electricalwiring
The smooth one is coax. I'm asking about the braided ones.
They're power, running from a metal junction box in my basement ceiling up to an outlet just above the pictured floor boards.
It's an old house, with lots of knob and tube. The box where this originates is fed by EMT.
You're referencing the wrong code section.
Romex is covered by article 334 Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS.
Article 382 covers a cable known as FlatWire, or Concealable Nonmetallic Extension (CNE) cable.
Notice that this cable has a single insulation layer over the conductors. Whereas NM cable has insulation on the conductors, and an outer insulated sheath.
The discussion in the comments revealed that the "planned addition" as described is perfectly fine but that the only detail that really needed additional confirmation was the conduit fill calculations, specifically regarding derating.
Since I'm using a T-style conduit body to branch a raceway for two fixtures there is not room to have a code-compliant splice between the common junction box and the fixtures themselves. This then means that separate neutrals need to be pulled and that there will therefore be more than 3 conductors in the shared part of the raceway. I had initially read that derating didn't come into effect for small wires (#14 and #12) until a much higher number of conductors are involved, but it was a good exercise to truly confirm this.
The NEC requires derating for 3-or-more conductors in one raceway. My initial impression for #14 THHN was that it's rated to only 15 amps, which would then mean that any derating requirements could be an issue for this 15 amp branch circuit. Looking deeper tough it turns out that 15 amps is only the required overcurrnet protection for circuits with #14 wire (from NEC from 240.4(D)(4)) and that #14 THHN is actually rated slightly above 15 amps at low temperatures. In my case I needed to derate 80% (for 4-6 conductors), and for #14 THHN at low temps this worked out to 80% of 25 amps = 20 amps, which still appears quite fine for the existing 15 amp branch circuit that I'm working with. This handy online tool, and of course the appropriate tables from the NEC (from 310.15(B)(16)), confirmed this.
So it looks like this planned addition is fine as stated assuming a wire gauge <= #14.
Best Answer
It is unusual for single conductor with cloth braid to be used except for feeder wiring like that. Cloth braid was common up to the 50’s but normally there were 2 wires in the cable when run loose. knob and tube also ran single insulated wires down walls but normally maintained separation of the conductors and was not allowed to wander like or to use the same hole through a wooden member . so it may have been a diy job of old or a addition to the system,
many states did not have the strict rules we have today even though the national electric code was established pre 1900 it Was probably an addition. If both wires enter the metal box in the same hole that would be more evidence that it probably was a later modification.