HP to amps calculator
Free HP to amps calculator: get estimated current from mechanical output, voltage, power factor, efficiency and current. With the formula shown.
Project inputs
Updates as inputs change. Unit changes preserve the same physical values.
Method & assumptions
Engineering estimate, not a code-table motor full-load current.
Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.
Sources and validation →Worked example
Conversion direction: Horsepower → amps · Mechanical output: 5 hp · Voltage: 230 V · Supply: Single phase · Power factor: 0.85 · Efficiency: 90 %
Estimated current: 21.191 A
This example uses the stated inputs; results for the entered values appear above.Estimated amps for common motor sizes
The table applies the calculator's formula, I = hp × 745.7 ÷ (k × V × PF × η), to common motor ratings from 1/2 hp to 200 hp, assuming a power factor of 0.85 and an efficiency of 90% for every size. The values are estimates of running current at full load. Single-phase figures stop at 10 hp, since larger motors are normally three phase.
| Motor (hp) | 115 V single phase | 208 V three phase | 230 V three phase | 460 V three phase |
|---|---|---|---|---|
| 1/2 | 4.2 | 1.4 | 1.2 | 0.6 |
| 3/4 | 6.4 | 2.0 | 1.8 | 0.9 |
| 1 | 8.5 | 2.7 | 2.4 | 1.2 |
| 1 1/2 | 12.7 | 4.1 | 3.7 | 1.8 |
| 2 | 17.0 | 5.4 | 4.9 | 2.4 |
| 3 | 25.4 | 8.1 | 7.3 | 3.7 |
| 5 | 42.4 | 13.5 | 12.2 | 6.1 |
| 7 1/2 | 63.6 | 20.3 | 18.4 | 9.2 |
| 10 | 84.8 | 27.1 | 24.5 | 12.2 |
| 15 | — | 40.6 | 36.7 | 18.4 |
| 20 | — | 54.1 | 48.9 | 24.5 |
| 25 | — | 67.6 | 61.2 | 30.6 |
| 30 | — | 81.2 | 73.4 | 36.7 |
| 40 | — | 108.2 | 97.9 | 48.9 |
| 50 | — | 135.3 | 122.3 | 61.2 |
| 60 | — | 162.3 | 146.8 | 73.4 |
| 75 | — | 202.9 | 183.5 | 91.8 |
| 100 | — | 270.6 | 244.7 | 122.3 |
| 125 | — | 338.2 | 305.9 | 152.9 |
| 150 | — | 405.9 | 367.0 | 183.5 |
| 200 | — | 541.1 | 489.4 | 244.7 |
Real motors differ from these assumptions. Small motors usually have a lower efficiency and power factor than assumed here, so they draw more current than the table shows; the motor nameplate gives the actual rated current.
Conductor and protection sizing
These estimates are not used to size motor circuits. Under NEC 430.6(A)(1), conductor ampacity and the ratings of switches and branch-circuit short-circuit and ground-fault protection are based on the full-load currents in the NEC motor tables, Tables 430.247 through 430.250, rather than on nameplate current or a calculated value; overload protection is then sized from the nameplate current under 430.32. The NEC tables are not reproduced here. The fuse size calculator starts from the table current, and the hp to kW converter converts motor power without a voltage.
Estimating and invoicing tool
Ampxa is developing an estimating and invoicing application for electrical contractors. The list below receives notice when it opens.