Shaft power is horsepower times 0.746 kW. Dividing by efficiency gives the electrical power the motor draws, and dividing that by power factor gives kVA. Take efficiency and power factor from the motor nameplate or datasheet at the load you expect. The starting kVA is several times higher than this running figure.
The HP to kVA formula
One horsepower is 0.746 kW of shaft power. The motor draws more than that because it has losses, and the supply carries more than the real power because of reactive current.
- Input kW equals HP times 0.746 divided by efficiency.
- kVA equals input kW divided by power factor, or HP times 0.746 divided by efficiency times power factor.
- Three phase amps equal kVA times 1000 divided by 1.732 times volts.
- kVAR equals the square root of kVA squared minus kW squared.
Worked example. A 100 HP motor at 94 percent efficiency and 0.88 power factor draws 100 x 0.746 / 0.94 = 79.4 kW and 79.4 / 0.88 = 90.2 kVA. On a 480 volt supply that is 90.2 x 1000 / (1.732 x 480) = 108.5 A.
HP to kVA chart, 1 to 500 HP
Computed at 90 percent efficiency, the same for every row so the columns compare cleanly, and at three power factors. Real motors vary, so treat this as a starting figure and use the calculator above with nameplate values for a design.
| Motor HP | Shaft kW | Input kW at 90% | kVA at PF 0.80 | kVA at PF 0.85 | kVA at PF 0.90 |
|---|---|---|---|---|---|
| 1 | 0.7 | 0.8 | 1.0 | 1.0 | 0.9 |
| 2 | 1.5 | 1.7 | 2.1 | 2.0 | 1.8 |
| 3 | 2.2 | 2.5 | 3.1 | 2.9 | 2.8 |
| 5 | 3.7 | 4.1 | 5.2 | 4.9 | 4.6 |
| 7.5 | 5.6 | 6.2 | 7.8 | 7.3 | 6.9 |
| 10 | 7.5 | 8.3 | 10.4 | 9.8 | 9.2 |
| 15 | 11.2 | 12.4 | 15.5 | 14.6 | 13.8 |
| 20 | 14.9 | 16.6 | 20.7 | 19.5 | 18.4 |
| 25 | 18.7 | 20.7 | 25.9 | 24.4 | 23.0 |
| 30 | 22.4 | 24.9 | 31.1 | 29.3 | 27.6 |
| 40 | 29.8 | 33.2 | 41.4 | 39.0 | 36.8 |
| 50 | 37.3 | 41.4 | 51.8 | 48.8 | 46.0 |
| 60 | 44.8 | 49.7 | 62.2 | 58.5 | 55.3 |
| 75 | 56.0 | 62.2 | 77.7 | 73.1 | 69.1 |
| 100 | 74.6 | 82.9 | 103.6 | 97.5 | 92.1 |
| 125 | 93.3 | 103.6 | 129.5 | 121.9 | 115.1 |
| 150 | 111.9 | 124.3 | 155.4 | 146.3 | 138.1 |
| 200 | 149.2 | 165.8 | 207.2 | 195.0 | 184.2 |
| 250 | 186.5 | 207.2 | 259.0 | 243.8 | 230.2 |
| 300 | 223.8 | 248.7 | 310.8 | 292.5 | 276.3 |
| 400 | 298.4 | 331.6 | 414.4 | 390.1 | 368.4 |
| 500 | 373.0 | 414.4 | 518.1 | 487.6 | 460.5 |
Free to cite with a link to this page. Every value is HP times 0.746 divided by 0.90 and by the power factor in the column heading. Rated efficiency and power factor change with motor size, speed and load, so the nameplate always wins over this table.
Running kVA from the NEC full load amps, 460 V
The code sizes circuits from table amps, not from the nameplate. Multiplying the table amps by 1.732 and the voltage gives a running kVA that already includes typical motor losses, and it lines up with a standard three phase transformer rating. The full table with wire and breaker sizes is on the motor full load amps chart.
| Motor | Table amps at 460 V | Running kVA | Next standard three phase kVA |
|---|---|---|---|
| 5 HP | 7.6 A | 6.1 | 15 kVA |
| 10 HP | 14 A | 11.2 | 15 kVA |
| 15 HP | 21 A | 16.7 | 30 kVA |
| 20 HP | 27 A | 21.5 | 30 kVA |
| 25 HP | 34 A | 27.1 | 30 kVA |
| 30 HP | 40 A | 31.9 | 45 kVA |
| 40 HP | 52 A | 41.4 | 45 kVA |
| 50 HP | 65 A | 51.8 | 75 kVA |
| 60 HP | 77 A | 61.3 | 75 kVA |
| 75 HP | 96 A | 76.5 | 112.5 kVA |
| 100 HP | 124 A | 98.8 | 112.5 kVA |
| 125 HP | 156 A | 124.3 | 150 kVA |
| 150 HP | 180 A | 143.4 | 150 kVA |
| 200 HP | 240 A | 191.2 | 225 kVA |
| 250 HP | 302 A | 240.6 | 300 kVA |
| 300 HP | 361 A | 287.6 | 300 kVA |
| 400 HP | 477 A | 380.0 | 500 kVA |
| 500 HP | 590 A | 470.1 | 500 kVA |
Free to cite with a link to this page. The next standard size is a floor for a single motor that runs and does not start often. Several motors on one transformer add up, and the starting of the largest usually decides the size.
Running kVA versus starting kVA
Everything above is running load. At start an across the line induction motor pulls locked rotor current, often around six times full load current for a short time. The nameplate code letter gives locked rotor kVA per horsepower. A transformer that carries the running kVA can still sag the voltage at start, so large motors on a single transformer are checked for voltage dip, or started with a soft starter or a variable frequency drive that holds the current down.
For a generator the same rule applies, and the starting kVA usually sets the generator size rather than the running figure. See the 500 kW generator and 1 MW generator pages for sizes.
Common questions
How do you convert HP to kVA
Multiply horsepower by 0.746 to get shaft kW, divide by the motor efficiency to get the kW the motor draws, then divide by the power factor to get kVA. A 50 HP motor at 90 percent efficiency and 0.85 power factor is 50 x 0.746 = 37.3 kW, 37.3 / 0.90 = 41.4 kW, and 41.4 / 0.85 = 48.8 kVA.
What is the HP to kVA formula for a three phase motor
kVA = HP x 0.746 / (efficiency x power factor). The three phase current is then kVA x 1000 / (1.732 x volts). Efficiency and power factor are written as decimals in the formula, so 92 percent is 0.92.
Is HP to kVA a fixed number
No. A kilowatt is a fixed amount of power but a horsepower of motor load needs more or fewer kVA depending on the motor. Small motors and lightly loaded motors have lower efficiency and power factor, so they need more kVA per horsepower. Use the nameplate or datasheet values for the motor you have.
What transformer size do I need for a motor
The running kVA is the floor. The starting current of an across the line motor is often around six times full load current, so a transformer serving one large motor is checked for voltage dip at starting, or the motor is started with a soft starter or drive. The full load amps chart lists the next standard transformer size for every motor from 5 to 500 HP at 460 V.
What is the difference between HP to kVA and HP to kW
HP to kW is a unit conversion at 0.746 kW per horsepower of shaft power. HP to kVA also accounts for the motor losses and the reactive current, which is why the kVA figure is always larger than the shaft kW.
Related tools. The kVA to kW calculator turns the result into real power, the kVA to amps calculator gives current for any transformer size, and the transformer sizing calculator sizes a whole service. Ready to buy for a plant with large motors, see 4160V to 480V transformers or request a quote.
Sources
- The factor of 0.746 kW per horsepower is the standard conversion for mechanical horsepower.
- NEC Table 430.250 full load currents as reproduced by Schneider Electric, Motor Control Solutions data bulletin, Full load current, three phase AC motors (2017 NEC Table 430.250). Source page
- Efficiency and power factor are entered by the reader. No typical values are claimed on this page.