Amps to HP Calculator
Convert full-load current (amps) to motor horsepower (HP) for single-phase and three-phase motors.
How to convert Amps to HP
The formula for a single-phase AC motor is HP = (Amps × Volts × PF × Efficiency) / 745.7.
For example, a motor drawing 21.6A at 240V with a 0.8 power factor and 90% efficiency produces exactly 5 HP. For three-phase systems, remember to multiply by the square root of 3 (1.732).
Common Amps to HP Conversions (240V, Single-Phase, 0.8 PF, 0.9 Eff)
| Current (Amps) | Voltage | Horsepower (HP) |
|---|---|---|
| 4.3 A | 240V | 1 HP |
| 8.6 A | 240V | 2 HP |
| 12.9 A | 240V | 3 HP |
| 21.6 A | 240V | 5 HP |
| 43.1 A | 240V | 10 HP |
Identifying Motor Size from Measured Current: Field Examples
Electricians and maintenance technicians often need to estimate a motor's horsepower rating from a clamp-meter reading when the nameplate is missing or unreadable. Comparing measured amps to NEC Table 430.250 (three-phase, NFPA 70) or Table 430.248 (single-phase) gives a reliable cross-check. Here are three realistic field scenarios:
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Unknown 460 V three-phase pump motor reading 14.5 A at full load.
NEC Table 430.250 lists 14 A FLA for a 10 HP motor at 460 V (3-phase). The slight difference (14.5 vs. 14 A) suggests this is a 10 HP motor running at near-rated load, or possibly a 15 HP motor (21 A NEC) running very lightly. Applying the formula: HP ≈ 14.5 × 460 × 1.732 × 0.85 × 0.90 ÷ 746 ≈ 11.1 HP, confirming a 10 HP motor running just above nameplate or a slightly oversized replacement winding. -
230 V single-phase compressor drawing 16.8 A.
NEC Table 430.248 lists 16 A FLA for a 3 HP single-phase motor at 230 V. Formula check: HP ≈ 16.8 × 230 × 0.85 × 0.88 ÷ 746 ≈ 3.1 HP. This is consistent with a 3 HP compressor motor running at rated load, which is the most common small-shop compressor size. -
208 V three-phase conveyor motor reading 10 A at partial load.
Formula: HP ≈ 10 × 208 × 1.732 × 0.85 × 0.90 ÷ 746 ≈ 3.7 HP. The nameplate likely says 5 HP — partial load (74 % of nameplate) is common for conveyors whose loads vary through the shift. Running a motor below its rated load reduces efficiency and can lower power factor, which is worth monitoring on large multi-motor lines.
Frequently Asked Questions
Why do I need efficiency and power factor?
Motors are not 100% efficient, and AC inductive loads have a power factor. These two values represent the real mechanical power produced vs. the total electrical power consumed.
What is a typical motor efficiency?
Standard electric motors range from 85% to 95% efficiency. We use 90% (0.9) as a standard default if the nameplate value is unknown.
What is 1 HP in watts?
One mechanical horsepower is exactly 745.7 watts.