hp to amps Calculator

Converting mechanical horsepower into electrical amperes requires bridging mechanical output power with electrical input power. Because motors are not 100% efficient and AC circuits involve phase relationships, we must factor in motor efficiency (η) and power factor (PF). First, horsepower is converted into watts where 1 hp equals 746 watts. The general formula divides the total required power by the product of voltage, efficiency, and system-specific multipliers: For direct current (DC), I = P / (V × η). For single-phase AC, I = P / (V × PF × η). For three-phase line-to-line (LL) configurations, the formula is I = P / (√3 × V × PF × η), and for line-to-neutral (LN) configurations, I = P / (3 × V × PF × η).

Let us calculate the current draw for a 5 hp three-phase AC motor operating on a 460V line-to-line voltage supply. Assume a motor efficiency of 85% (0.85) and a power factor of 0.90. First, convert horsepower to watts: 5 hp × 746 W/hp = 3,730 watts. Next, apply the three-phase line-to-line formula: I = 3730 / (√3 × 460 × 0.90 × 0.85). Calculating the denominator gives approximately 1.732 × 460 × 0.90 × 0.85 = 609.43. Finally, divide the wattage by this denominator: 3730 / 609.43 = 6.12 amps of running current.

When sizing circuit breakers, wires, and motor starters, always consult the motor's specific nameplate data rather than relying solely on theoretical calculations, as real-world efficiencies vary by manufacturer. Remember that the current calculated here represents running load current (FLC); starting current (inrush current) can be significantly higher—often 5 to 7 times the running amp draw—which must be accounted for when selecting protective devices.