Electrical

Motor Current Calculator

Estimate motor running current from shaft output, efficiency and power factor. Compare SI/US results and review the method, assumptions and related references below.

Change any input to update the calculation. Unit changes preserve the physical quantity.

CALCULATION SETUP

Inputs

V
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%
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YOUR CALCULATION

Results

Estimated running current

10.3772 A

Mechanical shaft output

5.5 kW7.3756 hp

Real electrical input

6.1111 kW8.1951 hp

Motor losses

0.6111 kW0.8195 hp

Apparent input

7.1895 kVA

Calculation breakdown

Result interpretation
Estimate motor running current from shaft output, efficiency and power factor

Assumptions & checks

A shaft-load estimate, not nameplate full-load current, locked-rotor current, protection selection or installation approval.

KEEP THE JOB MOVING

Where does this result go next?

Input lookupMotor Current Estimate Reference

Compare conditional current scenarios

Next calculationAWG Wire Size Chart

Look up AWG area

Input lookupMotor kW & HP Conversion Chart

Convert mechanical HP and kW

Next calculationElectrical Power Calculator

Check balanced power

Follow the full workflow: Motor cable voltage-drop check →Follow the full workflow: Three-phase motor power and current check →

Formula

Estimate motor running current from shaft output, efficiency and power factor

Pinput=Pshaft/η; I=Pinput/(V·PF) for single phase, or /(√3·VLL·PF) for balanced three phase. Reverse Pshaft=I·phaseFactor·V·PF·η. Worked example: 5.5 kW shaft output, 400 V three phase, PF 0.85 and efficiency 90% gives about 10.377 A.

Worked example

Using the default values shown in the calculator, the same formula gives the following result. This is a quick sanity check for the calculation, not a design recommendation.

Inputs

Supply system
Balanced three-phase AC
Supply voltage (line-to-line for three phase)
400 V
Power factor
0.85
Motor efficiency
90 %
Mechanical shaft output
5.5 kW

Result

Estimated running current
10.3772 A
Mechanical shaft output
5.5 kW
Real electrical input
6.1111 kW
Motor losses
0.6111 kW
Apparent input
7.1895 kVA

REAL INPUTS · CLEAR METHOD

Practical worked examples

Illustrative scenarios using this page’s calculation or verified reference model. Change the assumptions for your own job.

5.5 kW motor at 400 V three phase

Given

Supply system
Balanced three-phase AC
Supply voltage (line-to-line for three phase)
400 V
Power factor
0.85
Motor efficiency
90 %
Mechanical shaft output
5.5 kW
Calculation / lookup method

Pinput=Pshaft/η; I=Pinput/(V·PF) for single phase, or /(√3·VLL·PF) for balanced three phase. Reverse Pshaft=I·phaseFactor·V·PF·η. Worked example: 5.5 kW shaft output, 400 V three phase, PF 0.85 and efficiency 90% gives about 10.377 A.

Result

Estimated running current
10.3772 A
Mechanical shaft output
5.5 kW
Real electrical input
6.1111 kW
Motor losses
0.6111 kW
Apparent input
7.1895 kVA

The 5.5 kW is mechanical shaft output. Dividing by 90% efficiency gives electrical input before applying the balanced three-phase current equation. This is an operating estimate, not an ampacity/FLC table.

7.5 kW motor at 415 V three phase

Given

Supply system
Balanced three-phase AC
Supply voltage (line-to-line for three phase)
415 V
Power factor
0.86
Motor efficiency
91 %
Mechanical shaft output
7.5 kW
Calculation / lookup method

Pinput=Pshaft/η; I=Pinput/(V·PF) for single phase, or /(√3·VLL·PF) for balanced three phase. Reverse Pshaft=I·phaseFactor·V·PF·η. Worked example: 5.5 kW shaft output, 400 V three phase, PF 0.85 and efficiency 90% gives about 10.377 A.

Result

Estimated running current
13.3325 A
Mechanical shaft output
7.5 kW
Real electrical input
8.2418 kW
Motor losses
0.7418 kW
Apparent input
9.5834 kVA

Efficiency and power factor are separate stated assumptions. Replace them with operating/nameplate data and review starting current independently.

10 HP motor at 230 V three phase

Given

Supply system
Balanced three-phase AC
Supply voltage (line-to-line for three phase)
230 V
Power factor
0.85
Motor efficiency
90 %
Mechanical shaft output
10 hp
Calculation / lookup method

Pinput=Pshaft/η; I=Pinput/(V·PF) for single phase, or /(√3·VLL·PF) for balanced three phase. Reverse Pshaft=I·phaseFactor·V·PF·η. Worked example: 5.5 kW shaft output, 400 V three phase, PF 0.85 and efficiency 90% gives about 10.377 A.

Result

Estimated running current
24.4689 A
Mechanical shaft output
7.457 kW
Real electrical input
8.2856 kW
Motor losses
0.8286 kW
Apparent input
9.7477 kVA

Mechanical horsepower converts to watts in the same engine. 230 V is line-to-line; the result is not a single-phase motor estimate.

Technical references for this calculation

Reference lookupSingle & Three-Phase Power Reference

Frequently asked questions

What does this result tell me?

A shaft-load estimate, not nameplate full-load current, locked-rotor current, protection selection or installation approval.

How can I check the calculation?

Worked example: 5.5 kW shaft output, 400 V three phase, PF 0.85 and efficiency 90% gives about 10.377 A. Review each stated input and unit; use project-specific values rather than treating the editable example as a requirement.

Reference data & method sources

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