Voltage Drop Calculator
Check whether a cable run loses too much voltage.
Why cables lose voltage
Every cable has some resistance, so current flowing through it causes a drop in voltage. The longer the run, the higher the current and the thinner the cable, the bigger the drop. A drop above about 3% for final circuits is often considered too much.
Example
With the starting values shown (Current 20 A; One-way cable length 30 m; Conductor size 4 mm²; Material Copper; Supply voltage 230 V), Voltage drop comes out as 5.16. Other results include resistance of the cable round trip: 0.258; voltage drop as a share of the supply: 2.24%; voltage at the load: 224.8.
This is a single-phase estimate with a simple resistance model at 20 °C. Real cables get warmer under load, which raises their resistance, and wiring rules set specific limits. Always follow your local electrical code and consult a qualified electrician.
Frequently asked questions
What is an acceptable voltage drop?
Many standards use about 3% for lighting and about 5% overall, but the rules depend on your country and circuit.
Does it work for three-phase?
Not directly. For three-phase, multiply the single-phase drop by about 0.866.
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