How the values are calculated
d (mm) = 0.127 × 92^((36 − n) / 39)
A (mm²) = (π / 4) × d²
n = 0, −1, −2, −3 for 1/0, 2/0, 3/0, 4/0
1 kcmil = 1000 circular mils = 0.506707 mm²
The nearest IEC size is the standard cross-section whose ratio to the AWG area is closest to 1 on a logarithmic scale, so a size slightly smaller and a size slightly larger are treated evenly.
Worked example
10 AWG (n = 10):
d = 0.127 × 92^(26 / 39) = 2.588 mm
A = (π / 4) × 2.588² = 5.261 mm²
Nearest IEC size: 6 mm²
500 kcmil is 500 × 0.506707 = 253.4 mm². The nearest IEC size is 240 mm², which is smaller than the AWG conductor; where a replacement must carry the same current with the same voltage drop, the next larger size is the safe choice.
Notes and limits
- In 9 of the 23 rows the nearest IEC size is smaller than the AWG area. "Nearest" is a geometric match, not an equivalence.
- IEC 60228 defines each metric size by a maximum DC resistance at 20 °C rather than an exact area, so the real metal area of a metric conductor can differ slightly from its nominal value.
- Current-carrying capacity cannot be transferred by size between NEC tables and IEC 60364-5-52: insulation temperatures, installation methods and correction factors differ. Size the replacement cable with the rules that apply where it is installed.