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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These photos above will assist homeowners and inspectors in recognizing the knob & tube electrical wiring method as well as common safety defects for which an inspector should be alert.
The photo shows knob and tube electrical wires passing through a wall top plate in a 1920 New York home.
[Click to enlarge any image]
The sketch above, courtesy of Carson Dunlop Associates, shows the usual ways that knob and tube electrical wiring is connected in homes. Below: knob and tube electrical wiring, including the ceramic components, fasteners, and the electrical conductors themselves may look as if they're in great shape.
That's what I think of the K&T photo just below. But beware: the electrical wiring you see at first glance may be in terrible condition somewhere else in the building. Here are some things you need to know about knob and tube electrical wiring.
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.
As with any older electrical wiring but particularly where wiring is exposed such as in an attic floor, the wiring may have been mechanically damaged by foot traffic, building leaks, building movement (earthquakes for example) or by inattentive electrical or other contracting work.
Beyond the obvious snafus like broken wires or broken, lost ceramic knob and tube supports and insulation, look more carefully at wiring for cracking or damaged insulation.
Look also at the wiring support wherever there is visual access to do so. That may tell you to be more or less worried about the wiring sections that you cannot see.
Above: running this old electrical conductor over a nail to "hang" an attic light fixture is likely to damage the conductor over time - it is unsafe.
Look to the right of the "light pendant" at the plugs and sockets used to connect additional knob and tube circuits running along the roof: someone who's not an electrician has been at work here, raising a red flag suggesting more thorough inspection of the electrical wiring.
Below some had a similar attic lighting idea but worse: the light is suspended by hanging it from a knob and tube wire itself.
The second knob and tube wiring damage photograph below shows some of the cracks in the light wire insulation.
But more, knob and tube conductors are not intended to be used to hang the laundry, towels, nor other light fixtures.
Below: the original electrical fuse panel that powered a knob and tube wired Poughkeepsie, New York home is shown before and after we opened its panel door to have a look inside.
and now the mouse hotel:
K&T wire probably should be replaced if easily accessible such as in an attic or basement, or if the walls or ceilings are opened for other reasons.
It is highly recommended and K&T wire should be replaced if:
Ignoring these recommendations is not [a violation of the national electrical code], nor is it illegal, but is risky, unsafe, and potentially dangerous.
K&T (knob and tube) wiring has several major inherent disadvantages compared to modern wiring.
First, it has no third wire ground conductor to protect the user in case of an internal fault within an appliance that a user may plug into a receptacle.
Second, the wires and receptacles are not polarized. This means that the larger shell of an incandescent lamp or the chassis of a hifi system or computer could be at the 120 volts potential of the hot wire instead of the near zero volts of the neutral conductor.
Both of these conditions could lead to a user being shocked or electrocuted which would not happen with normal more modern polarized and grounded electrical system.
With age, the rubberized cloth insulation on K&T may decay with mold, may dry out and fracture, and sometimes even falls off, potentially exposing people to hot electrified wires.
Old K&T is legal in the sense that it has been grandfathered in. Codes do not mandate retroactive replacement. However, it is absolutely not permitted or legal to install [new knob and tube circuits] today, and in many or most locations, it cannot be extended or modified.
K&T wiring is probably okay if
- Ozzie, by private email 2017/03/31
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.
The contractor who blew cellulose loose-fill into the attic shown above he wanted to make the home warmer. You can see that covering the knob and tube electrical wiring makes it easy to step on it while clambering around in a dim attic: damaging the wiring, causing a fire, an event that will certainly warm the home. Consider also what two-word expletive you'll grunt while falling through the ceiling after you trip over a "hidden" electrical wire.
Where knob and tube electrical 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.
Model electrical codes do not prohibit the presence or use of knob and tube wiring circuits in buildings, but you will want to be sure that the specific knob and tube wired circuits in your building are in good condition, safe to use, and that the circuit has not been improperly modified.
Below is a photo of knob and tube wiring supporting a light fixture - well not exactly. If readers are not sure why this wiring makes me nervous use the page top or bottom CONTACT link to tell me what you think about running an old brittle light wire conductor over copper plumbing pipes.
Often the original knob and tube wired electrical circuit has been modified or extended by subsequent building owners/occupants - an improper practice that is may not be permitted depending on codes that apply where the building is located. Carson Dunlop's sketch, above illustrates how knob and tube circuits are often extended. Image provided courtesy of Carson Dunlop Associates, a Toronto home inspection & education company.
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.
Watch out: for improperly abandoned knob and tube electrical wiring, knob and tube circuits that have been extended to include new circuits and devices, damaged knob and tube wire and wire insulation, knob and tube wire that has been insulated-over or around, changing its heat rating and perhaps creating a fire risk, and other K&T damage or defects that make the wiring system unsafe.
Our photos below show a combination of errors, extending a knob and tube electrical circuit and a twist on connector electrical splice outside of an electrical junction box. Immediately below is a DIY extension of an existing knob and tube electrical circuit.
Extending an existing K&T circuit to add more circuits, wires, devices, switches, lights, receptacles is not permitted. The concern is that you're going to overload an old electrical conductor, causing overheating and a fire: a risk that is increased if the wiring has been insulated-over or has been gnawed bare by rodents.
Below: this knob and tube circuit has been extended by running it into an electrical box.
Above: the knob and tube wiring circuit has been run to an electrical box where it is used to extend to additional circuits in the building. Then there's more: the DIY wiring, missing box cover, overcrowded electrical box, unprotected knob and tube conductor run through the electrical box opening, damaged wiring conductors, exposed bare wires at splices, possible evidence of wire overheating, and more. [Click to enlarge any image]
The photo below 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.
Below: take a close look at this photograph and note the blackening of an old sill plate and ceramic knob. I think this electrical circuit may have been exposed to fire. If so the wire itself is likely to have suffered damage and may thus be unsafe.
Above: new knob and tube electrical wiring applied to a lighting circuit in Japan.
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