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Knob and Tube Wiring - an unbiased review

By Bill, KNT Electric, Contributer of Wikipedia.com's Knob and Tube Wiring page Updated Jan 26, 2014

You've all heard it before "don't buy a house with knob and tube wiring, you'll regret it, knob and tube wiring is a fire hazard, knob and tube wiring is dangerous, knob and tube wiring must be removed or else...". Are they right? The answer is maybe and maybe not.

Fact, since 1980, there has not been a single house fire that resulted in any loss of life or significant  property damage caused by knob and tube wiring. There has not been one reported case. This fact should speak volumes on itself.

Knob and tube wiring was used end of the 1800s all the way up to the 1950's. That is over 60 years of proof that knob and tube wiring works. In ways it's actually better than the new wiring methods we use today, but don't the home inspectors won't tell you that would they?

The Good

#1 - Heavier gauge copper wire

Knob and tube wiring uses a heavier gauge copper wire that are separated. I'm sure you know, in the past people actually built things better and used higher quality materials. The heavier gauge copper and individual wires allow more electrical throughput with less resistance which means less heat.

#2 - Taped connections

Knob and tube wiring uses taped connections after being spliced and soldered. If done correctly this is a very safe and stable method of connecting electrical wires. I've seen homes built in the 1940s who's knob and tube wiring is still functioning well even after 70 years?! If this isn't good craftsmanship, then I seriously don't know what is.

#3 - Codes still recognize it

Modern building codes still recognize knob and tube wiring as a safe and stable method of wiring a home. Even without the presence of a ground wire. In most applications around the home, a grounding wire is not necessary.

The Bad

#1 - Abused and damaged knob and tube wiring

Knob and tube wiring that has not been disturbed is usually safe and stable, HOWEVER, this is usually not the case in the vast majority of homes today. People move, people neglect, people upgrade, and rodents like to chew. These are the most common reasons why knob and tube wiring degrade over time and time is certainly a major factor here. As I've mentioned before rubber has a useful life of 25-30 years before it begins to brittle and lose it's elasticity. Abused and damaged knob and tube wiring is a safety hazard and has potential to start a house fire.

#2 - Over-fusing

Over-fusing essential means connecting too many electrical appliances to a given connection. This results in heat, flickering, blown fuses, smoking, sparks, and potentially a fire. Knob and tube wired homes are dated, they simply were not designed to power modern appliances and households that now include fully furnished basements. The long term consequences of over-fusing are, brittle wires, damage to insulation. This degrades knob and tube wiring very quickly.


Although undisturbed knob and tube wired homes may appear to be safe and working, I strong recommend homeowners hire a licensed electrician to inspect the system. It's an outdated method of wiring a home and despite the advantages of it, knob and tube wiring has been abandoned by the industry for over 60 years. The service life of existing installations have far-exceed it's useful life and I can only recommend the replacement of this good but antiquated system.

Disclosure: the author of this article is a Master Electrician with KNT Electric. If you have any questions or concerns please call Bill at 647-880-0881 anytime, or if you prefer to use email, please email "kntelectric@gmail.com"

An image of splicing knob and tube wiring with another wire. Very dangerous, but common by homeowner's who like DIY "crafty" solutions to save a few dollars.


Knob and Tube Wiring Primer

By Bill, KNT Electric March 19, 2012


Although there are even more primitive systems out there, the oldest surviving wiring folks like you might actually find in an old house today is the knob-and-tube system made up of individual rubber-coated conductors strung up on porcelain insulators. This electrical wiring method enjoyed wide use from the 1890s into the 1940s. At the time, knob-and-tube wiring was the latest technology, up until the 19040s.

Today, knob-and-tube wiring is perfectly legal – as long as it is in good condition. You should never assume that any knob-and-tube circuit is disconnected. Some of the wires may still carry current, spliced to cables or newer electrical upgrades. The most likely places you will find knob-and-tube wiring are attics and cellars/basements, where the conductors are accessible and uncovered by insulation or wall finishes.

Signs of knob-and-tube wiring

- Black rubber wires
- Porcelain cylinders
- Brittle/cracked wires, exposed single strands of wiring
- Presence of “attix cable” a woven cloth like material used to protect knob-and-tube conductors (also known as “rag wire”)
- BX Cable made of rubber, must be replaced/decommissioned (as mentioned earlier, rubber has a useful life of about 25 years).
- Black wiring that appears to be nibbled by rodents.

Special Attention

Rubber degrades faster when exposed to open air, sunlight, or moisture.

The most dangerous point is where the wiring enters the enclosure. Sometimes even the slightest movement will break and uncover live and raw electrical wires.


Problems poised by Knob-and-Tube wiring

The problem with knob-and-tube wiring is the same problem that plagues old wiring in general. Everything has a useful life and so does rubber which was the standard wiring insulating material until after World War II. Rubber tends to lose its flexibility after about 25 years, then the insulation can crack and break apart. Therefore any knob-and-tube wiring still in use is long past its useful life. You may not break any laws, but you are essentially gambling with your safety and your house by not decommissioning the knob-and-tube wiring.


Please call KNT Electric at 647-880-0881 and ask about our Knob and Tube Replacement Service anytime.



What exactly is Knob and Tube Wiring?

By Bill, KNT Electric Jan 3, 2012


Knob and tube wiring is usually found in unupdated residential properties throughout Toronto built before the 1960's. They call it Knob and tube wiring because of their componentry. The wires are isolated from objects by ceramic tubes that are lined in holes through floors and walls. Also based on my experience, homes with 60 amp service panels are usually of the Knob and tube standard.

The key difference between Knob and tube wiring and the electrial wiring we use today is the total absence of a ground wire. Knob and tube wiring consists of only a black hot wire and a white neutral wire. These two wires usually run separately to electrical outlets as opposed to current wiring where all three wires (hot, neutral, and ground) are contained in one plastic sheating.


All customers question the safety and function of Knob and tube wiring. The only one who can answer that question is a qualified electrician. A licenced and qualified electrician can inspect the wiring to determine it's level of safety and estimate remaining useful life as well as issue documentation showing this. This electricians report is very important for insurance companies, banks, and or potential buyers of the property. Insurers are especially concerned about the fact that there is no ground wire. Modern electrical appliances and equipment have exceeded the electrical output of 50 years ago. The effects are higher potential for fires, overloading fuses, and the question of fraying wires all pose severe fire and health risks to home owners.

Today most insurance companies have long began to decline insurance to homes with knob and tube wiring, however there still exists some insurers who will take on the risk, but with a higher cost and usually an electricians report.

If you plan to purchase a home in an area where knob and tube wiring was used then ask your realestate agent for advice on getting home insurance first.

Please call KNT Electric at 647-880-0881 and ask about our Knob and Tube Replacement Service anytime.




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