Voltage Drop Calculator – Free Online Tool

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About this calculator

Voltage drop along a cable run can cause motors to run hot, undervoltage trips and inefficient operation if the cable is too long or undersized. This calculator estimates the voltage drop using the cable's resistance per km (from cable catalogue tables), the load current and the one-way cable length.

Formula

1-phase: Vd = (2 × R × I × L) ÷ 1000 3-phase: Vd = (√3 × R × I × L) ÷ 1000

RCable resistance — ohms per km
ILoad current — amperes
LOne-way cable length — meters
VdVoltage drop — volts

Worked Example

Given: A three-phase circuit carries 25A over a 50m cable with resistance 1.15 Ω/km.

Result: Vd = (1.732 × 1.15 × 25 × 50) ÷ 1000 = 2.49 V.

How to Use

Select single-phase or three-phase.
Enter the load current in amps.
Enter the one-way cable run length in meters.
Enter the cable's resistance in ohms per km — this is listed in cable manufacturer catalogues by cross-sectional area.
Click Calculate to get the voltage drop in volts.

Important Assumptions

Uses cable AC resistance only; reactance is ignored, which is a reasonable simplification for smaller cable sizes and typical distances.
Assumes a purely resistive approximation — actual drop for large cables/long runs may differ slightly due to reactance.

Common Mistakes

Using the total cable length (there and back) instead of the one-way run length where the formula already accounts for the return conductor.
Using the wrong resistance value for the cable's actual cross-section — always check the manufacturer's table for the specific cable size.

Frequently Asked Questions

What is an acceptable voltage drop limit?

Many electrical codes recommend keeping total voltage drop under about 3–5% of supply voltage for the full circuit run, though exact limits depend on the applicable code.

Where do I find the cable resistance value?

Cable manufacturers publish resistance per km for each conductor size (mm² or AWG) in their technical catalogues or datasheets.

These calculators are provided for informational and estimation purposes. Results depend on the assumptions and input values used. Always verify critical engineering calculations against applicable standards, manufacturer specifications, site measurements, and qualified professional advice.