What is Knob and Tube Wiring?

Knob and tube wiring is an early method of electrical wiring used in buildings. Is it safe to keep in your antique home? Learn more from Mr. Electric.

An early method of electrical wiring, knob and tube is an outdated wiring that is unsafe for older, antique homes. What is knob and tube wiring, and just how old is it? Also called ‘open wiring,’ knob and tube was used in homes from as far back as the 1880s to the 1940s. Hidden away in the attic, basement, or spider webbed within the walls and only visible when plaster or wallboard is removed, many historic homes built in this timespan hide knob and tube wiring, unbeknownst to inhabitants.

How to Identify Knob and Tube Wiring

Knob and tube wiring is easy to identify, provided you can see it and it’s not hidden within the walls of your home. This obsolete form of wiring is identifiable by its white, ceramic, spool-like knobs. You’ll find the spools nailed to joists, with wiring run through the knobs, which are used to support strands of wires. Heavy and ceramic, the tubes protect loose wires where they run through the lumber of building joists. Knob and tube can also be hidden within walls, a process dubbed ‘spider webbing.’

Is Knob and Tube Really that Dangerous?

Knob and tube wiring is one of the Top 5 Causes of Electrical Fires. Just one of the many reasons most insurance companies refuse to cover houses with knob-and-tube. What makes knob and tube so unsafe?

  • Age and Neglect
    Age and neglect result in the deterioration of knob and tube wiring systems, rendering them dangerous. The rubberized cloth once protecting wires becomes brittle, cracking and falling off over time to leave exposed wires. Animal damage can speed this process. Compounding the problem, aging wires stretch and sag over time, coming into contact with surrounding materials. This presents a tremendous shock and fire hazard.
  • Unsafe Insulation Additions
    Knob and tube wiring was designed to be suspended away from key components, so that it can dissipate heat. Adding insulation into attics and wall cavities results in wires being covered, which can lead to overheating, as well as damage when wires buried in insulation are walked upon.
  • Shoddy Electrical Connections
    Improper connections made out of ignorance or laziness over the history of the home, such as connecting, soldering or splicing wires together without a junction box, hooking up wiring to fuses with amperage too high for this outdated wiring to accommodate, as well as improper taping, severely compromise safety.
  • No Ground
    Knob and tube wiring is often paired with 2-prong outlets (though that doesn’t stop some from improperly wiring knob and tube to 3-prong outlets). Either way, knob and tube lacks grounding providing only a hot and neutral wire. This puts appliances and electronics in a precarious position, giving voltage fluctuations and surges nowhere to go, and increasing the risk of destruction, shock, and electrical fires.
  • Incompatible with Moisture
    Knob and tube is dangerous in humid or wet locations such as baths, kitchens, and laundries, and is not rated for moisture.
  • Knob and Tube Can’t Keep up with the Demands of Today’s Electronics
    Knob and tube was not designed to handle the load of today’s glut of electronics and appliances, including air conditioners, dryers, dishwashers, microwaves, computers, gaming systems, home theatre equipment, smart and mobile devices. The heavily increased load caused by the demand for such a massive quantity of electronics strains the system, causing wires to overheat and creating fire hazards throughout your home.   

How Much Does it Cost to Replace Knob and Tube Wiring?

The cost to replace knob and tube wiring depends on its location, as well as the age and size of your home. It is not a do-it-yourself job, it is dangerous and requires the skills of a trained professional electrician. Expect to spend from $5,000 to $10,000 or more to upgrade knob and tube wiring within 1,500-3,000 square foot homes.  

A historic home is a beautiful thing. Historic wiring is not. Learn more about improving the electrical safety of your antique home with the help of Mr. Electric today.

This blog is made available by Mr. Electric for educational purposes only to give the reader general information and a general understanding on the specific subject above. The blog should not be used as a substitute for a licensed electrical professional in your state or region. Check with city and state laws before performing any household project.