Wire Resistivity: Copper & Aluminum K Factor

What the K factor is and where it comes from

The resistivity constant K is the single number that lets a simple voltage-drop formula stand in for a full physics calculation. It is expressed in circular-mil-ohms per foot, and it bundles a conductor material's intrinsic resistivity into a form that pairs directly with wire sizes measured in circular mils. Copper's K is about 12.9 and aluminum's about 21.2 at a typical operating temperature near 75 °C — and that ratio, roughly 1.6 to 1, is exactly why aluminum needs a larger conductor than copper for the same run.

K appears in both formulas the wire pillar depends on. Voltage drop for a single-phase run is (2 × K × I × L) / CM, and a conductor's resistance is (K × L) / CM, where I is current in amps, L is one-way length in feet and CM is the cross-section in circular mils. Because resistance rises slightly with temperature, K is quoted at a reference temperature; a conductor running hot drops a little more than the nominal figure, which is one more reason to keep a design margin rather than sizing to the exact limit.

These constants feed the Voltage Drop and Wire Resistance calculators. They are stable engineering values, not a live feed, so the tools built on them need no maintenance and always return the same answer for the same inputs.