HP to Amps Calculator — Free Online Calculator

How to Use This Calculator

Enter the motor horsepower rating, voltage, and phase. Adjust efficiency and power factor if known (typical motor efficiency is 85-95%, power factor 0.80-0.90).

The Formula Explained

First convert HP to watts: Watts = HP × 746. Then account for motor efficiency: Input Watts = Output Watts / Efficiency. Finally convert to amps: Single-phase: I = W / (V × PF). Three-phase: I = W / (V × √3 × PF).

Horsepower to Amps: Sizing Motor Circuits

Converting motor horsepower to amps is the daily bread of industrial electricians. Motors are specified by mechanical output power (HP or kW), but circuit design requires electrical input current. The conversion involves three factors: the mechanical-to-electrical unit conversion (1 HP = 746 watts), motor efficiency (usually 80-94%), and power factor (usually 0.80-0.92). For quick approximations you can use the formula I = HP × 746 / (V × efficiency × PF × sqrt(3) for three-phase or just V for single-phase).

But for real circuit sizing, NEC 430 directs you to use the nameplate full load amps (FLA) value rather than calculated values. This is because the calculated value depends on assumed efficiency and PF that vary between manufacturers and operating conditions, while nameplate FLA is the tested actual current at rated load. Always use the nameplate when available; use the HP calculation only for initial sizing when a specific motor has not been selected.

Worked Example: 25 HP Industrial Compressor

A 25 HP three-phase 480V air compressor has nameplate efficiency 90% and power factor 0.87. Electrical input power: 25 × 746 / 0.90 = 20,722 watts = 20.7 kW. Apparent power: 20.7 / 0.87 = 23.8 kVA. Current: 23,800 / (sqrt(3) × 480) = 28.6 amps.

For comparison, NEC Table 430.250 gives the standard FLA for a 25 HP 460V three-phase motor: 34 amps. The NEC table is higher than the calculated value because it is designed for conservative circuit sizing — it assumes worst-case efficiency and PF to ensure the wire and breaker can handle any motor of that rating. Use the NEC table value for wire and breaker sizing even when the actual nameplate is lower.

Circuit sizing: 34 amps × 125% (continuous) = 42.5 amps. Conductor: 8 AWG copper (50A at 75°C). Breaker: thermal magnetic set to 250% of FLA for motor starting (NEC 430.52), so 34 × 2.5 = 85A breaker. Overload relay set to 115% of FLA (NEC 430.32) = 39A.

Worked Example: 1.5 HP Pool Pump (Single-Phase)

A 1.5 HP pool pump at 230V single-phase. NEC Table 430.248 gives single-phase motor FLA: 10 amps for 1.5 HP at 230V. Note this is the FLA, which already accounts for typical motor efficiency and power factor.

Circuit sizing: 10 × 1.25 = 12.5 amps. The motor circuit needs to handle the continuous load plus starting surge. Conductor: 14 AWG copper (15A) is technically enough but 12 AWG (20A) is better for motor circuits because of starting surge and long-term reliability. Breaker: 20A instantaneous magnetic for short-circuit protection; thermal overload on the motor itself for running protection.

Common mistake: using a 15A breaker because the FLA is only 10A. On startup, the motor draws 6-7x FLA for about 1 second. A 15A breaker might trip nuisance-wise during startup even though it is code-compliant for steady-state operation. A 20A breaker handles starting surges more reliably.

Five HP Calculation Mistakes

1. Ignoring efficiency. 10 HP mechanical output does not equal 7.46 kW electrical input. A typical 10 HP motor at 87% efficiency draws 7.46 / 0.87 = 8.57 kW of electrical input.

2. Ignoring power factor for current calculation. Real power does not directly tell you current on reactive loads. You need apparent power (kVA) which requires dividing kW by PF.

3. Using calculated values instead of nameplate FLA. NEC 430 requires using nameplate FLA for circuit sizing. Calculated values are only a starting point.

4. Confusing HP input and HP output. Motor HP on the nameplate is mechanical output power. Input electrical power is higher due to losses. Do not multiply HP × 746 to get input watts without dividing by efficiency.

5. Forgetting starting surge on breaker selection. Motor starting current is 5-7x FLA for a few seconds. Instantaneous-trip breakers must be sized to ride through this surge without tripping. NEC 430.52 provides the multiplier for standard breakers (250% of FLA for most types).

NEC Motor FLA Tables Quick Reference

Single-phase 230V (NEC Table 430.248): 1/2 HP = 4.9A. 1 HP = 8A. 1.5 HP = 10A. 2 HP = 12A. 3 HP = 17A. 5 HP = 28A. 7.5 HP = 40A. 10 HP = 50A.

Three-phase 460V (NEC Table 430.250): 1 HP = 2.1A. 2 HP = 3.4A. 3 HP = 4.8A. 5 HP = 7.6A. 10 HP = 14A. 15 HP = 21A. 20 HP = 27A. 25 HP = 34A. 30 HP = 40A. 50 HP = 65A. 75 HP = 96A. 100 HP = 124A.

Use these values directly for circuit sizing. The table values are conservative and cover any manufacturer's motor at the listed HP/voltage. Nameplate FLA on a specific motor may be lower, and you can use the actual nameplate for wire sizing if it is marked as "Design B" or similar standard type. For overload protection sizing, always use the actual nameplate FLA.

NEC Article 430 Motor Circuit References

430.22 — conductor sizing (125% of FLA for continuous operation). 430.32 — overload protection (115-125% of FLA depending on motor type). 430.52 — short-circuit and ground-fault protection (250% of FLA for thermal-mag, higher for instantaneous-trip). Table 430.248 — FLA values for single-phase motors by HP and voltage. Table 430.250 — FLA values for three-phase motors by HP and voltage.

For motor control circuits, NEC 430.75 governs. For multi-motor installations, 430.24 allows feeder sizing at 125% of the largest motor plus 100% of the others, which provides significant savings compared to sizing for all motors at 125%. NEMA MG 1 covers motor design and efficiency standards that complement the NEC electrical installation requirements.

Frequently Asked Questions

Related Calculators