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Wire voltage drop calculator

Voltage lost in a copper or aluminum wire run by AWG size, length and current, with the voltage left at the load.

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Voltage drop 5.79 V

Voltage drop

  1. Good (under 3%)
  2. Borderline (3–5%)
  3. Too high (5%+): use thicker wire

How it works

Vdrop = k × L × I × R ÷ 1,000

Current flows out and back, so a single-phase or DC circuit has twice the one-way length of wire (k = 2); three-phase uses k = √3. Resistance per 1,000 ft comes from the National Electrical Code (Chapter 9, Table 8, at 75 °C). A common target is no more than 3% drop on a branch circuit, so lights don’t dim and motors don’t strain. If the drop is too high, go up a wire size.

  1. Look up the resistance per 1,000 ft 12 AWG → 1.93 Ω per 1,000 ft
  2. k × L × I × R ÷ 1,000 2 × 100 ft × 15 A × 1.93 ÷ 1,000 = 5.79 V
  3. As a share of the supply 5.79 ÷ 120 × 100 = 4.83%

Good to know

  • For sizing real installations, follow the NEC and local code; this checks voltage drop only, not ampacity.

Printed from rocketcalc.io/tech/wire-voltage-drop