Motor Current Calculator
Calculate motor full-load current from its rating, voltage, efficiency and power factor.
About Motor Current Calculator
Full-load current follows from the shaft rating once efficiency and power factor are accounted for. Cables are sized above this figure and overload relays set just above it to protect the winding without nuisance tripping. A 15 kW motor at 400 V draws around 27 A, and this is the starting point for every cable, contactor and protection selection.
Formula
Motor Current
I = P ÷ (√3 V η cos φ) for three phase
- η
- Motor efficiency
- overload setting
- Usually 110 to 120% of full-load current
Worked example
With the default values below, Motor Current Calculator returns Full-Load Current of 27.665. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Motor ratingminimum 0 kW
- 15 kW
- Supply voltageminimum 1 V
- 400 V
- Motor efficiencyaccepts 0–100 %
- 91 %
- Power factoraccepts 0.01–1
- 0.86
- Phasesone of Single phase, Three phase
- Three phase
Results
- Full-Load Current
- 27.665 A
- Electrical Input Power
- 16.4835 kW
- Minimum Cable Rating
- 34.6 A
- Typical Overload Relay Setting
- 31.81 A
- Apparent Power
- 19.1669 kVA
Step by step
- 1Values used
- 2Motor Current
- 3Full-Load Current= 27.665 A
- 4Electrical Input Power= 16.4835 kW
- 5Minimum Cable Rating= 34.6 A
- 6Typical Overload Relay Setting= 31.81 A
- 7Apparent Power= 19.1669 kVA
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Motor Current calculated?
I = P ÷ (√3 V η cos φ) for three phase Full-load current follows from the shaft rating once efficiency and power factor are accounted for. Cables are sized above this figure and overload relays set just above it to protect the winding without nuisance tripping.
Why does Motor Current matter?
A 15 kW motor at 400 V draws around 27 A, and this is the starting point for every cable, contactor and protection selection.
What values do I need to enter?
This calculator takes 5 inputs: Motor rating, Supply voltage, Motor efficiency, Power factor, Phases. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.