"Knob and tube" was the most cost-effective way to wire a home from about 1880 to the 1930s. It began gradually being phased out through the 1940s, displaced by electrical cables that bundled hot and neutral, and eventually ground, wires in a single flexible sleeve. We have not seen any homes built in 1950 or later with K&T in Florida.

    If a home has knob and tube wiring still functioning, or porcelain knobs or tubes abandoned in place in the attic or crawl space, that indicates it is pre-1950 construction when the actual date of construction is unknown. A home we recently inspected was listed as built in 1990, but one of the ways we were able to show the buyer that was a much older house that had been moved to the site and remodeled was the abandoned porcelain knobs and disconnected wires in the crawl space.

    To learn more about it, see our blog post What is "knob and tube" wiring? 

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Here’s links to a collection of our other blog posts about ELECTRICAL WIRING:

• Which house appliances need a dedicated electrical circuit?

• Can a short circuit cause a high electric bill?

• What is the maximum spacing requirement for securing NM-cable (nonmetallic-sheathed cable)?

• Is it alright to just put wire nuts on the end of unused or abandoned NM-cable or wiring?

• What causes copper wires to turn green or black in an electric panel?  

• What are typical aluminum service entrance wire/cable sizes for the electrical service to a house?

• Why is it unsafe to bond neutral and ground wiring at subpanels?

• Should I get a lightning rod system to protect my house?

• Why is a strain relief clamp necessary for the cord connection to some electric appliances?  

• Does a wire nut connection need to be wrapped with electrical tape?

• What is the minimum clearance of overhead electric service drop wires above a house roof?

• What are the requirements for NM-cables entering an electric panel box?

• What is the color code for NM cable (Romex®) sheathing?

• Why is undersize electric wiring in a house dangerous? 

• What causes flickering or blinking lights in a house?

• Why are old electrical components not always "grandfathered" as acceptable by home inspectors?

• How can I find out the size of the electric service to a house?

• What could cause an extremely high electric bill?

• Can old electrical wiring go bad inside a wall? 

• What is an open electrical splice?

• What are the most common electrical defects found in a home inspection? 

• What is the life expectancy of electrical wiring in a house? 

• What is an "open junction box"? 

• How dangerous is old electrical wiring? 

• What is a ground wire? 

• I heard that aluminum wiring is bad. How do you check for aluminum wiring? 

   Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

Diagram courtesy CodeCheck