$100 Reward

We have all learned that knob & tube wiring may not be covered with thermal insulation. The usual explanation is that it was designed to be installed in open air in order to dissipate heat. The problem that I see is that every reference I have come across is in retrospect, for example, a contemporary publication stating that K&T HAD been designed to dissipate heat… I have searched high and low for some proof of this. I believe it is plausible that K&T was developed because it was a fast, efficient way of installing single conductors with cotton cambric insulation. That being in open air facilitated soldered splices and protected conductors from abrasion and physical damage. I believe it is plausible that the K&T design has little to do with heat dissipation / ampacity.

The inachi article on K&T indicates the K&T has a higher ampacity than other conductors because of it’s open air design. Many HI’s would state that a #14 AWG copper conductor should be protected at 15 amps, regardless of it’s true ampacity and insulation composition. Also- what effect do long lengths of cambric loom have on the ampacity??

Now don’t get me wrong, I’m not looking to bring back the glory days of K&T. But this has bugged me since it first became in issue in the 80’s. Lots of people regurgitating something they heard from someone else who kind of knows. Never, in 26 years (including 13 as an AHJ) have I seen proof of this purported design limitation. Yes the NEC began to address it in the 80’s, but from what I can find on my own, the claims of the fire hazard seem to be unfounded, or at the very least, unreported or unpublished.

I would love to obtain an old electrician’s handbook, code book, electrical design book etc. that clearly states the K&T must be installed in open air for the purposes of cooling. Maybe an old patent or such.

I will pay $100 for an original, definitive publication. If you have and old document like this and would like to sell it, email me.



This is from the 1987 NECH. Under Uses Not Permitted it mentions heat dissipation in insulation.


I know that in my area of the Canadian Atlantic provinces there are thousands of homes over 90-100+ years old that have operating K&T in insulation…It never has become an issue here…yet!

Robert- I’m well aware of the NEC pubs. What I’m looking for is a publication from say… 1920, supporting this widely held, but so far historically unsubstantiated belief.

The original 1899 NEC states-**
18. Wires – Continued.
***rigidly *at least eight inches from each other, except within the structure of lamps, on hanger-boards, in cutout boxes, or like places, where a less distance is necessary.
e. Must, on side walls, be protected from mechanical injury by a substantial boxing, retaining an air space of one inch around the conductors, closed at the top (the wires passing through bushed holes), and extending not less than seven feet from the floor.

The knobs that were used kept the wire off the studs to allow the wire to dissipate the heat. It would have been easier to staple them to the wood like we do today. They knew that the insulation on the wire gets brittle from heat.

Interesting… but that was still in the infancy of residentail wiring and learning about this newly harnessed energy transferred in metal wires…

Many have said that they have not heard of problems of insulation and fires from K&T…could they have been overly concerned about the new untried systems back then?

Robert E, your excerpt and the balance of the 1899 NEC still says nothing about the disipation of heat as it relates to ampacity of the conductor. The spacing only refers to separation from combustible materials (which I’n not challanging). Even the except is refering to 1" spacing in “boxing” as protection from physical damage exposed on walls less below 7 feet. Again, I believe this is a clearance from combustibles issue, as most old “boxing” is wooden.

Ok here is more from the 1896 Meeting of the [FONT=Times New Roman][size=4]NATIONAL CONFERENCE ON STANDARD ELECTRICAL RULES[/size][/FONT]
[FONT=Times New Roman][size=4]
[/size][/FONT]Board of Fire Underwriters code. This table adds 25 percent to the first nine currents in
concealed work, and represents an elevation of temperature amounting roughly to about 14
degrees C. or 25 degrees F. at steady full load.
The recently issued rules of the British Board of Trade call for a maximum temperature
elevation of 30 degrees F., or about 17 degrees C., and are equivalent to a still further increase in the permissible current strengths, and a corresponding reduction in the margin of safety.
**Mr. Stone: **I would like to ask if there is any difference in the matter of disintegration or
injury to the insulation of the wires by any increase in the heat above 16 degrees C. or 25 degrees F.?
**[FONT=Times New Roman]Mr. Kennelly: **I am not aware that there is any danger at a temperature below that at
which you can hold your bare hand upon a wire.
**Mr. Greene: **It seems to me that this question resolves itself into a question of what is a
safe current carrying capacity of wires. Certainly, from the information given by Mr. Kennelly
the carrying capacities as shown in column 1, are very conservatively rated and there is a large
factor of safety, considerably larger than is the case with the capacities given in table 2

I think it would be much more satisfactory to everyone. I think it is quite proper that there
should be a distinction between open and concealed work, because it is a well known fact thatthe conductor can carry a larger amount of current when “open” than when concealed in a moulding, with the same temperature rise in each case.

[FONT=Times New Roman]The heating of the fusewill depend upon its resistance. Its contacts, which often afford a variable element and the rate of getting rid of heat will depend upon the size of the fuse and its contacts and the temperature. The time of melting and the current strength will also depend on the temperature of the surrounding air. A fuse in a boiler room, where the surrounding temperature is high, will melt with a small current.

[FONT=Times New Roman][FONT=Times New Roman]On ordinary wires, the heating limit is determined by the insulation on the wire. When we had
paraffin insulation, it was desirable to run the wires cool; with the present grade of wires they
may be run warmer. We would like to have insulation not affected by heat, so that they might be run warmer than is now proposed.


Fascinating stuff.
I have a couple books from 1910 and 1913 where both mention Knob and Tube being on its way out because of the NEC even though it costs 1/2 to 1/3 that of conduit.

The wire needs to stay away from moisture also and insulation gets wet.I doubt they had moisture barriers under insulation back then if there was insulation at all.


Go to Google Books and punch in keywords with Knob and tube plus install or something similar to find more.I saved 2 PDF versions which you can do on the site.
Here is the 1913 book from above.Electric wiring and lighting …: Part I–Electric wiring By American School (Chicago, Ill.), Charles Edwin Knox, George Carl Shaad
Just give me a kind word if you find what you are looking for.
P.S if they are made to disperse heat you would not want to cover them with flammable insulation…now would you.:slight_smile:


Knob and tube abolishment.jpg

OK- so here the NEC is acknowledging K&T being covered with thermal insulation with absolutely no reference to heat dissipation or ampacity adjustment.

Excerpt from 1930 NEC

Article 502 - Concealed Knob & Tube

J. When thermal insulation is used in the hollow spaces of walls and ceilings in which concealed knob-and-tube work is installed, only approved non-corrosive, non-combustible, non-conducting, materials shall be used, and these shall be applied in a manner not liable to place a strain upon the wires or supports.