What is motor starting current?

When an induction motor starts from rest, it draws 5–8 times its full-load current for a few seconds until it reaches operating speed. This inrush can cause voltage dips and trip overcurrent devices if not accounted for.

How does star-delta starting reduce inrush?

Star-delta starting connects the motor windings in star (Y) configuration during startup, reducing per-phase voltage to 1/√3 of full voltage. This cuts both starting torque and starting current to 1/3 of the DOL values.

What tap ratio should I use for an auto-transformer starter?

Common tap ratios are 50%, 65%, and 80% of full voltage. The supply line inrush current is proportional to the tap ratio squared (motor winding current is proportional to the tap ratio). The 65% tap is most common for general-purpose loads.

What multiplier does DOL starting use?

DOL (direct-on-line) starting applies full voltage immediately. The starting current is typically 6× FLC, though it can range from 5× to 8× depending on motor design. This calculator uses 6× as the standard factor.

How does a soft starter limit inrush?

A soft starter ramps up voltage gradually using thyristors, limiting current to a programmable multiple of FLC (commonly 2–3×). This also reduces mechanical shock on the driven load.

What is the difference between starting current at the motor and at the supply?

For auto-transformer starters, motor terminal voltage is reduced to tap × V, so motor-side current is tap × I_start_DOL. Because the transformer steps current down by another factor of the tap ratio, supply-side line current is tap² × I_start_DOL.

Motor Starting Current Calculator

Enter motor rated current and starting method to get the expected inrush (starting) current and reduction factor.

Motor Parameters

Full-Load Rated Current (FLC) — A

Found on the motor nameplate as FLC or I_n.

Starting Method

DOL Starting Current Multiplier (× FLC)

Standard is 6×. Check motor datasheet for exact value (range 5–8×).

Results

Enter motor rated current and click Calculate.

Starting Method Comparison

Method	Multiplier	I_start (A)

Summary

This calculator determines the starting (inrush) current drawn by an AC induction motor based on its full-load rated current and the starting method used. Direct-on-line (DOL) starting draws 5–8× rated current, while reduced-voltage methods like star-delta, auto-transformer, and soft starters limit inrush to protect equipment and reduce voltage dips. Use this tool to size circuit breakers, contactors, and cables for motor control circuits.

How it works

Enter the motor's full-load rated current (FLC) from the nameplate.
Select the starting method: DOL, star-delta, auto-transformer, or soft starter.
For auto-transformer, specify the tap ratio (50%, 65%, or 80%).
For soft starter, specify the current limit multiplier (typically 1.5–3× FLC).
The calculator applies the standard multiplier for the chosen method and returns the starting current.
Review the reduction factor to understand how much inrush is reduced versus DOL.

Use cases

Size circuit breakers and fuses for motor branch circuits.
Determine contactor and cable ratings for reduced-voltage starters.
Estimate voltage dip on the supply bus during motor start.
Compare starting methods to choose the best fit for a given load.
Verify that a generator set can handle the starting surge.
Document expected inrush values in electrical design reports.

Frequently Asked Questions

Last updated: 2026-06-19 · Reviewed by Nham Vu