Motor Starting Current Calculator
Estimate the inrush (locked-rotor) current an electric motor draws at start-up. Enter the full-load current and a starting multiplier for the starting method, and this calculator returns the starting current — and, if you add a voltage, the starting apparent power in kVA for checking voltage dip and generator sizing.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the motor's full-load current FLA in amps (from the nameplate).
- Enter the starting multiplier for your starting method, or leave it blank to use 7 (direct-on-line): DOL is about 6–8, star-delta about 2–3, and a soft-starter or VSD about 2–4.
- Optionally enter the supply voltage (and tick single-phase) to also see the starting apparent power in kVA.
How it works
Starting current is simply the full-load current multiplied by the starting multiplier: Istart = FLA × multiplier. When you supply a voltage, the starting apparent power is worked out the same way as any AC power: three-phase uses √3·V·Istart / 1000 and single-phase uses V·Istart / 1000, both giving kVA. That kVA is what the transformer, generator or upstream supply must momentarily deliver during the start.
Worked example
Worked example. A three-phase motor has a full-load current of 20 A and is started direct-on-line (multiplier 7). Starting current = 20 × 7 = 140 A. At 400 V line-to-line the starting apparent power = √3 × 400 × 140 / 1000 = 96.99 kVA — the brief demand placed on the supply while the motor runs up to speed.
Common mistakes
- Using the full-load current as the protection or cable rating and forgetting the much larger start-up surge — motor circuits need time-delay protection that rides through the inrush.
- Applying a direct-on-line multiplier (6–8) when the motor actually uses a star-delta or soft starter, which cuts the inrush to roughly 2–3 × FLA.
- Forgetting the √3 factor for three-phase kVA — leaving it out understates the starting apparent power by about 42%.
Frequently asked questions
Why is motor starting current so much higher than running current?
At the instant of starting the rotor is stationary, so the motor behaves almost like a short-circuited transformer (locked-rotor condition). With little back-EMF to oppose it, the current surges to several times the full-load value until the motor speeds up and the back-EMF builds, dropping the current back to normal.
How do I reduce the starting current?
Use a reduced-voltage starting method. Star-delta starting cuts the inrush to roughly one third of the direct-on-line value, while soft-starters and variable-speed drives (VSDs) ramp the voltage or frequency up smoothly to keep the starting current to about 2–4 × FLA and reduce mechanical shock.
Related tools
- Motor Full Load Amps Calculator
- Single Phase Power Calculator
- Three Phase Power Calculator
- Transformer Primary/Secondary Current Calculator
- Circuit Load Calculator
- Transformer Sizing Calculator
Explore more in Electrical, Electronics, Solar & Energy.
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.