What is motor starting current?

Motor starting current (inrush current) is the initial surge of current when a motor is first energized. For DOL (direct-on-line) starting, it is typically 6–8 times the rated full-load current and lasts until the motor reaches operating speed.

Why is starting current so high?

At standstill, the rotor does not generate back-EMF, so the motor winding impedance is very low, resulting in high current. As the motor accelerates, back-EMF builds up and current drops to the normal running value.

What is the difference between DOL and star-delta starting?

DOL (direct-on-line) applies full voltage immediately, giving 6-8× rated current. Star-delta starts in star connection (1/3 voltage per winding), reducing starting current to about 1/3 of DOL values (2-3× rated), then switches to delta for normal running.

How does a VFD reduce starting current?

A Variable Frequency Drive (VFD) controls both voltage and frequency, ramping them up gradually. This allows the motor to start with only 1–1.5× rated current while still providing adequate starting torque.

What is the NEMA code letter for locked rotor current?

NEMA code letters (A through V) indicate the locked rotor kVA per HP. For example, code letter F means 5.0–5.6 kVA/HP. The starting current can be calculated as I_start = (kVA/HP × HP × 1000) / (√3 × V).

Free Engineering Tool

Motor Starting Current Calculator

Calculate motor starting (inrush) current for DOL, star-delta, soft starter, and VFD methods. I_start = k_start × I_rated.

DOL 6–8× Star-Delta 2–3× VFD 1–1.5×

Rated Motor Power (kW)

Line Voltage (V)

Power Factor cos φ (0–1)

Efficiency η (0–1)

Starting Method

Start Multiplier k (×I_rated)

Quick presets

Results

Starting Current I_start

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Rated Full-Load Current I_rated

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Start Multiplier k

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Starting Apparent Power

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Starting Method

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Rated Current

Starting Current

Starting Methods Comparison

Method	Current Multiplier	Torque	Typical Use
DOL	6–8 × I_rated	Full	Small motors < 7.5 kW
Star-Delta	2–3 × I_rated	~33%	Pumps, fans, 7.5–75 kW
Soft Starter	2–4 × I_rated	Variable	Conveyors, compressors
VFD	1–1.5 × I_rated	Full	Any, best control

Example — 15 kW Motor, DOL Start

Given: P = 15 kW, V = 400 V, cos φ = 0.85, η = 0.91, DOL k = 7

I_rated = 15000 / (√3 × 400 × 0.85 × 0.91) = 28.0 A

I_start = 7 × 28.0 = 196.0 A

Motor Starting Current Calculator | I_start = k × I_rated | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026

Results

Rated Current

Starting Current

Starting Methods Comparison