium discounts of 10-20% after K&T wiring is replaced as it lowers the perceived risk. There may also be senior citizen discounts.
Seek referenced contractors - The insurer may only cover wiring replacement work performed by specific licensed electricians they trust to do the job properly.
Consider coverage riders - Optional riders like "Green Improvement" coverage can potentially help fund efficiency upgrades like wiring replacement. These come at an additional cost.
Bottom Line - While insurance can offset some costs, homeowners should expect to pay at least part of a knob and tube rewiring project out-of-pocket. But this investment pays dividends in safety.
Just How Expensive is it to Replace Knob and Tube Wiring?
As you might expect, rewiring an entire older home with modern electrical infrastructure is a major undertaking that commands a substantial price tag. Some key factors impacting project costs include:
Home size - The number of square feet, rooms, and circuits to update will obviously drive overall cost. Rewiring larger houses costs exponentially more.
Accessibility - How readily wiring can be accessed impacts labor time. Fishing new wires through finished walls and ceilings is time consuming compared to open framing.
Peripheral upgrades - Upgrading the electrical panel or installing new lighting and switches while walls are open adds incremental costs but improves functionality.
Contractor rates - Contractor hourly rates, travel fees, material markups all vary. Get multiple quotes before selecting a provider.
In terms of ballpark estimates, a full knob and tube rewire for an average 2,000 sq. ft. home often ranges from $8,000 to $15,000. High labor costs in some regions can drive this even higher. Spreading the work over time helps manage costs.
Is a DIY Knob and Tube Wiring Replacement Realistic?
Some competent and experienced homeowners take on DIY wiring projects in their own homes in order to control costs. However, knob and tube replacements have some distinct challenges:
Permitting - Extensive wiring replacement typically requires opening walls and ceilings throughout the home. Securing proper municipal permits for this level of demolition is mandatory but difficult as a homeowner.
** Skill level** - Safe electrical procedures, circuit mapping, making concealed connections and splices all require specialized abilities beyond those of an enthusiastic amateur. Faulty work risks catastrophe.
Time requirements - A whole home rewiring project is a lengthy, labor-intensive process. While incremental DIY is good for saving money, projects left incomplete are unsafe.
Insurance implications - Insurance claims down the road could be denied if wiring was replaced improperly and contributed to fire or shock damage. Their inspectors will look for professional work.
Bottom Line - The complexity and safety considerations inherent to knob and tube replacement make a licensed electrician the only prudent choice for nearly all homeowners. The benefit of experience and accountability outweighs potential savings.
How to Select the Best Electrician for a Knob and Tube Replacement Project
Once the decision is made to hire a professional electrician, take time to identify the best contractor for the job:
Verify licenses - Electricians must hold a valid license confirming education, experience and passing relevant exams. This is mandatory.
Check credentials - Search for contractors certified in historical rewiring or endorsed by local historical preservation boards. This indicates specialist expertise.
Seek referrals - Friends, realtors, insurance agents can recommend electricians they know from past knob and tube replacements.
Review examples - Ask contractors to provide before and after photos of previous rewiring projects to evaluate the quality of their work.
Interview thoroughly - Meet in-person and come prepared with questions to determine the right fit. Gauge their responsiveness and knowledge.
Compare bids - Get estimates from multiple electricians before deciding. Beware extremely high or low bids.
Doing diligence ensures the best contractor match and increases the likelihood of a successful wiring overhaul. Don't rush the selection process.
While antiquated knob and tube wiring lingers in millions of older homes, it presents a substantial fire and shock risk that only grows over time as insulation deteriorates. Homeowners should educate themselves on identifying these obsolete electrical systems and work towards replacing them completely with modern wiring. The expense can be significant but is a prudent investment in safety. Involving a licensed electrician and your insurance provider helps ease the process. With updated electrical infrastructure, homeowners gain much needed peace of mind knowing their family and property are no longer at risk.