How do I use the electric motor starting current calculator?

Enter the motor power (in kW), supply voltage (in V), and the starting current multiplier (typically between 4 and 8) to estimate the nominal and inrush currents for single-phase or three-phase motors.

What is the starting current multiplier?

The starting current multiplier represents how many times the motor draws its nominal current during startup. Typical values range from 4 to 8, depending on the motor type and starting method.

How is the nominal current calculated?

For single-phase motors, the nominal current is calculated as: I_nom = (1000 × P) / (U × η × cos φ), where P is the motor power in kW, U is the supply voltage in V, η is the efficiency, and cos φ is the power factor.

How is the inrush current calculated?

The inrush current is calculated as: I_start = k_start × I_nom, where k_start is the starting current multiplier and I_nom is the nominal current.

What are typical values for nominal and inrush currents?

For a 1.5 kW single-phase motor with 240 V supply, assuming 92% efficiency and 88% power factor, the nominal current is approximately 8.38 A, and the inrush current is approximately 46.09 A.

Electric Motor Starting Current Calculator

Motor power, kW:

Supply voltage, V:

Starting current multiplier, 4–8:

This online tool estimates steady state and inrush currents for electric motors connected as single phase or three phase. It also provides practical guidance for selecting protective devices and basic capacitor sizing for single phase motors. Calculations run in the browser and produce numeric results and charts for quick comparison.

Table of Contents

How to use

Pick the motor connection type: single phase, three phase star, or three phase delta.
Enter motor rated power in kilowatts, supply voltage in volts, and start current multiplier.
Click Calculate to show nominal current, inrush current and phase currents.
Use the output to choose circuit breakers, contactors and cable sizes that tolerate starting currents.

Key parameters

Rated power P in kilowatts
Supply voltage U in volts
Start multiplier k_start that expresses how many times the nominal current the motor draws at start
Assumed motor efficiency η and power factor cos φ are used when converting power to current

Core formulas

Single phase nominal current

$$I_{nom} = \frac{1000\,P}{U\,\eta\,\cos\varphi}$$

Three phase nominal current

$$I_{nom} = \frac{1000\,P}{\sqrt{3}\,U\,\eta\,\cos\varphi}$$

Start (inrush) current

$$I_{start} = k_{start}\;I_{nom}$$

Phase start current for delta connection

$$I_{phase} = \frac{I_{start}}{\sqrt{3}}$$

Worked examples

Single phase motor

Input

P = 1.5 kW
U = 240 V
k_start = 5.5
Assumed efficiency η = 0.92 and power factor cos φ = 0.88

I_nom = 1000 × 1.5 / (240 × 0.92 × 0.88) ≈ 8.38 A

I_start = 8.38 × 5.5 ≈ 46.09 A

Three phase motor in star

P = 4.2 kW
U = 415 V
k_start = 6.5
Assumed efficiency η = 0.93 and power factor cos φ = 0.90

I_nom = 1000 × 4.2 / (√3 × 415 × 0.93 × 0.90) ≈ 6.62 A

I_start = 6.62 × 6.5 ≈ 43.03 A

Phase start current, star = 43.03 A

Three phase motor in delta

P = 9.0 kW
U = 480 V
k_start = 6.0
Assumed efficiency η = 0.91 and power factor cos φ = 0.87

I_nom = 1000 × 9.0 / (√3 × 480 × 0.91 × 0.87) ≈ 21.0 A

I_start = 21.0 × 6.0 ≈ 126.0 A

Phase start current, delta = 126.0 / √3 ≈ 72.7 A

Reference table — typical values

The table below gives approximate nominal and start currents for selected motor powers. Values assume 0.9 efficiency and 0.85 power factor for the conversions shown.

Power	Connection	Nominal current	k_start	Start current
0.37 kW	single	2.10 A	6	12.60 A
0.75 kW	single	4.26 A	6	25.56 A
1.1 kW	single	6.25 A	6	37.50 A
1.5 kW	single	8.53 A	6	51.18 A
2.2 kW	three star	4.15 A	6	24.90 A
3.0 kW	three star	5.66 A	6	33.96 A
4.0 kW	three delta	7.55 A	6	45.30 A
5.5 kW	three star	10.38 A	6	62.28 A

✍ When choosing protective devices consider both the inrush current and the duration of the surge. Contactors and motor circuit breakers with adjustable long time delay often prevent nuisance trips. Cable sizing must account for continuous running current, temperature rating and short term heating caused by starts.