Three-phase motor full-load current calculator
Estimate the full-load current (FLA) of a three-phase induction motor from its rated power — in kilowatts or horsepower — plus line voltage, power factor and efficiency. The result is the per-phase current used to size contactors, breakers, thermal overload relays and variable-frequency drives.
Three-phase motor full-load current
Formula: I = (kW × 1000) / (√3 × V × PF × η)
Rated full-load current is the current the motor draws at rated torque and speed — the figure a contactor, breaker, thermal overload relay or variable-frequency drive must be sized for (motors draw 5–8× FLA on startup, so protective devices are usually selected at or above the rating plate current). AO Ctrl stocks the switching and protection gear and drives for three-phase motors:
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Frequently asked questions
How is full-load current calculated for a three-phase motor?
FLA = (kW × 1000) / (√3 × V × PF × η), with PF and η from the nameplate — the tool defaults to 0.85 and 0.90. For horsepower the input is converted with hp × 745.7 W first. At the default 15 kW, 400 V, the computed FLA is about 28 A per phase.
Why does the default value matter?
The presets are typical for a 15 kW four-pole motor at 400 V — a common IEC 50 Hz rating. Substitute your own nameplate PF and efficiency, since a 0.05 error in power factor moves the result by several percent. FLA is the figure used to size contactors, breakers, thermal overload relays and variable-frequency drives.
How much current does a motor draw at startup?
A three-phase induction motor typically draws 5-8× its full-load current during startup, so protective devices are usually selected at or above the rating-plate current while the running load stays at the FLA you computed here. The FLA is the per-phase current at rated torque and speed.