Electrical Motors - Full Load Amps
As a "rules of thumb" amps horsepower rating can be estimated to
- 115 volts motor - single-phase : 14 amps/hp
- 230 volts motor - single-phase : 7 amps/hp
- 230 volts motor - 3-phase : 2.5 amps/hp
- 460 volts motor - 3-phase : 1.25 amps/hp
Always check nameplate information before designing protective devices, wiring and switch gear.
Single-Phase Motors - HP and Full-Load Currents
A motor of a given rated horsepower is expected to deliver that quantity of mechanical power at the motor shaft. Be aware that the motor efficiency is not calculated in to the values for kW and amps below. Motor efficiency must be considered to avoid undersized power supply.
| Power | Full Load Current (amp) | |||
|---|---|---|---|---|
| (hp) | (kW) | 115 V | 208 V | 230 V |
| 1/6 | 0.13 | 4.4 | 2.4 | 2.2 |
| 1/4 | 0.19 | 5.8 | 3.2 | 2.9 |
| 1/3 | 0.25 | 7.2 | 4.0 | 3.6 |
| 1/2 | 0.38 | 9.8 | 5.4 | 4.9 |
| 3/4 | 0.56 | 13.8 | 7.6 | 6.9 |
| 1 | 0.75 | 16 | 8.8 | 8 |
| 1 1/2 | 1.1 | 20 | 11 | 10 |
| 2 | 1.5 | 24 | 13.2 | 12 |
| 3 | 2.3 | 34 | 18.7 | 17 |
| 5 | 3.8 | 56 | 30.8 | 28 |
Note that most electric motors are designed to run at 50% to 100% of rated load and that maximum efficiency usually is near 75% of rated load. For a 1 horsepower (hp) motor the load should typically be in the range 1/2 to 1 hp with maximum efficiency at 3/4 hp.
Typical load ranges:
- acceptable for a short-period: 20 - 120%
- acceptable for operating: 50 - 100%
- optimum efficiency: 60 - 80%
A motor with a service factor can occasional be overloaded. Overloading will over time reduce motor efficiency.
Three-Phase Motors - HP and Full-Load Currents
| Power | Full-Load Current (amp) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Induction Type Squirrel-Cage and Wound Motor | Synchronous Type Unity Power Factor | |||||||||
| (hp) | (kW) | 115 V | 230 V | 460 V | 575 V | 2300 V | 230 V | 460 V | 575 V | 2300 V |
| 1/2 | 0.38 | 4 | 2 | 1 | 0.8 | |||||
| 3/4 | 0.56 | 5.6 | 2.8 | 1.4 | 1.1 | |||||
| 1 | 0.75 | 7.2 | 3.6 | 1.8 | 1.4 | |||||
| 1 1/2 | 1.1 | 10.4 | 5.2 | 2.6 | 2.1 | |||||
| 2 | 1.5 | 13.6 | 6.8 | 3.4 | 2.7 | |||||
| 3 | 2.3 | 9.6 | 4.8 | 3.9 | ||||||
| 5 | 3.8 | 15.2 | 7.6 | 6.1 | ||||||
| 7 1/2 | 5.6 | 22 | 11 | 9 | ||||||
| 10 | 7.5 | 28 | 14 | 11 | ||||||
| 15 | 11 | 42 | 21 | 17 | ||||||
| 20 | 15 | 54 | 27 | 22 | ||||||
| 25 | 19 | 68 | 34 | 27 | 53 | 26 | 21 | |||
| 30 | 23 | 80 | 40 | 32 | 63 | 32 | 26 | |||
| 40 | 30 | 104 | 52 | 41 | 83 | 41 | 33 | |||
| 50 | 38 | 130 | 65 | 52 | 104 | 52 | 42 | |||
| 60 | 45 | 154 | 77 | 62 | 16 | 123 | 61 | 49 | 12 | |
| 75 | 56 | 192 | 96 | 77 | 20 | 155 | 78 | 62 | 15 | |
| 100 | 75 | 248 | 124 | 99 | 26 | 202 | 101 | 81 | 20 | |
- 1 hp (English horse power) = 745.7 W = 0.746 kW = 550 ft lb/s = 2,545 Btu/h = 33.000 ft lb/m = 1.0139 metric horse power ~= 1.0 KVA
Direct Current Motors - Power and Full Load Currents
| Power | Direct Current (A) | ||
|---|---|---|---|
| (hp) | (kW) | 230 V | 440 V |
| 1/4 | 0.19 | 0.81 | 0.42 |
| 1/3 | 0.25 | 1.1 | 0.56 |
| 1/2 | 0.37 | 1.6 | 0.85 |
| 3/4 | 0.56 | 2.4 | 1.3 |
| 1 | 0.75 | 3.2 | 1.7 |
| 1 1/2 | 1.1 | 4.9 | 2.5 |
| 2 | 1.5 | 6.5 | 3.4 |
| 3 | 2.2 | 9.7 | 5.1 |
| 5 | 3.7 | 16 | 8.5 |
| 7 1/2 | 5.6 | 24 | 13 |
| 10 | 7.5 | 32 | 17 |
| 15 | 11 | 49 | 25 |
| 20 | 15 | 65 | 34 |
| 30 | 22 | 97 | 51 |
| 50 | 37 | 162 | 85 |
| 75 | 56 | 243 | 127 |
| 100 | 75 | 324 | 170 |
- for 115V - the amps is twice the amps for 230V
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