Voltage Drop Calculator for Copper and Aluminium Cables
Enter supply voltage, current, one-way cable length and the cable size. You get the voltage drop, the voltage reaching the load, power lost as heat and the longest run for a 3% drop.
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Voltage drop
3.13%
Between 3% and 5%- Drop in volts
- 7.2 V
- Voltage at load
- 222.8 V
- Power lost in cable
- 115.2 W
- Longest run for 3% drop
- 24 m
For guidance only. Electrical work must follow IS 732 and the cable makers’ tables, and be done or checked by a licensed electrician.
Resistance only (ρ copper 0.0225, aluminium 0.036 Ω·mm²/m). Large cables have slightly more drop from reactance.
Show working
- ΔV = 2 × 25 × 16 × 0.0225 ÷ 2.5 = 7.2 V
- 7.2 ÷ 230 × 100 = 3.13%
How to use
- Choose single- or three-phase and enter voltage and current.
- Enter the one-way cable length.
- Pick copper or aluminium and the cable size.
Good to know
Single-phase drop = 2 × length × current × ρ ÷ area (current goes out and back). Three-phase drop (line to line) = √3 × length × current × ρ ÷ area. ρ is taken for cable at working temperature: 0.0225 Ω·mm²/m for copper, 0.036 for aluminium.
A drop up to about 3% is a common design target for final circuits (IEC 60364 allows 3% for lighting and 5% for other uses from a public supply). Too much drop makes motors run hot, lights dim and electronics misbehave, and the lost power is paid for on your bill.
This uses resistance only. For large cables (above about 25 sq mm) and low power factor, reactance adds a little more drop. Follow IS 732 and get a licensed electrician to choose cables.
Questions
What is an acceptable voltage drop?
About 3% for final circuits is a common target; up to 5% overall is often accepted. Check your design rules.
Why multiply by 2 for single-phase?
Current flows out on the phase wire and back on the neutral, so the cable length counts twice.
How do I reduce voltage drop?
Use a thicker cable, shorten the run or split the load across circuits.
Does voltage drop waste electricity?
Yes. The lost watts heat the cable and are billed. The tool shows this loss.
Is length one way or both ways?
One way, from the board to the load. The formula accounts for the return path.