Use the free Three-Phase Voltage Drop Calculator below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
What makes three-phase different
Three-phase power delivers electricity through three conductors whose currents peak at different times, and that timing changes the voltage-drop maths. Instead of current travelling out and back along two wires, as in single-phase, the three phases partly share the return path, and the drop formula uses the square root of three, about 1.732, times the line current, the length and the resistance, adjusted by power factor. For the same load power, a three-phase system also draws less current per conductor than a single-phase system, which is why factories, large workshops, farms and big heat pumps use it.
This calculator works with line voltage, the voltage measured between two phases, which is 400 volts in most of Europe and 208 or 480 volts in many North American commercial settings. Do not enter the phase-to-neutral voltage, such as 230 volts, by mistake: entering the wrong voltage changes the percentage, which is the number you judge by. The model is simplified and resistive, like the single-phase version, so it suits planning and comparison. Large installations, long feeders and motor-starting studies need a full calculation from an electrical engineer.
How the formula behaves in practice
The square-root-of-three factor means three-phase drop is less than the single-phase drop for the same conductor, current and length would suggest at first glance, because the phases help each other. But the real advantage is the lower current: a 12-kilowatt load at 400 volts three-phase draws about 17 amps per phase at unity power factor, while the same load at 230 volts single-phase draws about 52 amps. Lower current means dramatically lower drop, which is why three-phase is the natural choice once loads grow beyond a few kilowatts.
A worked example
Take a small workshop machine drawing 20 amps per phase, 50 metres from a 400-volt three-phase supply, wired in 6 AWG copper at a power factor of 0.9. The calculator gives a drop of about 2.0 volts, or 0.5 percent. That is an excellent result with plenty of margin, and it shows how forgiving three-phase can be at moderate distances. Now stretch the same load to 150 metres without changing the cable. The drop triples to about 6.1 volts, or 1.5 percent, still acceptable, which would be a very different story on a single-phase 230-volt circuit at the same current.
A tighter example shows where the limit appears. A 60-amp load, 80 metres away, on 4 AWG copper at a power factor of 0.9, drops about 5.1 volts, or 1.3 percent of 400 volts. That passes comfortably. The same load on 10 AWG copper drops about 20 volts, or 5 percent, which is at the edge and would cause starting trouble for motors. The lesson is the same as everywhere else in this batch: cable size is the lever, and the calculator lets you pull it before the cable is buried or pulled through conduit.
Common mistakes with three-phase drop
The first mistake is entering phase voltage instead of line voltage. On a 400-volt system the phase-to-neutral voltage is 230 volts, and entering 230 volts makes the percentage look nearly twice as bad as it is. The second mistake is using the total three-phase power to derive current incorrectly. Line current equals power divided by the square root of three, times line voltage, times power factor, and skipping that square root inflates the current by 73 percent.
Frequently asked questions
Why is three-phase drop lower than single-phase? Partly the square-root-of-three geometry, but mostly the lower current per conductor for the same power. Can I use this for a 208-volt system? Yes, enter 208 as the line voltage and your actual current per phase. What about the neutral wire? In a balanced three-phase system the neutral carries almost no current, so it contributes almost no drop, which is why the formula uses the line quantities. For heavily unbalanced loads, get a professional calculation.
Safety first: an estimate, not a code approval
Every electrical tool on this page is an educational estimator only. It is never professional advice and never a substitute for a licensed electrician. Electrical codes differ by country, region and building type, and they change over time. A result that looks fine here can still fail a local inspection, and a result that looks tight here can still be acceptable under a rule this simple calculator does not model, such as temperature correction, grouping, or a special location rule. Before you buy cable, cut a wall, or touch a panel, check your local electrical code and talk to a licensed electrician. Breaker, conduit and box-fill results are estimates, not code-compliance guarantees. If you are unsure about any part of a job, stop and ask a professional. Electricity can injure or kill, and no free online calculator changes that fact.
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