Wire Resistivity: Copper & Aluminum K Factor
The resistivity constant K (circular-mil-ohms per foot) for copper and aluminum used in the voltage-drop and wire-resistance formulas.
| Metal | K Factor | Notes |
|---|---|---|
| Copper | 12.9 cmil·Ω/ft | Soft-drawn copper, ~75°C — used in VD = 2·K·I·L / CM |
| Aluminum | 21.2 cmil·Ω/ft | AA-8000 series, ~75°C — ~64% the conductivity of copper |
Sources: NEC Chapter 9 Table 8 (conductor resistance), NEC 310.16 (ampacity), NEC Chapter 9 Tables 1/4/5 (conduit areas & THHN), ASTM B258 (AWG geometry), NREL (peak sun hours) · All sources