ToolnozaENGINEERING TOOL
Motor Starting Current Calculator
Estimate balanced three-phase motor running current, starting current and starting apparent power with a supplied current multiplier.
Motor Starting Current Calculator
FREE ONLINE TOOLSTEP BY STEP
How to use motor starting current calculator
- Check the input units and assumptions.
- Review the result and example.
IN PRACTICE
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.
GOOD TO KNOW
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.
Formula and worked example
Irated (A) = 1000Pout/[√3 × Vline × (η/100) × PF]; Istart = Irated × multiplier; Sstart (kVA) = √3 × Vline × Istart/1000.
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.
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.
What assumptions limit this calculation?
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.
How to Calculate Motor Starting Current: Formula & Example