Wire Resistance Chart — Conductor Properties

Every rule-of-thumb voltage-drop formula — VD = 2 × K × I × L ÷ cmil — is this table compressed to one constant: K is just resistance × circular mils ÷ 1000, which lands near 12.9 Ω·cmil/ft for copper and 21.2 for aluminum on every row. When you need the real thing, the voltage drop calculator works from the code's AC data (Chapter 9, Table 9) with power factor and raceway effects; this chart is the right source for DC circuits — battery banks, PV strings, control wiring — and for checking any conductor's series resistance directly. The circular-mil column is the same one on the wire ampacity chart, and the diameter and area here are the bare conductor — for insulated dimensions use the wire dimensions chart.

These values assume Class B concentric stranding at 75°C. Hotter conductors are more resistive — Note 2's correction runs about 0.32% per °C for copper — so a conductor at its 90°C limit reads roughly 5% above the table. Coated (tinned) copper is listed separately because the tin raises resistance a few percent. Compact and compressed stranding shrink the bare diameter about 9% and 3% respectively without changing resistance. And as always: these are calculation values from the code — a specific cable's datasheet governs when precision matters.