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Knob-and-tube wiring, what you should know

Electrical Tip of the Day: Knob and tube electrical wiring was installed in homes from the 1920’s to the mid 1960’s – later in some rural locations.  As indicated by the name the instillation method used a combination of porcelain “knobs” and “tubes” as insulators to install the wire.  Here are the basic characteristics of a knob and tube wiring system:

  • No ground: Only a hot and neutral wire are used… the circuit does not have a ground conductor
  • Wire insulation: Electrical wires are wrapped in a rubberized cloth or ascroll fiber insulation.  In some cases a second rubber / fiber insulator was also slid over the primary conductors.
  • Connections or splices were made outside of electrical junction boxes, typically by twisting the wire at a “tap” point, soldering the connection, and insulating it with tape.
  • Hot and Neutral conductors separated on framing member:  Hot and neutral conductors were not in a common cable like modern romex wire.  They were separated onto opposite sides of framing members.

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What you should know.  Currently the NEC code and most local jurisdictions do not specifically say that knob and tube wiring is illegal, however they do have some very specific requirements if it is to be left installed and in use (NEC 2008 – article 394). Here are the basic guide lines that must be followed:

  • Over current protection to be on hot conductors only
  • Over current protection not to exceed 15 amps
  • No open splices – original splices may remain if soldered and insulated with tape
  • New extension wire connections to be made in junction boxes
  • Knob and tube wiring to be supported only by non-conductive stand off supports – it may not be secured in contact with combustible materials
  • Knob and tube wiring is only rated to be used in free air, un-insulated spaces.  It is not permitted to have knob and tube wiring covered or concealed by insulation of any kind

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So what makes knob and tube such a major safety concern today?   What it really comes down to is that it does not have a ground, the age of the conductor insulation, changes to the building or original installation (such as the way it has been spliced), and the addition of building insulation over or around the knob and tube.  Here are the key concerns:

  • No electrical ground: The circuit is only comprised of a “hot” and a “neutral” conductor – making it less safe than a modern grounded electrical circuit.  Devices that use a grounded plug (three prong) should not be connected on an un-grounded circuit, specifically appliance loads.
  • Damage : In most cases knob and tube is past the “safe” useable life span. Heat damage, cracked and rotten insulation, exposure to leaks, cracked or missing insulators, chewing rodents, or damage from being steeped on all add up over time.  We don’t find much knob and tube that is in safe useable condition.
  • New work and open splices: We often find improper “new” extensions added to original knob and tube circuits.  Electrical splices are to be made in an approved junction box per NEC code.  The very basic reason for this is to contain a fire if the junction fails.
  • Building insulation: The fire safety of knob and tube wiring relied on the fact that the wires were typically separated on framing members, suspended in free air between knobs, and passed thru combustible materials in ceramic tubes. Where original conductors were installed in walls or in attic floors, and where then later covered in building insulation, the knob and tube wires then no longer meet code.  They can become hotter than intended, and may be a fire hazard due to the proximity to combustible materials.
  • Switched Neutral: It was the excepted install practice at the time to switch the neutral conductor on a knob and tube circuit.  It poses a increased risk for homeowners to have switched neutrals.  A switch can be in the off position and still have full voltage at the load (like a light fixture). Switched neutrals on a circuit can also cause voltage irregularities that can effect today’s sensitive modern electronics.
  • Homeowners insurance: Most homeowner’s insurance policies have specific language regarding knob and tube wiring.  Is some cases they call for its removal in order for the policy to be valid.  In other cases there may be increased rates and/ or changes in coverage.  If your home has knob and tube wiring you will want to specifically ask your insurance agent how this affects your coverage.

So, that was a lot to digest.  If you have questions or concerns about knob and tube I would be happy to answer them for you.  Contact me here or thru the Artisan web page.

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