Knob and tube wiring with illegal extension (C) Daniel FriedmanKnob & Tube Electrical Wiring
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  • KNOB & TUBE WIRING - CONTENTS: How to inspect, troubleshoot, & repair knob & tube electrical wiring.What is knob and tube wiring? Is knob and tube electrical wiring safe? How do I repair knob and tube wiring?
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Guide to knob and tube electrical wiring: This article answers basic questions about Knob and Tube electrical wiring. We define knob and tube wiring, we include photographs that aid in recognition of this generation of electrical wiring, and we describe both proper and improper K&T wiring installations, repairs, or circuit extensions.

This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe electrical conditions. Our page top photo shows a home inspection client pointing out knob and tube electrical wiring in an older home.

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Knob-and-tube wiring in older homes: description, inspection, repair

Knob and tube wiring with illegal extension (C) Daniel Friedman Knob and tube electrical wiring detailed photo (C) Daniel Friedman

Knob and tube electrical of wiring has been installed in homes from the 1920s right up into the 1970's in some jurisdictions. Our photos above will assist homeowners and inspectors in recognizing this wiring method.

Article Contents

Characteristics & Safety Considerations of a knob and tube electrical circuit

Ceramic tubes used in knob and tube wiring at Justin Morrill Smith home (C) Daniel Friedman

  • No ground: Only a hot and neutral wire are provided.
    Watch out: A knob and tube electrical circuit has no electrical ground path.
  • Wire insulation: The individual electrical wires are wrapped in a rubberized cloth. That was fine when the wires remained suspended in air and had not been chewed by a squirrel.
  • Knob and tube wiring connections or splices were made outside of electrical junction boxes. In normal practice knob and tube wiring splices are soldered and also taped.

    We describe these splices at "Taps" in ELECTRICAL SPLICES, HOW TO MAKE.
  • Ceramic tubes to route knob and tube electrical wires through building framing members:

    Wherever the knob and tube circuit wires pass through building framing lumber a ceramic tube is used to insulate the wire from the wood.

    These thick ceramic tubes are a good insulator separating electrical wire from wood - if both are un-damaged. Our photo at left shows knob and tube electrical wiring in the Justin Morril Smith historic home in Strafford VT. Also see our earlier photo at above right where electrical wires pass through a wall top plate in a 1920 home.

Ceramic tubes supporting knob and tube electrical wiring Justin Morrill Smith house Strafford VT (C) Daniel Friedman
  • Ceramic knobs to support knob and tube electrical wires passing over building framing surfaces: Where the knob and tube wiring is surface mounted in a building it is attached using a ceramic "knob".

    These ceramic knobs are a good insulator provided they have not been damaged or modified.
  • Our photo at left shows knob and tube electrical wiring in the Justin Morril Smith historic home in Strafford VT. Also see ceramic tubes supporting knob and tube circuits in other photos and sketches on this page.

 

 

Illustrations of Proper and Improper Knob and Tube Wiring Details

Sketch of knob and tube electrical wiring connetion methods (C) Carson Dunlop Associates

The sketch at left, courtesy of Carson Dunlop, shows the usual ways that knob and tube electrical wiring is connected in homes.

  • Knob and tube electrical circuits are not "illegal" and there is not a code requirement that they be replaced. However this wiring method is considered obsolete.

  • No electrical ground is provided - the circuit is less safe than a modern grounded electrical circuit and appliances and devices that use a grounded plug should not be connected on an un-grounded circuit.
  • The knob and tube wiring may have become damaged by age, exposure to leaks, or to chewing rodents. In attics, for example, we often see that this wiring has been damaged by having been stepped-on or by chewing rodents.
  • The safety of the knob and tube circuit may have been affected by building changes such as adding insulation (discussed next) and modifications to the original circuit (discussed below).

Effect of Building Insulation on Knob and Tube Electrical Circuit Safety

Knob tube wiring safely routed in air (C) Daniel Friedman T Hemm Knob and tube wiring less safe where insulated and maybe damaged (C) D Friedman T Hemm

  • Adding building Insulation changes the knob and tube wire game: The fire safety of knob and tube wiring relied on the fact that the wires were generally routed through the air, suspended by knobs and protected by a heavy ceramic tube where passing through wood.

    The two photos above, courtesy of Tim Hemm, show a knob and tube circuit that has not been covered with insulation (though we wonder about the significance of those leak stains on the ceiling joist forming the attic floor), while the knob and tube circuit in the right photo has been partly buried in blown-in cellulose insulation.

    We pose that the same insulation project may have filled wall cavities with insulation too, possibly leading to overheating of knob and tube circuits that run in the building's exterior walls. We're also concerned that where knob and tube wires have been run in an attic floor and later covered by insulation, there's a good chance that someone walking in the attic has stepped-on and damaged the wires - a condition we've found often.

    Where such wires were routed in walls or in attic floors, and where later those building cavities have been insulated, the knob and tube wires are no longer suspended in air, can become hotter than intended, and may be a fire hazard for that reason.

Safety & Electrical Code Concerns with Knob and Tube Electrical Circuits that Have been Extended or Spliced-into

Our photo at below left shows a combination of errors, extending a knob and tube electrical circuit and a twist on connector electrical splice outside of an electrical junction box. The photo also shows three types of electrical circuit wiring: knob-and-tube, armored "BX" cable, and plastic cable wires. The photo shows therefore three generations of electrical wiring (and probably other modifications) in this building. Carson Dunlop's sketch at below right shows how these circuits might be extended.

Illegal extension of knob and tube electric wiring (C) Daniel Friedman How to extend knob and tube wiring if permitted (C) Carson Dunlop Associates

  • Often the knob and tube circuit has been modified or extended by subsequent building owners/occupants - an improper practice that is not permitted in most codes.

    Our photo at the top of this page shows an inspection client pointing to a modern plastic NM electrical cable that has been used to extend an older knob and tube electrical circuit.

    In some jurisdictions, extending an existing knob and tube circuit is not recommended, and is even an illegal installation: by adding load to the knob and tube circuit, risks increasing the temperature of the wiring and possibly causing a fire.

Advice about improving the safety and reliability of knob and tube electrical wiring

  • Inspect the whole electrical: An expert should inspect the condition of the building electrical wiring, including the wires, connections, devices like receptacles, switches, and overcurrent protection by fuses or circuit breakers.
  • Replace bad circuits: Knob and tube circuits that have been modified, damaged, or covered with insulation should be replaced with a modern grounded electrical circuit.
  • Ground fault protection GFCI circuit protection and possibly arc fault protection can be added on two-wire un-grounded electrical circuits to reduce the chances of electrical shock or fire - steps that we recommend.

    See AFCIs ARC FAULT CIRCUIT INTERRUPTERS. (A homeowner cannot safely test a GFCI while it is in place, installed on an electrical circuit that has no ground but the GFCI can still be expected to work correctly if it is wired properly.)

 

Continue reading at OLD HOUSE ELECTRICAL WIRING or select a topic from the More Reading links shown below.

or see FALSE GROUND at RECEPTACLES

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Frequently Asked Questions (FAQs) about knob & tube electrical wiring inspection, troubleshooting, repairs, extensions

Question: the circuit I was working on does not seem to be connected to a circuit breaker - it only turns off when main power is off

Our home has a combination of 14/2 and knob and tube wiring - with an updated circuit breaker box. I wanted to change a kitchen light fixture and when I went to shut-off the breaker found that the light I wanted to change wasn't connected to a breaker? When I shut-ff the main breaker,the light circuit was off? Don't understand - help! -Mike - Joyce.k.smalley@gmail.com - 8/12/2011

Reply:

Mike - Joyce, this is not good, and if you cannot quickly see what's going on I strongly recommend hiring an experienced electrician to trace down your wiring.

From your description, and assuming you correctly identified your main breaker and have only one such in the home and panel, then I suspect your main breaker is defective and did not turn off internally.

Here are some other possibilities:

1. Switch off the MAIN circuit breaker in the home. If that does NOT turn off ALL power in the home then one of these problems is happening:

- you have not correctly identified the main breaker, or in some older panels, there may be multiple "Mains"

- your main breaker is a brand that often fails to actually turn off inside even if the switch is in the "OFF" position, such as FPE Stab-Lok (search our website for Federal Pacific Electric ) for details.

- your home has more than one electrical service or (as I have found on some old buildings in very dense urban neighborhoods, usually now used as businesses) on occasion someone has wired one or more circuits in a building by "borrowing" a circuit or power from a neighbor.

If on closer inspection your MAIN breaker turns off the light in question, but the individual circuit breaker that you thought controlled that light does not, then either you've not correctly ID'd the individual circuit breaker, or as I described above, the breaker is defective and is not turning off the circuit in question.

Question: how would you track down knob and tube in the walls of an old house

If you were an insulation person, how would you track down knob and tube in the walls of an old house so you could generally ascertain which bays NOT to insulate. I am thinking you begin in the basement -- though some of our insulation guys start in the unfinished attic and go down. I am a novice at electricity -- and having trouble identifying a proper plan of "attack." - MBT 9/7/2011

Reply: Tracking down just where knob and tube wiring is found in a building: tips

There are several electrical test instruments that can spot metal running through a wall, including both metal pipes and metal wiring. The trouble with the idea of leaving some bays uninsulated to avoid overheating the K&T wiring is that in walls wiring typically runs horizontally - through most or all stud bays.

I'm not sure, other than tracing wiring, how you could reliably infer that wiring in building cavities is the original knob and tube or if some areas have been re-wired. Certainly if you open a junction box and see that there is a ground wire entering the box, or if you see armored cable entering the box, at least right there it's not knob and tube.

But we may not know exactly where new circuits were installed and where old ones remain.

For example, I would not rely on seeing grounded receptacles as an indication that the wiring is newer than K&T. It's just too easy for someone who is uninformed about good practice to have installed grounded electrical receptacles even though the circuit may not include a ground (and may be knob and tube).

In sum: make visual inspection of the entire building to identify where knob and tube wiring is visible, trace those circuts as far as you can into individual rooms; use instruments (such as the Tic Tracer) to trace down wiring paths in walls, ceilings, floors.

Reader follow-up:

Thanks, DanJoe for your suggestions. I had been told that there was also something that you could plug into the electric panel --then go around the house to see where knob and tube (or at least ungrounded stuff) was. Have you heard of this -- and do you recommend it?

Reply:

MBT there is a variety of circuit tracing tools including what you describe. The simplest is a stand-alone device like the tic tracer by Tif that senses electrical wires in walls; schemes using transmitters also work though where I've seen them used more often it was to track down a specific circuit. Yes, it's a useful approach.

Question:

Do you know what they call the knob and tube ceramic junction box above the light bulb fixture that supplies neutral & hot to a drop light bulb fixture. My teacher said the word starts with R - Ray 10/21/2011

Reply:

Electrical junction box.

Question:

Love your site.

I am in Los Angeles in a house that was built in 1921. It has knob and tube and is still running electricity that way. We've had a licensed electrician come and inspect all the wires, install a brand new breaker box and ground all the outlets. We asked him if cellulose could be blown in over the knob and tube in the attic only and he said that'd be fine because the wires are still in good shape.

I called an insulation company with experience in knob and tube and they said it's fine, as well. Especially because cellulose is non-flammeable.

I still have reservations, though. A lot of forums say to replace the knob and tube completely.

Should I be worried? We are planning on only blowing in the cellulose in the attic, not the walls.

Thank you! - Richard 10/28/2011

Reply:

x

Question: is it ok to run romex wire off of a knob and tube circuit?

isit ok to run romex off knob &tube - Jack 7/11/2012

Reply: No. Do not extend knob and tube electrical circuits

Mixing knob & tube with romex is not safe - Bruse

k&t should never be spliced into to extend the circuit. When k&t was around it was used primarily only for lighting and other light load on the wiring. Don't even think of using it for bathrooms, hair dryers, coffee makers, kettles and toasters, etc. With the age of the wiring (splices were taped), the splices are brittle and often deteriorated.

Adding a new circuit as illustrated above can be very dangerous, depending on new load added to the circuit. Splices overheat and can be a potential fire hazard. Many insurance companies in Quebec Canada will not insure a home with k&t or charge and insane premium for it.

If they do insure it, they demand that the system be inspected by an electrician prior to insuring the home. I believe in Ontario Canada if you have a home with k&t it has to be replaced, they will not insure your home! (Can someone confirm this?). The insurance companies are doing this due the the higher risk of fire with homes that have k&t (statistically).

I personally would never add a circuit modify the wiring or cover it with insulation. It can be performing well right now, but the modification can over heat the wiring and cause a fire. I would make a plan to slowly replace the wiring. If insulating an attic replace the wiring, its just safer, why take a chance.

This is my personal opinion based on past experience. Always consult an expert. - Kosta (professional home inspector)

- Kosta Trivlidis Home Inspector (AIBQ) National Home Inspector (NHICC) CSI Inspections Inc. csiinspections.ca kosta@csiinspections.ca

Thanks Kosta, we agree that extending K&T electrical circuits is improper. Most jurisdictions permit homes to continue to use K&T circuits but as you will see in the article above, even though as originally installed (widely separated hot and neutral wires, ceramic knob and tube support with no direct wood contact, suspension in air for cooling) made these circuits perform well, in older homes where conditions have changed, the circuits, even legally in place, may not be as safe and reliable. Reasons that K&T circuits cannot and should not be extended include:

  • the absence of an electrical ground
  • the added electrical load on an old circuit whose wiring, particularly where it passes thorugh hidden spaces, may no longer be as fire-safe as when new
  • electrical code prohibitions

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