GlenCalc

Voltage drop calculator

Long wire runs lose voltage. Enter the wire size, the one-way length and the load to see the drop, and whether it stays within the commonly recommended 3%.

Your answer

Voltage drop6.18 V
Percent drop5.15 %Over 5%: use a larger wire
Voltage at the load113.8 V
Wire resistance1.9315 Ω per 1,000 ft
  • Wire must also be rated for the current (ampacity) under your electrical code; voltage drop is a separate check.

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How we worked it out

Updated as you change the numbers.

  1. Resistance of 12 AWG copper at 75 °C ≈ 1.9315 Ω per 1,000 ft.
  2. Drop = 2 × length × current × resistance ÷ 1,000.

Method and assumptions

Resistance comes from the conductor’s cross-section (from the AWG formula) and resistivity at 75 °C, matching NEC Chapter 9 Table 8 values to within about 2%. Single-phase drop counts both wires (2 × one-way length); three-phase uses √3. The NEC’s informational notes suggest at most 3% on a branch circuit and 5% in total.

Questions

What is the voltage drop on 100 ft of 12 AWG copper at 16 A, 120 V?

About 6.2 V, or 5.2%, so 10 AWG would be a better choice.

Sources

Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.

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