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Ω/km
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Output
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Voltage Drop Calculator
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This Voltage Drop Calculator computes how much voltage is lost over a cable run, based on the current, one-way cable length, the conductor's resistance per kilometer, and whether the system is single-phase or three-phase. Excessive voltage drop over long cable runs can cause equipment to underperform or overheat, and most electrical codes recommend keeping total drop under about 3-5% of the source voltage. Enter your circuit details and the calculator shows both the voltage lost and the percentage, flagging whether it's within the common 3% guideline. Everything runs locally in your browser. Scroll down to enter your circuit details.
Features
- Calculates voltage drop in volts and as a percentage
- Supports both single-phase and three-phase systems
- Flags results against the common 3% recommendation
- Instant recalculation as you adjust inputs
Why use this voltage drop calculator?
- Helps catch undersized cable runs before installation
- No manual formula lookup or algebra required
- Fully private — nothing uploaded
- Free with unlimited use
Frequently asked questions
What's an acceptable voltage drop percentage?
Many electrical codes and guidelines recommend keeping total voltage drop under 3% for branch circuits and 5% for the combined feeder and branch circuit, though specific limits depend on your local code.
Where do I find my conductor's resistance per km?
Reference tables (like the one used in this site's Wire Size Calculator) list resistance per km by wire gauge and material — copper and aluminum have different values for the same gauge.
Why is single-phase multiplied by 2?
Single-phase voltage drop accounts for the round trip through both the outgoing and return conductors, so the resistance is effectively doubled compared to a one-way calculation.
Why is three-phase multiplied by √3 instead of 2?
Three-phase circuits use a different vector relationship between line currents and voltages, and √3 (≈1.732) is the standard multiplier used in the three-phase voltage drop formula.
What happens if voltage drop is too high?
Equipment may run inefficiently, motors can overheat, and lighting may dim — the typical fix is using a larger-gauge (lower resistance) conductor or shortening the cable run.