Table of Contents
What Is HP to Amps Conversion?
Horsepower (HP) measures the mechanical output power of a motor, while amperes (amps) measure the electrical current the motor draws. Converting HP to amps is essential for sizing circuit breakers, selecting wire gauges, and designing electrical panels. One mechanical horsepower equals 746 watts.
The conversion depends on several factors: the supply voltage, motor efficiency (how effectively the motor converts electrical energy to mechanical energy), and for AC motors, the power factor (the ratio of real power to apparent power). Typical motor efficiencies range from 80% to 95%, and power factors from 0.75 to 0.95.
Conversion Formulas
Reference Table
| HP | 120V 1-Phase | 230V 1-Phase | 460V 3-Phase |
|---|---|---|---|
| 1 | 9.8 A | 5.1 A | 1.5 A |
| 3 | 29.3 A | 15.3 A | 4.4 A |
| 5 | 48.9 A | 25.5 A | 7.4 A |
| 10 | 97.8 A | 51.0 A | 14.7 A |
| 25 | 244.4 A | 127.6 A | 36.8 A |
Factors Affecting Current
- Voltage: Higher voltage means lower current for the same power output.
- Motor Efficiency: Less efficient motors draw more current to produce the same mechanical output.
- Power Factor: A lower power factor increases the apparent current drawn from the supply.
- Starting Current: Inrush current can be 5-8 times the full-load amperage during startup.
- Load: A motor running below rated load draws less current than at full load.
Frequently Asked Questions
How many amps does a 5 HP motor draw?
A 5 HP single-phase motor at 230V with 90% efficiency and 0.85 power factor draws approximately 25.5 amps. At 460V three-phase, the same motor draws about 7.4 amps. Always check the motor nameplate for exact ratings.
Why is the starting current much higher than running current?
At startup, the motor has no back-EMF (electromotive force) to oppose the supply voltage, so the winding impedance alone limits the current. As the motor accelerates, back-EMF builds up and reduces the current to the normal running level.
What wire size do I need for a given amperage?
Wire sizing depends on current, distance, and ambient temperature. As a general guide: 14 AWG handles up to 15A, 12 AWG up to 20A, 10 AWG up to 30A, 8 AWG up to 40A, and 6 AWG up to 55A. Always consult the NEC tables and local codes.