Voltage drop calculator

Estimate voltage drop in DC, single-phase or balanced three-phase wiring from cable size or resistance.

Get started

Choose a model. For three phase, enter line-to-line voltage and line current. Enter one-way distance once.
The distance limit uses your chosen drop percentage. Check ampacity, temperature and installation requirements separately.

Example

Input

120 V, single phase, 500 ft, 1 A and 1.2 Ω/km

Expected result

0.37 V drop, 0.30%

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Good to know

The calculation runs in your browser. Use consistent units in fields with no explicit unit.

Calculation method

Use one-way length and identical parallel conductors. Multiply I·L·R by 2 for DC or single phase and √3 for balanced three phase. For lagging AC, R and X contribute through the power factor. Wire-size estimates use nominal resistance near 20 °C; use manufacturer data for the actual cable.

\Delta V=\frac{k I L R_{\rm eff}}{n},\quad k=2\ \text{(DC, 1)}\phi,\quad k=\sqrt{3}\ \text{(3)}\phi

R_{\rm eff}=R\cos\theta+X\sin\theta,\quad \%\Delta V=100\frac{\Delta V}{V_s}

Calculation mode

Known resistance or impedance

Resistance or impedance per length
Unit per length · Ω/km / Ω/m / mΩ/m / Ω/1000 ft / Ω/ft / mΩ/ft
Voltage · V
Circuit · DC / Single-phase AC / Three-phase AC
Conductors per phase
One-way distance
Length unit · ft / m / mi / km
Current · A
Drop limit for distance estimate · %

Estimate from wire gauge

Material · Copper / Aluminum / Silver / Gold
Wire size
Voltage · V
Circuit · DC / Single-phase AC / Three-phase AC
Conductors per phase
One-way distance
Length unit · ft / m / mi / km
Current · A
Drop limit for distance estimate · %

Estimate from area

Material · Copper / Aluminum / Silver / Gold
Cross-sectional area · mm²
Voltage · V
Circuit · DC / Single-phase AC / Three-phase AC
Conductors per phase
One-way distance
Length unit · ft / m / mi / km
Current · A
Drop limit for distance estimate · %

Resistance and reactance

Resistance
Reactance per length
Unit per length · Ω/km / Ω/m / mΩ/m / Ω/1000 ft / Ω/ft / mΩ/ft
Lagging power factor
Voltage · V
Circuit · DC / Single-phase AC / Three-phase AC
Conductors per phase
One-way distance
Length unit · ft / m / mi / km
Current · A
Drop limit for distance estimate · %