electric_bolt Motor Full Load Current Calculator (kW ↔ HP) star

How It's Calculated

Full load current comes from rearranging the electrical power equation. Single-phase: P = V × I × PF × η, so I = P ÷ (V × PF × η). Three-phase: P = √3 × V × I × PF × η, so I = P ÷ (V × √3 × PF × η) — where P is shaft (mechanical output) power in watts, V is line voltage, PF is power factor and η is efficiency.

Worked example: a three-phase 15 kW motor on a 400 V supply, with a typical power factor of 0.85 and efficiency of 0.90. Current is I = 15,000 ÷ (400 × 1.732 × 0.85 × 0.90) = 15,000 ÷ 530.6 ≈ 28.3 A — matching the roughly 28 A this calculator returns and the figures published in manufacturer motor tables for that frame size.

What You Should Know

Power factor and efficiency are the two variables that make this calculation only an estimate unless you use the real numbers from the motor's own nameplate. Both are properties of a specific motor's design, winding and load point — not universal constants. Power factor in induction motors typically runs 0.8 to 0.95, dropping noticeably when the motor runs lightly loaded, since the magnetizing current stays roughly constant while the in-phase (work-producing) current shrinks. Efficiency typically runs 80% to 95%, rising with motor size and with modern IE3/IE4 efficiency-class designs, and falling somewhat below full load and well below rated speed.

Because both factors are load-dependent, a motor running at half its rated power doesn't simply draw half the calculated current — power factor collapses faster than power does, so the current-to-power ratio actually gets worse at partial load. This is exactly why using catalog "typical" values instead of nameplate figures gives only an approximate result, useful for early sizing but not for final protection settings.

• Nameplate data always wins. The plate on the physical motor states its actual rated current directly — when it's available, use it instead of recalculating from power, voltage, PF and efficiency, since those last two vary by winding design and manufacturing tolerance.
• Starting current is a separate number entirely. Direct-on-line starting draws roughly 5 to 8 times full load current for a second or two while the rotor accelerates from standstill — breakers, contactors and cable are sized with this inrush in mind, not just the steady-state full load amps this calculator produces.
• kVA is not the same as kW. Apparent power (kVA) is what the electrical supply — transformer, generator, cable — must be sized to deliver; it's inflated above the real mechanical work (kW) by exactly the power factor and efficiency losses this formula accounts for.