AWG Conductor Geometry (Diameter, Circular Mils, mm^2)
The bare-conductor size behind a gauge number, from the AWG definition: d = 0.005 x 92^((36 - n)/39) inches, so every 6 gauges roughly doubles the diameter. #12 AWG is 0.0808 in (2.05 mm), 6,530 circular mils, 3.31 mm^2; 4/0 is exactly 0.460 in and 211,600 cmil. Circular mils are (diameter in mils)^2, the diameter-squared area unit NEC tables use; the mm^2 is the true pi (d/2)^2 cross-section. Bare copper/aluminum, solid-equivalent -- not the over-insulation diameter (see conduit-fill). A reference; the manufacturer's conductor dimensions govern.
Formula and source
d_in = 0.005 * 92^((36 - n)/39); d_mm = 25.4 d_in; area_cmils = (1000 d_in)^2; area_mm2 = pi (25.4 d_in / 2)^2. Aught sizes 1/0..4/0 use n = 0, -1, -2, -3.
The American Wire Gauge geometric definition (ASTM B258), by name; a bare, solid-conductor size.
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