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How to Identify Knob and Tube Wiring

white ceramic knobs attached to joists and white ceramic tubes through joists, with wires running through knobs and tubes

Does your home have knob and tube wiring? These open wire systems were added to homes with the first wave of residential electrical installations of the early 1900s through the 1940s, though some jurisdictions allowed this type of wiring as late as the 1970s.

If you’re considering buying an older house, you need to know how to identify knob and tube wiring.

Keep reading to learn about the safety issues and challenges you may face when purchasing or residing in a home with knob and tube wiring.

What Does Knob and Tube Wiring Look Like?

Knob and tube wiring is still present in many older homes. It is easy to identify. Look for knob and tube wiring in your basement or attic. You can identify it by its white, ceramic, spool-like knobs and tubes. Electric wires snake through the knobs, which support individual wiring strands. Heavy ceramic tubes protect wires where they run through joists.

Don’t See Knob and Tube Wiring? That Doesn’t Mean You Don’t Have It

Unfortunately, it's not uncommon for knob and tube to be hidden behind in the walls of homes. Dubbed “spider webbing,” knob and tube wiring is frequently hidden by previous homeowners (intentionally or otherwise). The only ways to identify it in these instances is by making holes in walls or sending a tiny camera inside the walls to look around.

Is Knob and Tube Wiring Safe?

While knob and tube electrical wiring is not illegal, it is an obsolete wiring method. Home wiring in the early 1900s was not designed to handle the glut of electronics used today. Even 50 years ago, most homes did not have air conditioners, microwaves, or dishwashers, let alone computers, home theater equipment, and personal electronics. The heavy electrical load from these devices strain knob and tube wires, pulling too large a load for safe operation and resulting in a fire hazard.

And that is not the only danger of knob and tube:

  • No Electrical Ground
    Knob and tube wiring only has two wires: a hot and a neutral wire. This lack of a grounding wire creates an enormous safety risk for your appliances—and you.
  • Compounding Damage (and Dangers) with Age
    Knob and tube wires are wrapped in rubberized cloth. Over time, that sheath becomes brittle, cracks and falls off, leaving wires exposed to create both a shock and fire hazard. Exposure to water leaks and animal damage further compound the wire exposure safety issue.
  • Building Changes Affect Safety Knob and tube wiring was designed to remain suspended in the air and protected by heavy ceramic tubes. Certain changes to the house, however, can disrupt this structure, including:
    • Added Insulation
      Adding insulation in attics and wall cavities can lead to the overheating of knob and tube wires, which were designed to remain suspended in open air. Covering them also commonly leads to damage from wires being unknowingly walked on by persons moving around in the attic.
    • Improper Connections
      Knob and tube wiring is a simple, often accessible system. This has led many homeowners to believe they can alter the wires themselves. Improper connections and splices, including taping and soldering additions without the use of junction boxes, greatly compromise safety. Modifications and extensions such as these are not permitted by most building and electrical codes.

Can I Get Homeowner’s Insurance with My Knob and Tube Wiring?

Is knob and tube wiring an insurance issue if you’re buying or selling an old house? Yes, it is. Most insurance companies will not insure or renew homeowner’s insurance policies on houses with knob and tube wiring. Therefore, it can be nearly impossible to secure financing to buy a home with knob and tube wiring. But if you’re not concerned about insurance or financing, you may be able to make the knob and tube wiring safe without going through the expense of having it removed entirely and replaced with modern wiring.

To make sure your knob and tube is safe:

  • Get a professional inspection.
    Have a licensed electrician verify the condition of the wiring, connections, receptacles, switches, and fuses or circuit breakers.
  • Replace bad circuits.
    Circuits that have been damaged, incorrectly modified, or covered with insulation should be replaced with a modern grounded electrical circuit.
  • Add protection to reduce shock and fire risk.
    Installing ground fault protection (GFCI) and possibly arc fault protection (AFCI) can enhance the safety of your knob and tube wiring. While the GFCI may be ungrounded, it will still be able to sense excess electrical flow and cut power to the outlet, thereby preventing electric shock. An AFCI circuit breaker can prevent house fires caused by loose knob and tube wiring connections.

Do you suspect you have knob and tube wiring in your home? Ensure your family’s safety with the help of Mr. Electric®. Schedule your residential electrical inspection online today. Or call us at (844) 866-1367 and a member of our team will connect you with your local Mr. Electric.

For further reading, from Mr. Electric:

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Lint-filled dryer vents may be another unseen danger behind your home's walls. Read about fire prevention through professional dryer vent cleaning from our fellow Neighborly® home services brand, Mr. Appliance.