American Wire Gauge runs backwards: 14 AWG is a small branch-circuit wire, 1/0 is a hefty feeder, and beyond 4/0 the naming gives up on gauge numbers entirely and switches to thousands of circular mils — 250 kcmil means 250,000 cmil by definition. A circular mil is the area of a circle 0.001 in across, chosen so that area in cmil equals the mil diameter squared with no π involved, which is exactly why the classic K-factor voltage-drop formula can divide directly by the cmil area.
The scale has a useful internal logic: every three gauge steps halves the resistance, and every ten steps changes the cross-sectional area by a factor of ten. The physical constants — cmil area, stranded diameter, and DC resistance (3.14 Ω/1000 ft for 14 AWG copper down to 0.0608 for 4/0, at 75 °C) — live in NEC Chapter 9, Table 8 and feed voltage-drop and fault-current calculations; copper resistance also shifts about 0.39% per °C, and for 250 kcmil and up skin effect makes the AC values in Table 9 the ones to use.
Current ratings are a separate table with separate rules: NEC 310.16 assigns each size its allowable ampacity by insulation temperature column, and a famous small-conductor quirk applies — 12 AWG copper shows 30 A in the 90 °C column yet NEC 240.4(D) caps its breaker at 20 A, with the higher table number surviving only as the starting point for derating.