Electrical Resistivity of Copper at 20 °C
| Value | 1.678e-8 Ω·m |
| Status | Measured: ± 5.00e-11 Ω·m (0.003 relative) |
| Source | IEC 60028 (IACS) / CRC Handbook of Chemistry and Physics |
| Categories | Material PropertiesEngineering & Tradeelectrical-trade |
| ohm meter | 1.6780000e-08 Ω·m |
| ohm centimeter | 0.000001678 Ω·cm |
| microohm centimeter | 1.678 μΩ·cm |
| nanoohm meter | 16.78 nΩ·m |
Learning zone
Two numbers circulate and they are not the same. Pure annealed copper measures 1.678 × 10⁻⁸ Ω·m (1.678 µΩ·cm) at 20 °C; the International Annealed Copper Standard, set in 1913 as the 100 % IACS reference for commercial conductor, is 1.7241 × 10⁻⁸ Ω·m — about 2.7 % higher, because commercial copper is not laboratory-pure. Modern high-conductivity copper actually exceeds 100 % IACS. The NEC's conductor resistance tables and the classic K = 12.9 Ω·cmil/ft voltage-drop constant are built on the IACS figure with an allowance for stranding lay, so use 12.9 for code work and 1.678 × 10⁻⁸ for physics.
The 20 °C qualifier is not decoration. Copper's resistance rises about 0.393 % per kelvin, so a conductor at its 75 °C insulation rating has 22 % more resistance than the table value — 22 % more voltage drop and 22 % more I²R heat than a cold-copper calculation predicts. That is why motor winding temperature is measured by resistance change, and why voltage-drop calculations on heavily loaded feeders should use the hot resistance. Alloying is brutal in the other direction: brass is four times as resistive as copper, and that is why terminals matter.