HP to Amps Calculator

What a Horsepower-to-Amps Estimate Is For

A motor's horsepower rating describes the mechanical power it delivers at the shaft, but the supply has to provide more than that: some energy is lost as heat inside the motor, and on AC circuits some of the current flowing back and forth does no useful work at all. That's why a "1 hp" motor on a 230 V line draws noticeably more than the 3.2 A you'd get by just dividing 746 watts by 230 volts. This calculator folds in efficiency and power factor to give a realistic running-current estimate — handy for comparing motors, checking whether a generator or inverter can carry a load, sizing a battery bank for a DC motor, or sanity-checking a nameplate.

What it is not for: in the United States, conductors, overload protection, and branch-circuit breakers for motors must be sized from the full-load current values in the National Electrical Code, Tables 430.247 (DC), 430.248 (single-phase), and 430.250 (three-phase) — not from a formula and not from the nameplate (NEC 430.6(A)(1)). The code tables deliberately use conservative figures: a 10 hp, 460 V three-phase motor is listed at 14 A, somewhat higher than a typical formula estimate. If you are wiring a motor circuit, look up the table value or have a licensed electrician do it.

What You Need Before You Start

• Horsepower (or measured amps, for the reverse conversion) from the nameplate.
• Supply voltage. For three-phase, use the line-to-line voltage (208, 230, 460, 575 V and so on).
• Efficiency. Nameplates often list it; if not, 85% is a reasonable general default. Small fractional-horsepower motors can be down around 60–75%, large premium-efficiency motors reach 93–96%.
• Power factor (AC only). Defaults to 0.85; induction motors at full load typically fall between about 0.75 and 0.90, and it drops sharply when the motor is lightly loaded. DC motors don't have one, so the field is hidden.

The Logic Behind the Calculation

One mechanical horsepower equals 746 watts. Input power is output power divided by efficiency, and current follows from the electrical power formula for each supply type:

• DC: I = hp × 746 ÷ (V × efficiency)
• Single-phase AC: I = hp × 746 ÷ (V × efficiency × PF)
• Three-phase AC: I = hp × 746 ÷ (√3 × V × efficiency × PF)

The reverse mode rearranges the same equations: hp = I × V × efficiency (× PF, × √3 for three-phase) ÷ 746. For example, 10 hp on 460 V three-phase at 90% efficiency and 0.85 PF gives 7,460 ÷ (1.732 × 460 × 0.9 × 0.85) ≈ 12.2 A. The same motor at the default 85% efficiency comes out at about 13.0 A.

When the Number Might Not Match Reality

The result is running current at full rated load. Starting (inrush) current on an across-the-line start is typically five to eight times higher for a fraction of a second, which matters for generator sizing. A motor running below its rated load draws less current than calculated, but not proportionally less, because power factor falls off. Voltage also matters: a motor on a supply that sags below nameplate voltage draws more current to deliver the same power. And the nameplate FLA, where printed, is the manufacturer's measured figure for that exact motor — when you have it, it beats any formula for everyday estimates, while the NEC tables remain the rule for wiring.