Reading the numbers

An AWG gauge fixes the conductor diameter, and from there everything else follows:

d = 0.127 mm \times 9 2^{(36 - n) /39} A = \frac{π}{4} d^{2}

Copper resistance per unit length is then $R / ℓ = ρ / A$ with $ρ = 1.724 \times 1 0^{- 8} Ω \cdot m$ . Note a run carries current and back, so voltage drop uses twice the one-way length.

Ampacity itself is empirical, not a clean formula — it depends on bundling, insulation rating and airflow. The two columns follow the common reference values: chassis wiring (single conductor, open air, short runs) is generous; power transmission (bundled, in conduit, long runs) is conservative. Pick the one that matches your build and keep margin.

Wire Ampacity Calculator

Reading the numbers