Motor Starting Current Calculator
A 30 kW motor at 380 V runs at about 58 A — and pulls around 350 A starting direct-on-line. Star-delta cuts that to ~117 A (but cuts torque the same way), a soft starter to ~175 A, a VFD to ~70 A.
Enter the motor and see all four starting methods side by side: amps, kVA, the torque catch, and what each means for your breaker, transformer or generator. Multipliers per IEC 60034-12 (Design N: 6–8× DOL).
Your Motor
Starting Methods Compared
| Method | Starting Current | Starting kVA | Starting Torque | The Catch |
|---|
The Formulas
Full-load current (three-phase; for single-phase drop the √3):
Starting current and kVA (k from the method — DOL per IEC 60034-12):
Method & Sources
- DOL starting current 6–8× FLC — IEC 60034-12 (Design N); NEMA MG 1 code letters are the US equivalent. Default 6×, editable to match your motor's locked-rotor data.
- Star-delta = DOL ÷ 3 — winding voltage drops by √3, so both current and torque fall to one-third. The changeover transient is real: a motor that has not reached speed slams to full DOL current when it switches to delta.
- Soft starter 2.5–4× — current-limit setting on the unit; torque falls with the square of the voltage reduction.
- VFD ≈1.2× — the drive controls current directly and delivers full torque from standstill. Allow 2–3% drive losses and check harmonics on generator supplies.
- Starting kVA = √3 × V × Istart — apparent power drawn during the surge; the locked-rotor power factor (~0.2) means most of it is reactive, which is exactly why alternators and transformers feel it so hard.
- Nameplate beats calculation — if the plate gives FLC or locked-rotor current, use it.
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Frequently Asked Questions
How much current does a motor draw starting direct-on-line?
6–8× full-load current for a standard Design N motor (IEC 60034-12). A 30 kW / 380 V motor runs at ~58 A but pulls 350–470 A for the first second or two — that surge is what dims lights, trips marginal breakers and stalls generators.
How do I calculate full-load current?
I = P ÷ (√3 × V × η × cosφ). Example: 30 kW at 380 V, 92% efficiency, 0.85 PF → 58.3 A. The nameplate overrides any calculation.
Does star-delta starting have a catch?
Yes: it cuts torque to one-third along with the current. A motor starting against load may never accelerate in star — then slams to full DOL current at changeover anyway. It suits loads that start light: fans, centrifugal pumps, unloaded compressors.
Is a VFD always the best choice?
For starting, it is the gentlest (~1.2× FLC, full torque control) and it pays again on variable loads — fan and pump power falls with the cube of speed. The catches: capital cost, 2–3% drive losses, and harmonics that may need filtering on sensitive sites or genset supplies.
Why does starting method matter for generator sizing?
The genset must ride the surge without a deep voltage dip — one DOL motor can force a set two frames larger than the running load needs. A soft starter or VFD on that one motor often shrinks the genset by more than the starter costs. Run it in the Genset Sizing Calculator.
Specifying Compressors or Pumps?
Karnot heat pumps use inverter compressors — they ramp up at about 1.2× running current instead of slamming on at six. Smaller breakers, smaller gensets, no light-dimming.
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