Motor Starting Current Calculator (DOL, Star-Delta, Soft Starter)
By Santaji GadepatilinUpdated October 28, 2026
A three-phase induction motor started direct-on-line (DOL) draws 6–8 times its full-load current for a few seconds, which can dip the supply voltage and trip protection. Reduced-voltage starters cut that inrush — compare the options in DOL vs star-delta starter and motor starting methods compared. This motor starting current calculator estimates the inrush for each method.
How to use the motor starting current calculator
Enter the motor’s full-load current from the nameplate, the DOL starting current ratio (IA/IN from the datasheet) and choose the starting method. You get the DOL inrush, the line current with your starter and the starting torque as a percentage of DOL torque.
Motor starting current formula
IDOL = k × IFL · Istart = IDOL × (Vstart/Vrated)²
For star-delta the factor is (1/√3)² = 1/3; for autotransformers it is the tap ratio squared.
Comparison of starting methods
| Method | Line current (× DOL) | Torque (× DOL) | Typical line current |
|---|---|---|---|
| Direct-on-line | 1.00 | 1.00 | 6–8 × FLC |
| Star-delta | 0.33 | 0.33 | 2–2.7 × FLC |
| Autotransformer 80% | 0.64 | 0.64 | 4–5 × FLC |
| Autotransformer 65% | 0.42 | 0.42 | 2.5–3.4 × FLC |
| Soft starter (current limit) | adjustable | low | 2.5–4 × FLC |
| VFD | — | up to 1.5 × rated | 1–1.5 × FLC |
Example
A 11 kW, 415 V motor with 20 A FLC and a 6.5 × starting ratio:
- DOL starting current = 130 A
- Star-delta starting current = 43 A, with one third of the DOL starting torque
Choosing a method
- DOL suits small motors (typically up to 7.5–11 kW) on a stiff supply.
- Star-delta suits pumps and fans that start lightly loaded, but causes a current spike at changeover.
- Soft starters give smooth acceleration and reduce mechanical shock.
- VFDs give the lowest current and full torque, plus speed control.
Check the voltage dip a large motor causes on a generator or weak supply — see the generator size calculator.
Frequently asked questions
What is the starting current of a motor?
Started direct-on-line, an induction motor draws its locked-rotor current — typically 6 to 8 times full-load current — until it approaches full speed.
How much does a star-delta starter reduce starting current?
To one third of the DOL value. Each winding sees 1/√3 of line voltage in star, and line current is one third. Starting torque also falls to one third.
Why does starting torque fall as much as current?
Torque is proportional to voltage squared. With an autotransformer, line current and torque both scale with the tap ratio squared.
What about soft starters and VFDs?
A soft starter typically limits current to 2.5–4 × FLC; a VFD can start a motor at about 1–1.5 × FLC with full torque.