ToolnozaInputs and measurement basis
Enter rated mechanical output kW, line-to-line voltage, full-load efficiency and full-load power factor. Enter the starting-to-running current ratio from the motor or starter data sheet; the sample ratio is illustrative.
Rated shaft output (kW); Line-to-line voltage (V); Full-load efficiency (%); Full-load power factor (0–1); Starting / rated current ratio.
Formula and calculation
Irated (A) = 1000Pout/[√3 × Vline × (η/100) × PF]; Istart = Irated × multiplier; Sstart (kVA) = √3 × Vline × Istart/1000.
Estimate balanced three-phase motor running current, starting current and starting apparent power with a supplied current multiplier.
Worked example
Pout = 15 kW; V = 400 V; η = 90%; PF = 0.85; multiplier = 6 → Irated = 28.30 A; Istart = 169.81 A; Sstart = 117.65 kVA.
How to interpret the result
The multiplier describes a specific motor and starting arrangement. A direct-on-line motor, soft starter and drive can have different current profiles; choosing a label alone does not determine a valid multiplier.
Starting kVA is useful for comparing scenarios but cannot size a generator alone. The allowable voltage dip, motor torque-speed curve and start duration are also needed. Verify the actual data with the equipment supplier.
Assumptions and limits
Balanced three-phase estimate at rated conditions. Nameplate current is preferable when available. Starting apparent kVA is not real kW. Voltage sag, acceleration time, harmonics, protection and generator selection are excluded.
Frequently asked questions
Can I use input electrical kW as rated output? No. This formula expects shaft output power. Electrical input already includes efficiency losses and must not be divided by efficiency again.