HP to Amps Calculator

About the HP to Amps Calculator

This HP to amps calculator converts motor horsepower into full-load amps for single-phase, three-phase, or DC motors. Use it to size breakers, conductors, and overload protection, or to verify a motor nameplate against expected current draw. Built using the standard NEC formula with default values for typical NEMA-rated motors.

Defaults match common Canadian and US industrial motor ratings (90% efficiency, 0.85 power factor). Adjust the values for high-efficiency Premium motors (92–96%), specialty motors, or motors operating at part-load.

How HP to amps conversion works

Three formulas based on phase configuration:

3-phase: I = (HP × 746) ÷ (V × η × PF × √3)
Single-phase: I = (HP × 746) ÷ (V × η × PF)
DC: I = (HP × 746) ÷ (V × η)

The constant 746 comes from the definition of horsepower (746 watts). Efficiency (η) and power factor (PF) reduce input power requirements compared to ideal motors. The √3 factor (1.732) accounts for the three-phase line voltage relationship.

Worked example

A 5 HP, 460V three-phase motor at 90% efficiency and 0.85 PF: I = (5 × 746) ÷ (460 × 0.9 × 0.85 × 1.732) = ~6.5 amps. To size the branch circuit per NEC 430.22, multiply by 1.25: 6.5 × 1.25 = 8.1 amps minimum conductor and breaker rating.

When to use this calculator

• Sizing motor branch circuits (conductors and breakers per NEC 430)
• Verifying measured motor amperage against nameplate
• Right-sizing overload relays and motor starters
• Specifying VFDs by required output amperage
• Calculating peak demand for utility billing
• Estimating wire and conduit costs for new installations
• Cross-referencing NEC Table 430.250 quick lookups

Frequently Asked Questions

How do you convert HP to amps for a 3-phase motor?

Multiply HP by 746, then divide by (voltage × efficiency × power factor × 1.732). For a 5 HP, 460V motor at 90% efficiency and 0.85 PF: I = (5 × 746) ÷ (460 × 0.9 × 0.85 × 1.732) ≈ 6.5 A.

What is the formula for HP to amps single-phase?

I = (HP × 746) ÷ (V × η × PF). For a 1 HP, 120V motor at 80% efficiency and 0.85 PF: I = (1 × 746) ÷ (120 × 0.8 × 0.85) ≈ 9.1 A.

Should I use this calculator or NEC tables for sizing?

For code-required sizing of branch circuits and overloads, use NEC Tables 430.248 (single-phase) and 430.250 (three-phase). This calculator gives expected operating amps based on actual nameplate values; NEC tables are conservative for safety. Use NEC values for code compliance.

What efficiency and power factor should I use if I don’t know them?

Default to 90% efficiency and 0.85 PF for typical industrial motors. NEMA Premium motors are 92–96% efficient. Power factor at full load is 0.85–0.92 for most motors; at light load it drops to 0.5–0.7.

Why is the calculated value different from the NEC table?

NEC tables use conservative values to ensure circuits are sized safely across a range of motor conditions. Your specific motor’s nameplate FLA may be lower than the NEC table value but you must size the circuit to the larger of the two.

Does this work for variable-frequency drives (VFDs)?

For sizing the wiring between a VFD and motor, yes — use motor nameplate HP and voltage. For sizing the supply wiring TO the VFD, add 25% margin per NEC 430.22 to account for harmonic distortion.

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Sizing a motor circuit? Browse General Purpose, Explosion Proof, or HVAC motors from Baldor, WEG, Marathon, and U.S. Motors.

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