HP to Amps Calculator — Horsepower to Current
Convert motor horsepower to current draw in amperes. Works for DC circuits, single-phase AC and three-phase AC motors — enter HP, supply voltage, motor efficiency and power factor.
HP
Phase type
V
%
5 HP • 240 V • Single-phase AC
- 1
Power in watts
5 × 745.7 = 3,728.5 - 2
Denominator V × η × PF
240 × 0.9 × 0.85 = 183.6 - 3
Current I = W ÷ denominator
3,728.5 ÷ 183.6 = 20.31
How does this calculator work?
Convert horsepower to amps: multiply HP by 745.7 to get watts, then divide by voltage × efficiency for DC, or by voltage × efficiency × power factor for single-phase AC, or by √3 × voltage × efficiency × power factor for three-phase AC. Always add a 25% safety margin when sizing conductors.
Formula
How this is calculated
One mechanical horsepower equals 745.69987 watts (per IEC 60034). To find the current the motor draws from the supply, we divide the input power (output ÷ efficiency) by the supply voltage — and, for AC circuits, also by the power factor.
For a DC motor the formula is simply I = (HP × 746) ÷ (V × η), where η is efficiency as a decimal (e.g. 90% = 0.9). For single-phase AC, the power factor PF is also in the denominator: I = (HP × 746) ÷ (V × η × PF), because AC loads draw apparent power at an angle relative to real power. For a balanced three-phase supply the line voltage is √3 (≈ 1.732) times the phase voltage, so the denominator gains that factor: I = (HP × 746) ÷ (√3 × V × η × PF), where V is the line-to-line voltage.
Efficiency is typically 85–95% for modern NEMA or IEC frame motors at full load; 90% is a reasonable default. Power factor at full load is commonly 0.80–0.92. Using optimistic values will underestimate the actual current — always size wiring and breakers with a safety margin per NEC Article 430 (typically 125% of full-load current for motor branch circuits).
Frequently asked questions
With 90% efficiency and 0.85 power factor: I = (5 × 745.7) ÷ (240 × 0.9 × 0.85) ≈ 18.1 A. The NEC 430.248 table lists 28 A for a 5 HP, 240 V single-phase motor as the full-load current — slightly higher to account for nameplate variability and provide a wiring safety margin.
In a balanced three-phase system the line (phase-to-phase) voltage is √3 times the phase voltage. Expressing power as V_line × I × √3 × PF accounts for this geometry, so the denominator includes √3 when V is the line voltage.
No. NEC Article 430 requires motor branch-circuit conductors to be rated at 125% of the motor full-load current, and the overcurrent device at 150–250% depending on type. Use this calculator for a starting estimate, then apply NEC rules and consult a licensed electrician for final sizing.
Also known as
TG we-Calculate Editorial Team. (2026). HP to Amps Calculator — Horsepower to Current [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/hp-to-amps-calculator
TG we-Calculate Editorial Team. "HP to Amps Calculator — Horsepower to Current." TG we-Calculate. 2026. https://we-calculate.com/calculator/hp-to-amps-calculator.
TG we-Calculate Editorial Team, "HP to Amps Calculator — Horsepower to Current," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/hp-to-amps-calculator
@misc{wecalculate_hp_to_amps_calculator, title = {HP to Amps Calculator — Horsepower to Current}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/hp-to-amps-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?