K is the resistance of an imaginary conductor one foot long and one circular mil in area. Here is where the number comes from, how to use it, and where it stops being trustworthy.
Resistivity of annealed copper at 20 °C is about 10.37 Ω·cmil/ft. Conductors run hot: correcting to 75 °C with copper's temperature coefficient — R₇₅ = R₂₀ × (234.5 + 75)/(234.5 + 20) ≈ R₂₀ × 1.216 — gives 12.6, and the conventional K = 12.9 adds a small allowance for stranding (stranded wire is slightly longer than its run). Aluminum starts near 17.0 Ω·cmil/ft and lands at K = 21.2 by the same path. The ratio 21.2/12.9 = 1.64 is the famous "61% more resistance" rule.
Drop: Vd = 2 × K × I × L / CM. Sizing (the reason K exists): CM = 2 × K × I × L / Vd — solve for circular mils, round up on the AWG chart. Example: 30 A, 150 ft, 240 V at 3% (7.2 V allowed): CM = 2 × 12.9 × 30 × 150 / 7.2 = 16,125 cmil → next size up is 8 AWG (16,510 cmil). The solver agrees.
Full formula context on the formulas page.
No — it's a derived industry constant consistent with NEC Chapter 9 Table 8 data at 75 °C. The NEC publishes the resistance table; K is the exam-room and tailgate shortcut built from it.
Resistance at 90 °C runs ~5% above 75 °C, so K ≈ 13.5 copper / 22.2 aluminum — rarely used, since terminations usually cap designs at the 75 °C column anyway.