Wire resistance calculator
Pick size, material and length. Resistance comes from NEC Chapter 9 Table 8 (75 °C) and is corrected to your temperature with the table’s own equation.
Where the numbers come from
NEC Chapter 9, Table 8 lists DC resistance at 75 °C for stranded, uncoated copper and for aluminum. Its Note 2 corrects for temperature: R2 = R1 × [1 + α × (T2 − 75)], with α = 0.00323 for copper and 0.00330 for aluminum. At 20 °C, 12 AWG copper works out to 1.98 × (1 − 0.00323 × 55) ≈ 1.63 Ω per 1,000 ft, close to the 1.62 Ω figure on manufacturer spec sheets.
Solid conductors (sizes 18 through 8 AWG are also made solid) are slightly lower: 12 AWG solid copper is 1.93 Ω/kft. Tinned (coated) copper is slightly higher.
Use the result with Ohm's law: voltage lost in a conductor = current × resistance. The voltage drop calculator does this for a whole circuit.
Questions people ask
What is the resistance of 12 AWG copper wire?
1.98 Ω per 1,000 ft for stranded uncoated copper at 75 °C (NEC Chapter 9 Table 8), or about 1.62 Ω per 1,000 ft at 20 °C.
How does temperature change wire resistance?
NEC Chapter 9 Table 8, Note 2: R2 = R1 × [1 + α × (T2 − 75)], with α = 0.00323 for copper and 0.00330 for aluminum.