IEC 60028: The International Standard of Resistance for Copper (1925)

Why Every Cable Cross-Section Calculation Traces Back to a 1925 Standard

IEC 60028 (1925) established the standard resistivity of annealed copper — the 100% IACS (International Annealed Copper Standard). Every cable selection calculation indirectly references this benchmark.

Core data: At 20 °C, standard annealed copper resistivity = 0.017241 Ω·mm²/m (conductivity 58 MS/m). All conductor materials are rated as % IACS — pure aluminum ~61% IACS, copper-clad aluminum (CCA) ~62–68% IACS.

Engineering reality: Commercial copper typically achieves 97–101% IACS due to impurities and cold-working variations — meaning same-cross-section cables can differ by 3–4% in resistance. Conservative designs use 97% IACS. Also critical: copper resistivity increases ~0.4% per °C above 20 °C. At rated current (conductor temperature ~90 °C), resistance is ~28% higher than at 20 °C — cable sizing must use operating temperature, not room temperature.

TN Lab — A 1925 copper benchmark still governs every cable sizing calculation today.