Electrical Health and Safety Knob and Tube Wiring

Home Inspector with Hud Certified 203K Consultant nachi10010504


Knob and TubeHistory of knob and tube wiring Knob and tube wiring was the go-to method for electricians in the United States from the 1880s to the late 1930s. Many pros continued to use this method through the 1950s, '60s and even '70s for new home construction. Many homeowners looking to purchase a home will find that they will need to (homes being sold as is) upgrade their electrical system or complete a renovation are discovering "hidden" knob and tube wiring in their homes and wondering how to proceed. We have uncovered many homes and building unsafe electrical system doing our certified inspection services (see the photos in this post. Knob and tube wiring gets its name from the ceramic knobs used to hold wires in place and ceramic tubes that act as protective casings for wires running through wall studs or floor joists. Instead of the three wires found in modern electrical installations, knob and tube wiring has only two — a black (hot) wire and a white (neutral) wire.

Mixed with Knob and Tube

This means there is no ground wire in the system for excess charge or in the event of a short. National Code have changed many times since the installation of knob and tube over the years. How do you know you have an out dated electrical system without a certified home inspection? As a result of active knob and tube (KB), outlets in a KB home will have two prongs, not three. Many electrical contractors will repair and mix knob and tube wiring into the branch wiring, this is not the case with State Licensed Contractors and a Certified Home Inspector will make sure that you have a safe system before you The hot and neutral wires in a KB setup are run and sheathed separately and placed approximately one inch apart, rather than being bundled together as with new remix wiring. Most KB installations are restricted to a 60-amp service. A Certified Home Inspector will advise you on the correct safe electrical wiring system. Problems with knob and tube wiring If a KB system is intact and working, it poses no immediate risk to you and your family, If it is not mixed with a 3 romex branch system wire system. One of the most common problems with this kind of wiring is its insulation, which is made of rubber instead of plastic. Over time, the rubber degrades, exposing bare wires to air and moisture, in turn increasing the chance of a short or a fire. Extra circuits are also a problem because basic KB installations only allowed for 12 circuits in a home. Often, homeowners who needed extra circuits would pay contractors to add new circuits at the panel or simply splice into an existing wire. Both of these modifications run the risk of overloading the system. No Certified Home Inspector is going to declare a home free of a KB branching wiring system unless it is truly safe to do so. Health and Safety Issues are the concerns of a Certified Home Inspector The 2008 National Electric Code addressed some issues with KB Branch wiring, most notably its high heat dissipation that poses a fire hazard when combined with fiberglass insulation. As a result, the NEC now requires that knob and tube wiring not be in "hollow spaces of walls, ceilings and attics where such spaces are insulated by loose, rolled or foamed-in-place insulating material that envelops the conductors." It's important to note, however, that this code is not mandatory. States can choose to follow it at their own discretion. Thus the reason homes in the Long Beach and Los Angeles county still have KB wiring for the past 100 years.


KB Branch wiring also poses a problem for insurance companies. Some demand higher premiums from customers with this kind of wiring in their homes, while other companies refuse to insure homeowners at all until the wiring is upgraded from KB branch wiring. In fact some insurance companies will insure if the safe knob and tube is above insulation instead of buried under insulation and is not mixed with Romex Branch wiring this can only be analyzed by a Certified Home Inspected or a State Licensed Electrical Contractor here in the Los Angeles/ Long Beach area. Once a certified home inspector’s written report on the condition of the branch wiring is established, the next question is, How Much Does It Cost to Replace Cloth Wiring? That is where you contact 3 State Licensed Electrical Contractors to provide a proposal of cost to replace the service panel and the branch wiring. If you are purchasing a home in the Los Angeles Area. Expect to pay around $5,000 to $15,000 (approx.) to rewire a 1,500- to 3,000-square-foot home. You'll also need to upgrade the service to your home to at least from 70 amps to 100 amps according to most California State Licensed Contractors, though many State Licensed Electrical contractors and HVAC Contractor will recommend 200-amp service to account for any future electrical needs. If wiring insulation has cracked and caused any fire damage, your costs will increase. By today’s standards for safe electrical standards KB is not safe and should be replaced if it is present in a home you are going to purchase or renovate.


Fred Sweezer Sr

Certified Master Home Inspector 

HUD Certified 203k Consultant SO712

FHA Complaince Inspector T477





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Barbara Todaro
RE/MAX Executive Realty - Retired - Franklin, MA
Previously Affiliated with The Todaro Team

Good morning, Fred Sweezer Sr. the electrical system of a home is THE most important to me....with every purchase I've made, the first separate inspection is with my electrician..... I want what I buy to be up to the present code......of course, at my expense....and I spent the money.... 

Sep 30, 2019 02:39 AM #1
Fred Sweezer Sr.
Hud Certified 203K Consultant - Long Beach, CA
Certified Home Inspector

Thanks for your support Barbera, it is a same to learn about such an expense after the purchase and not before.

Sep 30, 2019 06:47 AM #2
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Fred Sweezer Sr.

Certified Home Inspector
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