How many amps does a 1 HP motor draw?

It depends on voltage and efficiency. At 120V single-phase with 85% efficiency and 0.85 PF: I = 746 / (120 × 0.85 × 0.85) ≈ 8.6 A. At 240V, it drops to about 4.3 A.

What is the difference between single-phase and three-phase current?

Three-phase systems distribute power across three wires with 120° phase offset. For the same power, three-phase requires √3 times less current per line than single-phase, enabling thinner wires and smaller components.

Why does motor efficiency matter for current calculation?

A motor with 85% efficiency consumes more power (and thus more current) than one with 95% efficiency to produce the same mechanical output. Lower efficiency means more current and more heat dissipation.

What is power factor?

Power factor is the ratio of real power (watts) to apparent power (VA). Motors are inductive loads with power factors below 1 (typically 0.80-0.95). Low power factor means higher current for the same real power output.

What is 746 in the HP to amps formula?

One mechanical horsepower equals exactly 746 watts. This conversion factor translates the motor's mechanical output rating (HP) into electrical power units (watts) needed for the current calculation.

Should I size the circuit breaker for the calculated amps?

No. Per NEC Article 430, motor branch circuits require 125% of full-load amps for continuous-duty motors. Also consider starting current (which can be 5-8× full load) when selecting the breaker type and rating.

HP to Amps Calculator

Convert horsepower to amps for single-phase and three-phase motors. Accounts for motor efficiency and power factor. Includes NEMA reference tables.

Horsepower (HP)

Phase

Voltage

Motor EfficiencyTypical: 80-96%

Power FactorTypical: 0.80-0.95

Current (Amps)

20.30 A

Line current

Power Output

3,730 W

5 HP × 746 W/HP

Power Input

4,239 W

At 88% efficiency

Apparent Power

4.87 kVA

Power factor: 0.87

Reactive Power

2,402 VAR

Due to inductive load

Heat Loss

509 W

12.0% of input

Power Triangle

Real: 4,239 W

Reactive: 2,402 VAR

Apparent: 4,872 VA

Quick Reference: HP to Amps at Common Voltages

HP	120V 1Φ	240V 1Φ	240V 3Φ	480V 3Φ
0.5	4.1	2.0	1.2	0.6
1	8.1	4.1	2.3	1.2
2	16.2	8.1	4.7	2.3
3	24.4	12.2	7.0	3.5
5	40.6	20.3	11.7	5.9
7.5	60.9	30.4	17.6	8.8
10	81.2	40.6	23.4	11.7
15	121.8	60.9	35.2	17.6
20	162.4	81.2	46.9	23.4
25	203.0	101.5	58.6	29.3
50	406.0	203.0	117.2	58.6

Motor Efficiency by HP Rating (NEMA Premium)

HP	Efficiency (%)	Power Factor
1	85.5%	0.82
5	89.5%	0.86
10	91.7%	0.88
25	93%	0.89
50	94.1%	0.9
100	95.4%	0.91
200	96.2%	0.92
500	96.8%	0.93

Planning notes, formulas, and examples

About the HP to Amps Calculator

Converting horsepower to amperage is one of the most common calculations in electrical engineering and motor installation. Since motors are rated in horsepower but circuit breakers, wire gauges, and fuses are rated in amperes, this conversion is essential for safe and code-compliant electrical installations.

The conversion is not a simple fixed ratio because it depends on the supply voltage, motor efficiency, power factor, and whether the system is single-phase or three-phase. A 10 HP motor on a 240V single-phase circuit draws significantly more current than the same motor on a 480V three-phase system.

This calculator handles both single-phase and three-phase conversions, accounts for motor efficiency and power factor, and computes the full power triangle (real, reactive, and apparent power). It includes reference tables for NEMA Premium efficiency ratings and a quick-reference HP-to-amps table at common voltages. Electricians, HVAC technicians, and maintenance professionals will find This calculator indispensable for motor circuit design.

When This Page Helps

The HP-to-amps formula involves efficiency, power factor, and a √3 factor for three-phase that people frequently forget or misapply. Under-estimating current leads to tripped breakers, overheated wires, and fire hazards. Over-estimating wastes money on oversized components. This calculator keeps the answer tied to the full formula and shows exactly how efficiency and power factor affect the current draw.

How to Use the Inputs

Enter the motor horsepower rating.
Select single-phase or three-phase operation.
Enter the supply voltage (common values: 120V, 240V, 480V).
Enter the motor efficiency percentage (check the nameplate; typical 80-96%).
Enter the power factor (check the nameplate; typical 0.80-0.95).
Read the required amperage, power input/output, apparent power, and reactive power.
Use the reference table to compare with standard values at other voltages.

Formula used

Single-Phase:
  I = (HP × 746) / (V × η × PF)

Three-Phase:
  I = (HP × 746) / (√3 × V × η × PF)

Where:
  I = current in amperes
  HP = horsepower
  746 = watts per horsepower
  V = voltage
  η = motor efficiency (decimal)
  PF = power factor (decimal)

Example Calculation

Result: 20.3 A

A 5 HP single-phase motor at 240V with 88% efficiency and 0.87 power factor: I = (5 × 746) / (240 × 0.88 × 0.87) = 3730 / 183.7 = 20.3 amps. The apparent power is 4.87 kVA.

Tips & Best Practices

Always use the nameplate efficiency and power factor, not estimated values, for accurate current calculation.
Three-phase motors draw significantly less current per line than equivalent single-phase motors.
Wire sizing and breaker selection should use 125% of the full-load amps per NEC Article 430.
Power factor can be improved with capacitor banks, reducing line current without changing real power.
Motor starting current (locked rotor amps) is typically 5-8× the full-load current, affecting breaker selection.
Higher voltage reduces current proportionally — 480V three-phase systems require much thinner wiring than 240V single-phase.

Understanding Motor Nameplate Data

Every electric motor has a nameplate with critical information: HP rating, voltage, full-load amps (FLA), efficiency, power factor, RPM, and frame size. The HP-to-amps calculation should always be verified against the nameplate FLA, which represents the manufacturer's tested value. The calculated value is most useful when selecting motors or during preliminary design when the actual motor has not yet been chosen.

Power Triangle: Real, Reactive, and Apparent

Motors are inductive loads that create a phase difference between voltage and current. Real power (W) does useful work, reactive power (VAR) sustains the magnetic field, and apparent power (VA) is what the utility must deliver. The power factor tells you what fraction of apparent power is real power. Improving power factor with capacitors reduces apparent power (and thus current) without changing the motor's output.

NEC Motor Circuit Requirements

The National Electrical Code (NEC Article 430) requires motor branch circuit conductors to carry at least 125% of the motor's full-load current. Overcurrent protection devices must handle the motor's starting current transient. These requirements exist because motors routinely draw more current than their nameplate rating during starting and under overload conditions. Always consult NEC tables and local codes when designing motor circuits.

Sources & Methodology

Last updated: March 7, 2026

Frequently Asked Questions

It depends on voltage and efficiency. At 120V single-phase with 85% efficiency and 0.85 PF: I = 746 / (120 × 0.85 × 0.85) ≈ 8.6 A. At 240V, it drops to about 4.3 A.