Enter your motor's voltage (V), power rating (kW or hp), power factor, and efficiency to calculate the full load current (amps) drawn by a 3-phase motor. Results include current in amps plus a breakdown of how each parameter affects the draw — useful for cable sizing, breaker selection, and motor protection.
Full Load Current
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Motor Power (kW)
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Apparent Power (kVA)
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Input Power (kW)
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Power Losses (kW)
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For a 3-phase motor, the full load current (I) is calculated using: I = (1000 × P_kW) / (√3 × V × PF × η/100). Where P_kW is the motor's rated power in kilowatts, V is the line-to-line voltage, PF is the power factor, and η is the efficiency expressed as a decimal. The √3 factor (≈1.732) accounts for the three-phase relationship.
Most 3-phase induction motors operate with a power factor between 0.80 and 0.92 at full load. A commonly used design value is 0.86. At partial loads, the power factor tends to drop, which is why running motors at or near their rated load is recommended for efficiency.
Motor efficiency (η) is the ratio of mechanical output power to electrical input power, expressed as a percentage. A less efficient motor must draw more current to deliver the same shaft output. For example, a motor at 90% efficiency draws about 5.6% more current than one at 95% efficiency, all else being equal. Modern premium efficiency motors typically achieve 93–97%.
A 15 kW motor at 415V with a power factor of 0.86 and efficiency of 95% draws approximately 27.3 A. Using the formula: I = (1000 × 15) / (1.732 × 415 × 0.86 × 0.95) ≈ 27.3 A. This result can vary depending on actual power factor and efficiency values.
First convert horsepower to kilowatts: 1 hp = 0.7457 kW. Then apply the 3-phase current formula: I = (1000 × P_kW) / (√3 × V × PF × η/100). For example, a 20 hp motor at 460V, PF 0.86, and 95% efficiency draws approximately: 20 × 0.7457 = 14.914 kW, giving I ≈ 25.1 A.
In a balanced 3-phase system, the total power is the sum of power in all three phases. Because the three phases are 120° apart, the combined power works out to √3 (approximately 1.732) times the product of line voltage and line current. This factor distinguishes 3-phase calculations from single-phase ones.
For 3-phase motor calculations using the standard formula with the √3 factor, always use the line-to-line (phase-to-phase) voltage. Common 3-phase supply voltages include 208V, 230V, 380V, 400V, 415V, 460V, and 480V. The line-to-neutral voltage is used only for single-phase calculations from a 3-phase supply.
The calculated full load current (FLC) is your baseline for protection and wiring. Circuit breakers for motor circuits are typically sized at 250% of FLC for inverse-time breakers, or 175% for thermal-magnetic types, per NEC guidelines. Cable ampacity should be at least 125% of FLC for continuous motor loads. Always consult local electrical codes and standards for final design.