How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manfuactured and modular homes

Is knob and tube wiring illegal?

Tuesday, March 12, 2019

Existing knob and tube wiring is not illegal and not required to be removed by the building codes. But because of the safety issues with a wiring type that is obsolete, ungrounded, and all original installations are at least 70 years old, insurance companies will not write a new policy for a home with K&T still in place. That’s the big problem with buying a house with knob and tube, since nowadays you must submit a four-point inspection report to apply for insurance for an older home and the obsolete wiring will be noted in the report.


    The National Electrical Code (NEC) devotes Article 394 - Concealed Knob-And-Tube Wiring to this wiring type. Here are the key limitations:

  • Can only be used for extensions of existing installations. No new construction, except by special permission from the AHJ (local building department)—which is extremely unlikely.
  • Not for commercial or hazardous locations.
  • Cannot be used in hollow spaces with loose, rolled, or foamed-in-place insulation. Must have clearance air around it.
  • Where the required clearance around the wires cannot be maintained, the individual wires must be enclosed in flexible non-metallic tubing.


    The Florida Building Code—Existing Building, 6th Edition (2017) does not directly address knob and tube wiring, but states at 607.1 that “existing electrical wiring and equipment undergoing repair shall be allowed to be repaired or replaced with like material."

    The most common defect we find when knob-and-tube is still in place in an older home is that it is buried under insulation, like in the photo below.

 
   To learn more, 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. 

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