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An AFCI is a great addition to K&T wiring. While the conductors in K&T are separated by large distances, and even studs, they do come together at junction boxes which are often metallic. An AFCI adds a layer of peace of mind to the situation.

Be sure to measure your K&T wire to determine gauge. It can take slightly more current than the modern equivalent at the same wire size, but to be safe stick with the modern values. You'll probably need a 2-pole AFCI, as K&T was often wired with a shared neutral. Two pole AFCI for knob and tube wiring Fora more complete writeup of K&T retrofit see https://diy.stackexchange.com/a/20279/5960

Note: when I replace a fixture in a K&T circuit I address the metal box weak point with some new loom, slipped over the wires as they come into the box. The old loom, which could easily be 50 to 100 years old, is sometimes but not always brittle. I'll also pull out the wire and wrap the exposed area with friction tape. This is the only point in the entire K&T system that even needs insulation.

Bryce
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