Horsepower does not convert to amps with one formula, because the current also depends on the voltage, the motor’s efficiency and its power factor. That is why the code uses a table. This calculator gives you both numbers: the NEC table value that is used to size circuits, and an estimate of the current the motor actually draws at full load.
Two different answers
Estimate, three-phase: I = hp × 745.7 / (√3 × V × efficiency × PF)
Estimate, single-phase: I = hp × 745.7 / (V × efficiency × PF)
The table current is a standard value for each horsepower and voltage. The NEC uses it to size the conductors and the short-circuit protection, whatever is on the nameplate. It is deliberately a little higher than a typical motor draws.
The estimate applies the formula, with the efficiency and power factor you enter. It is an approximation of the running current at full load. For a real motor, the nameplate current is better, and it is the figure used to set the overload protection.
Worked example
A 10 hp three-phase motor on 460 V:
- Table current: 14 A.
- Estimate at 90 % efficiency and 0.85 power factor: 7,457 / (1.732 × 460 × 0.9 × 0.85) = 12.2 A.
The estimate is lower than the table value, which is normal. The next step, the conductor, uses the table: 1.25 × 14 = 17.5 A. The motor circuit calculator carries on from here to the overload and the breaker.
Three-phase motors: full-load current in amps
| hp | 208 V | 230 V | 460 V | 575 V |
|---|---|---|---|---|
| 1 | 4.6 | 4.2 | 2.1 | 1.7 |
| 2 | 7.5 | 6.8 | 3.4 | 2.7 |
| 3 | 10.6 | 9.6 | 4.8 | 3.9 |
| 5 | 16.7 | 15.2 | 7.6 | 6.1 |
| 7.5 | 24.2 | 22 | 11 | 9 |
| 10 | 30.8 | 28 | 14 | 11 |
| 15 | 46.2 | 42 | 21 | 17 |
| 20 | 59.4 | 54 | 27 | 22 |
| 25 | 74.8 | 68 | 34 | 27 |
| 50 | 143 | 130 | 65 | 52 |
| 100 | 273 | 248 | 124 | 99 |
Single-phase motors: full-load current in amps
| hp | 115 V | 230 V |
|---|---|---|
| 1/6 | 4.4 | 2.2 |
| 1/4 | 5.8 | 2.9 |
| 1/3 | 7.2 | 3.6 |
| 1/2 | 9.8 | 4.9 |
| 3/4 | 13.8 | 6.9 |
| 1 | 16 | 8 |
| 1.5 | 20 | 10 |
| 2 | 24 | 12 |
| 3 | 34 | 17 |
| 5 | 56 | 28 |
| 7.5 | 80 | 40 |
| 10 | 100 | 50 |
The NEC tables list more horsepower ratings than shown here, and the calculator above has all of them. Supply voltages are matched to the nearest table column: a 480 V supply normally uses the 460 V column and a 240 V supply the 230 V column.
Common mistakes
- Using the estimate to size the circuit. Under the NEC the conductor and the short-circuit device are sized from the table current.
- Using the table to set the overload relay. The overload setting comes from the nameplate current of the motor you have.
- Choosing the wrong voltage column. A 480 V supply normally uses the 460 V column, and a 240 V supply the 230 V column.
- Expecting the nameplate to match. A nameplate can show a different figure from the table, and that is fine. The table is a sizing basis, not a measurement.
Questions
How many amps is a 5 hp motor?
On a three-phase 460 V supply the table gives 7.6 A. On 230 V three-phase it is 15.2 A, and a single-phase 230 V motor has 28 A.
How many amps is a 1 hp motor?
The table gives 2.1 A at 460 V three-phase, 4.2 A at 230 V three-phase, and 8 A at 230 V single-phase.
Why is the table current higher than my measured current?
The table is a standard value used for sizing. A real motor’s current depends on its efficiency, power factor and how heavily it is loaded, so a lightly loaded motor can draw much less.
Can I use this for a motor with a drive?
Motors on variable-speed drives have their own requirements. Check the drive manufacturer’s data and the NEC provisions for adjustable-speed drives.
Related calculators
Found a wrong value or formula? Report an error. Confirmed fixes are listed on the corrections log.