Free online tool
Three-Phase Cable Size Calculator
For balanced three-phase circuits: motor feeders, industrial distribution boards, and three-phase supplies. Uses line current and the √3 voltage-drop factor instead of the single-phase formula.
Recommended conductor size (per phase)
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Fill in the form to see your result.
Engineering estimate for planning purposes only. Motor circuits typically also need separate overload and short-circuit protection sized to the motor, not just the cable. Confirm against your local electrical code and equipment documentation.
Why three-phase sizing isn't just single-phase × 3
The conductor itself doesn't care how many phases share the circuit: ampacity depends on the current in that one conductor and how the heat it generates gets away, so the same ampacity tables apply. Voltage drop is where three-phase differs. A single-phase or DC circuit's current travels out on one conductor and back on another, doubling the effective resistance (factor of 2). A balanced three-phase circuit's return current is shared across the other two conductors, which works out mathematically to a factor of √3, roughly 1.73, instead of 2. That's a real difference: for the same current, length and cable size, a three-phase circuit drops less voltage than a single-phase one would.
Frequently asked questions
Why does three-phase use √3 instead of 2 for voltage drop?
In a DC or single-phase circuit, current travels out on one conductor and back on another, so the resistance is effectively doubled: a factor of 2. In a balanced three-phase circuit, the three conductors share the return path between them rather than each doubling back, which works out to a factor of √3 (about 1.732) instead of 2.
What current do I enter for a three-phase load?
Enter the line current per phase, the figure on a motor nameplate or feeder schedule (often shown as FLA, full-load amps). For a load given in kW, line current is roughly kW × 1000 ÷ (√3 × voltage × power factor); a typical industrial power factor is 0.8-0.9 if the exact value isn't known.
Does a three-phase motor need extra cable sizing margin?
Motors draw a starting current well above their running current, often 5-8 times the full-load figure for a few seconds. That surge is handled by the starter or overload protection rather than by upsizing the cable for it, but it's worth applying a duty-cycle margin (the continuous-load option above) for motors that run for long uninterrupted periods.