Calculation reviewed: 4 October 2026 · Engineering Desk Tools
How to use the Motor shaft output to input current calculator
Estimate motor input power, apparent power and line current from shaft output, efficiency and power factor. Start with measurements or documented values for the equipment and reporting period you actually want to assess. The calculator does not fill in a typical installation for you. Its blank fields help keep a demonstration value from being mistaken for your own operating data.
Choose the required options, enter the values in the labelled units and select Calculate. If a field is rejected, check its range and unit before changing it. The result is accompanied by the equation and assumptions so you can decide whether it answers your engineering question.
Inputs and measurement basis
- Supply. Use three-phase for a balanced three-phase supply and single-phase for a two-wire circuit.
- Motor shaft output power (kW). Shaft output from the nameplate or a measurement, in kW. Convert horsepower first. The value must be greater than zero.
- Motor efficiency at this load (%). Take it from the manufacturer data at the real load point. It is usually lower at light load. Allowed range: 1 to 100.
- Power factor (decimal, up to 1). Power factor as a decimal, from the nameplate or a measurement at the real load. The allowed maximum is 1. The value must be greater than zero.
- Supply voltage (V). Supply voltage at the motor terminals; line-to-line for three-phase. The value must be greater than zero.
Record the measurement source and operating condition. Retain unrounded values for verification.
Method and interpretation
A motor nameplate normally states shaft output in kW or horsepower. The supply must also provide the motor losses, so electrical input is the output divided by efficiency. Current then follows from input power, voltage and power factor. Efficiency and power factor both fall at partial load, so full-load nameplate values understate the current drawn by a lightly loaded motor for the power it delivers.
Input power = shaft output / (efficiency / 100). Three-phase current I = input kW × 1000 / (√3 × V × PF). Single-phase current I = input kW × 1000 / (V × PF). Apparent power = input kW / PF.
Use the efficiency and power factor for the actual load point, not only the nameplate full-load values: both fall at light load. This is a steady-running estimate. It excludes starting current, service factor and cable or breaker sizing. Compare with the nameplate current and a clamp-meter reading.
Worked example
This example does not populate the form. Enter your own installation data.
Calculated result: 20.7544 A. Estimated running current at the entered load. Starting current and protection settings need separate assessment.
Check the arithmetic independently using the displayed equation. Retain intermediate precision and compare the final value with your equipment documentation.
Quick-reference table
These illustrative rows retain the worked example assumptions except for the varied input. They are not certified ratings or manufacturer data. Use the form for intermediate values.
Common mistakes
Entering electrical input power as shaft power; using full-load efficiency for a lightly loaded motor; typing 85 instead of 0.85 for power factor; treating the running current as the starting current. Before relying on the output, check the decimal format, unit scale and source of each value. Recheck unusual results against the formula.
Frequently asked questions
Are the example values used automatically?
No. Inputs remain blank when the tool opens or when you clear them. The worked example and table are reading material below the calculator. Enter the actual values you want to check each time, and label saved reports so their context is not lost.
What does the result mean?
The result is the line current the motor should draw while delivering the entered shaft power. Compare it with the nameplate current and a clamp-meter reading; a large difference suggests a different load or a data error.
Can I use a zero or a negative value?
The allowed range follows the meaning of each input. A divisor needs a positive value; other fields may allow zero. Check the form labels and error message rather than forcing an invalid value. A rejected input must be corrected before a result can be shown.
Is this a final design or equipment approval?
No. Use the efficiency and power factor for the actual load point, not only the nameplate full-load values: both fall at light load. This is a steady-running estimate. It excludes starting current, service factor and cable or breaker sizing. Compare with the nameplate current and a clamp-meter reading. Use the calculation as support for a checked decision, and review equipment documentation and applicable requirements before changing an installation or operating setting.
How can I save or share a calculation?
Calculate first, then use Copy result or Print report. Some maintenance tools also provide a CSV summary. Reports contain your own entered data, so review them before sharing. You can share the public page URL without including plant records or confidential measurements.
How do I report an unexpected result?
Use the Contact page and include the tool URL, input values, units, expected output and the reference you used. Explain the measurement basis. Exclude confidential information.
Related tools and records
Use related calculations with consistent units, periods and equipment scope.
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