Voltage Drop Calculator

Work out voltage drop and percentage drop over a cable run for copper or aluminium conductors. Supports single-phase AC, three-phase AC and DC, in AWG or mm2, with a power-factor input and a check against the 3% recommended limit.

How It Works

The formula

Voltage drop is the conductor impedance multiplied by the current and the length of the run. For a single-phase or DC circuit the current travels out and back, so the run length is counted twice:

  • DC: Vdrop = 2 × L × I × R
  • Single-phase AC: Vdrop = 2 × L × I × (R·cosφ + X·sinφ)
  • Three-phase AC: Vdrop = √3 × L × I × (R·cosφ + X·sinφ)

R and X are the conductor resistance and reactance per unit length (from standard conductor tables), L is the one-way run length, I is the load current and cosφ is the power factor.

Note what happens on low-voltage DC. The volts lost are the same for a given cable and current, but they are a share of a much smaller supply, so a drop that is negligible at 240 V is severe at 12 V. Solar and battery runs live in that region, and the DC voltage drop calculator at Solar Sizing Calculator is set up for it, with 12V, 24V and 48V run-length tables.

Worked example

A 100 ft run of #10 AWG copper carrying 20 A at 120 V single-phase, unity power factor: with R about 3.9 ohm/km and L = 0.0305 km, Vdrop = 2 × 0.0305 × 20 × 3.9 ≈ 4.8 V, or about 4%. That exceeds the 3% guideline, so a larger conductor would be chosen.

Maximum Run Length Chart (3% Voltage Drop)

The longest one-way run each copper conductor can serve before voltage drop reaches 3%, on a 240 V single-phase circuit. Halve these figures for 120 V, since the same drop in volts is twice the percentage.

Maximum one-way run length in feet at 3 percent voltage drop, 240 V single-phase copper
Copper size 15 A 20 A 30 A 40 A 50 A 60 A 100 A 150 A
14 AWG 77 ft 57 ft — — — — — —
12 AWG 119 ft 89 ft — — — — — —
10 AWG 201 ft 151 ft 100 ft — — — — —
8 AWG 307 ft 230 ft 153 ft 115 ft 92 ft — — —
6 AWG 489 ft 366 ft 244 ft 183 ft 146 ft 122 ft — —
4 AWG 771 ft 578 ft 385 ft 289 ft 231 ft 192 ft — —
2 AWG 1,270 ft 952 ft 635 ft 476 ft 381 ft 317 ft 190 ft —
1/0 AWG 2,018 ft 1,514 ft 1,009 ft 757 ft 605 ft 504 ft 302 ft 201 ft
2/0 AWG 2,386 ft 1,789 ft 1,193 ft 894 ft 715 ft 596 ft 357 ft 238 ft
4/0 AWG 3,878 ft 2,909 ft 1,939 ft 1,454 ft 1,163 ft 969 ft 581 ft 387 ft

Greyed cells are current levels the conductor cannot carry at a 75°C termination, so run length is moot. Figures use the AC resistance of NEC Chapter 9 Table 9 at unity power factor and ignore reactance, which is the conservative simplification for the small conductors most branch circuits use. The 3% branch-circuit and 5% total figures are NEC informational-note recommendations rather than enforceable requirements. Need the size rather than the length? Use the wire size calculator or pick your breaker rating below.

20 A 30 A 40 A 50 A 60 A 70 A 80 A 100 A 125 A 150 A 200 A 400 A

Frequently Asked Questions

What is an acceptable voltage drop?

A common guideline is no more than 3% on a branch circuit or feeder and 5% total from the supply to the load. Local wiring rules take precedence.

Why is run length counted twice for single-phase and DC?

Current flows out along one conductor and back along the other, so it passes through twice the one-way length of conductor.

Why does three-phase use the square root of 3 instead of 2?

In a balanced three-phase circuit the line-to-line voltage drop works out to √3 times the per-conductor drop, not twice it.

Does power factor affect voltage drop?

Yes. A lower power factor increases the contribution of conductor reactance, raising the drop. DC and unity-power-factor AC depend on resistance only.

Copper or aluminium?

Aluminium has roughly 1.6 times the resistance of copper for the same size, so an aluminium conductor drops more voltage and is usually sized up to compensate.

Related Calculators

Continue with these related electrical tools

Wire Size Calculator

Smallest conductor that meets ampacity and the voltage-drop limit (AWG/mm2)

AWG Conversion Calculator

Convert AWG to mm2, circular mils, kcmil and diameter

Watts Amps Volts Calculator

Convert between watts, volts and amps (DC, 1-phase, 3-phase)

Ohm's Law Calculator

Solve voltage, current, resistance and power from any two

Embed This Calculator

Add this free calculator to your website. Copy the code below and paste it into your page.

Electrical Calculators Electrical Calculators

Fast, free calculators for electricians and engineers

More Calculators

© 2026 Electrical Calculators. All rights reserved.

Privacy Policy Terms of Service Cookie Policy