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Recently I found an article saying all Knob and Tube circuits need to be protected with a 15-amp breaker or fuse. I know the modern small conductor wiring rule (NEC 240.4(D)) limits 14 AWG to 15A for building-wiring and other general purposes, no matter the insulation type, but the statement startled me because the article didn't differentiate any particular wire sizes. I had never heard this before and had thought it was correct to protect 12 AWG with 20A breakers/fuses and 14 AWG with 15A. I searched the NEC and online but couldn't find a rule about protecting Knob and Tube wiring specifically.

The plot only gets thicker. In my research, I located a free download for the original 1897 NEC codebook, I discovered that back when these systems were first being installed in homes, when the wiring was brand new, they only trusted a rubber coated 14 "B.& S.G"(the older name for American Wire Gauge) to be safely protected at 12 amps, not 15! Likewise, a rubber coated 12 AWG was supposed to have been limited to 17 amps.

How many electricians know this? I sure didn't. I suspect most electricians are going along, doing panel upgrades and, if they aren't completely rewiring, they are putting portions of these older circuits on modern protection, using rules meant for modern thermoplastic insulated conductors. K&T often has crusty, brittle insulation that must be carefully managed when splicing to new branch circuitry. Folding the last accessible few inches of Knob and Tube conductors into a junction box usually needs a lot of tape or shrink tubing to keep it intact. (By the way, does anyone know the exact requirement or best method for doing this? I realize it is supposed to be sleeved with non-metallic tubing from its last support before entering the box with the sleeve clamped inside the box and showing its last 1/4-inch, but what about keeping the insulation from falling off? Would electrician's tape or shrink tubing on the last six inches nullify the original rating of the insulation?)

Please straighten me out, if I need to be, because this seems like a major concern. Does anyone know of a specific NEC (or other official) allowance saying we can use lesser protection for an old Knob and Tube rubber coated conductor that is already doubtful? Or, should we rather find some way of keeping them protected the way they were originally intended, at 12A and 17A respectfully? I once saw an overcurrent protective device with a switch. It was apparently meant to snap into a modern breaker panel, but instead of breaker components, it was a fuse-holder. I don't remember what brand it was. Are these available for any manufacturer?

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    Think back in 1897 they probably did not have much testing data to go on and the quality of gauge size was not has good as todays. NEC has decades of testing to go by now. Most safety standards have a factor built in. Imagine 14G is safe at 15 amps, but the amperage it will start burning is higher(maybe 25 or30 amps).
    – crip659
    Apr 2 at 23:09

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but what about keeping the insulation from falling off?

As you point out, NEC does not allow continuing in service any wiring which has been degraded.

Or, should we rather find some way of keeping them protected the way they were originally intended, at 12A and 17A respectfully?

10A breakers are in the catalog. Any auxiliary overcurrent protection would need to be UL listed.

Recently I found an article saying all Knob and Tube circuits need to be protected with a 15-amp breaker or fuse.

the statement startled me because the article didn't differentiate any particular wire sizes.

People say stuff. This person's logic is probably that "it's hard to tell #12 from #14".

The plot only gets thicker. In my research, I located a free download for the original 1897 NEC codebook,

Wrong edition. You need to follow the edition that was in force when the permit was pulled. They may have raised the ampacity from 1897 to 1921 as better science came in.

K&T often has crusty, brittle insulation that must be carefully managed when splicing to new branch circuitry.

Most K&T is ungrounded. Can't extend an ungrounded circuit.

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    I was hoping you'd answer this. Do you know where I can get a hold of the old code books? I found a few archives on NFPA.org, but nothing that old. Apr 3 at 1:11
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An AFCI or preferably AFCI/GFCI breaker (at 15A)(or a standalone AFCI and a standalone GFCI on modern wiring at the head of the circuit) would give me a lot more sense of "treating this safely until it can be replaced" than a 12A fuse.

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