Motor Starting Current Calculator

Estimate motor starting current, inrush current, locked rotor current and starter method current for DOL, star-delta, autotransformer, soft starter and VFD applications.

⚡ Field Rule: DOL motor starting current is commonly several times full-load current. Use this with the Motor Current Calculator, future Contactor Size Calculator, Breaker Size Calculator, Voltage Unbalance Calculator, and Voltage Drop Calculator.

🚀 Motor Starting Current — FLA to Inrush

Motor Rating

Line Voltage

Power Factor

Efficiency

Starting Method

Starting Multiplier

Start Duration

Supply / Starter Limit Optional

FLA is estimated from kW/HP, voltage, PF and efficiency. Nameplate current is better when available.

Presets:5.5kW DOL15kW pump55kW soft

Motor Full Load Current

Starting Method

Starting Multiplier

Locked Rotor Code Optional

Starter / Breaker Current Limit

Start Duration

Use this tab when you know the motor nameplate FLA. It is usually more accurate than estimating from kW.

Starting Current

Supply Short-Circuit Current

Nominal Voltage

Allowed Voltage Dip

Power Factor During Start

This is a simplified voltage dip estimate using starting current compared with available fault current. For final design use system impedance study.

📐 Formula Reference

3-Phase FLA

I = kW × 1000 ÷ (√3 × V × PF × η)

HP to kW

kW = HP × 0.746

Starting Current

Istart = FLA × starting multiplier

Approx Voltage Dip

Dip % ≈ starting current ÷ available fault current × 100

📋 Quick Reference

Typical Starting Current

DOL5–8× FLA

Star-delta~2–3× FLA

Soft starter2–4× FLA

Starter Notes

DOLhigh torque

Star-deltalow torque

VFDcontrolled

Check Also

Voltage dipsupply

Breaker curvetrip

Cable dropstart

📚 Engineering Notes

DOL current is highDirect-on-line starting can draw 5 to 8 times FLA, depending on motor design and supply condition.

Star-delta reduces current and torqueStar-delta lowers starting current, but starting torque also drops. Heavy loads may not accelerate properly.

Check protection and contactorsUse the future Contactor Size Calculator and Breaker Size Calculator.

Check supply qualityHigh inrush can cause voltage dip. Use Voltage Drop Calculator, Short Circuit Current Calculator and Voltage Unbalance Calculator.

What is a Motor Starting Current Calculator?

A motor starting current calculator estimates the inrush current drawn by an induction motor during startup. It helps in starter selection, breaker checks, cable voltage drop checks and supply voltage dip estimation.

How motor starting current is calculated

The calculator first estimates motor full-load current from motor kW or HP, voltage, power factor and efficiency. Then it multiplies the full-load current by a starting multiplier based on the selected starting method.

Starting method comparison

DOL starting has the highest current but gives strong starting torque. Star-delta and autotransformer starters reduce current but also reduce torque. Soft starters and VFDs provide more controlled starting depending on settings and load type.

Important limitation

This calculator is a planning aid. Real starting current depends on motor design code, locked rotor current, load torque, acceleration time, supply impedance, starter settings, cable impedance and protective device curves.

❓ Frequently Asked Questions

A DOL started induction motor commonly draws around 5 to 8 times full-load current. The exact value depends on motor design and supply impedance.

Yes. Star-delta starting typically reduces line starting current to about one-third of DOL current, but starting torque is also much lower.

Usually yes. A VFD ramps voltage and frequency, so starting current can be controlled near rated current if the drive and load are sized correctly.