Convert horsepower to amps for single-phase and three-phase motors. Accounts for motor efficiency and power factor. Includes NEMA reference tables.
Converting horsepower to amperage is one of the most common calculations in electrical engineering and motor installation. Since motors are rated in horsepower but circuit breakers, wire gauges, and fuses are rated in amperes, this conversion is essential for safe and code-compliant electrical installations.
The conversion is not a simple fixed ratio because it depends on the supply voltage, motor efficiency, power factor, and whether the system is single-phase or three-phase. A 10 HP motor on a 240V single-phase circuit draws significantly more current than the same motor on a 480V three-phase system.
This calculator handles both single-phase and three-phase conversions, accounts for motor efficiency and power factor, and computes the full power triangle (real, reactive, and apparent power). It includes reference tables for NEMA Premium efficiency ratings and a quick-reference HP-to-amps table at common voltages. Electricians, HVAC technicians, and maintenance professionals will find this tool indispensable for motor circuit design.
The HP-to-amps formula involves efficiency, power factor, and a √3 factor for three-phase that people frequently forget or misapply. Under-estimating current leads to tripped breakers, overheated wires, and fire hazards. Over-estimating wastes money on oversized components. This calculator ensures accurate results every time and shows exactly how efficiency and power factor affect the answer.
Single-Phase: I = (HP × 746) / (V × η × PF) Three-Phase: I = (HP × 746) / (√3 × V × η × PF) Where: I = current in amperes HP = horsepower 746 = watts per horsepower V = voltage η = motor efficiency (decimal) PF = power factor (decimal)
Result: 20.3 A
A 5 HP single-phase motor at 240V with 88% efficiency and 0.87 power factor: I = (5 × 746) / (240 × 0.88 × 0.87) = 3730 / 183.7 = 20.3 amps. The apparent power is 4.87 kVA.
Every electric motor has a nameplate with critical information: HP rating, voltage, full-load amps (FLA), efficiency, power factor, RPM, and frame size. The HP-to-amps calculation should always be verified against the nameplate FLA, which represents the manufacturer's tested value. The calculated value is most useful when selecting motors or during preliminary design when the actual motor has not yet been chosen.
Motors are inductive loads that create a phase difference between voltage and current. Real power (W) does useful work, reactive power (VAR) sustains the magnetic field, and apparent power (VA) is what the utility must deliver. The power factor tells you what fraction of apparent power is real power. Improving power factor with capacitors reduces apparent power (and thus current) without changing the motor's output.
The National Electrical Code (NEC Article 430) requires motor branch circuit conductors to carry at least 125% of the motor's full-load current. Overcurrent protection devices must handle the motor's starting current transient. These requirements exist because motors routinely draw more current than their nameplate rating during starting and under overload conditions. Always consult NEC tables and local codes when designing motor circuits.
It depends on voltage and efficiency. At 120V single-phase with 85% efficiency and 0.85 PF: I = 746 / (120 × 0.85 × 0.85) ≈ 8.6 A. At 240V, it drops to about 4.3 A.
Three-phase systems distribute power across three wires with 120° phase offset. For the same power, three-phase requires √3 times less current per line than single-phase, enabling thinner wires and smaller components.
A motor with 85% efficiency consumes more power (and thus more current) than one with 95% efficiency to produce the same mechanical output. Lower efficiency means more current and more heat dissipation.
Power factor is the ratio of real power (watts) to apparent power (VA). Motors are inductive loads with power factors below 1 (typically 0.80-0.95). Low power factor means higher current for the same real power output.
One mechanical horsepower equals exactly 746 watts. This conversion factor translates the motor's mechanical output rating (HP) into electrical power units (watts) needed for the current calculation.
No. Per NEC Article 430, motor branch circuits require 125% of full-load amps for continuous-duty motors. Also consider starting current (which can be 5-8× full load) when selecting the breaker type and rating.