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Typical motor efficiency: 80-95%
Typical motor PF: 0.80-0.90
kVA = (HP × 0.746) ÷ (η × PF)
Where:
The kVA (apparent power) rating is always higher than kW (real power) because motors require reactive power. The power factor represents the ratio of real power to apparent power. Transformers and generators are rated in kVA because they must supply both real and reactive power.
Motor HP to kVA at 90% efficiency with different power factors:
| HP | kVA (PF=0.90) | kVA (PF=0.85) | kVA (PF=0.80) |
|---|---|---|---|
| 1 | 0.97 | 1.03 | 1.10 |
| 2 | 1.95 | 2.06 | 2.19 |
| 3 | 2.92 | 3.09 | 3.29 |
| 5 | 4.87 | 5.15 | 5.48 |
| 7.5 | 7.30 | 7.73 | 8.21 |
| 10 | 9.73 | 10.31 | 10.95 |
| 15 | 14.60 | 15.46 | 16.43 |
| 20 | 19.47 | 20.62 | 21.90 |
| 25 | 24.34 | 25.77 | 27.38 |
| 30 | 29.20 | 30.92 | 32.85 |
| 40 | 38.94 | 41.23 | 43.80 |
| 50 | 48.67 | 51.54 | 54.76 |
| 75 | 73.01 | 77.31 | 82.13 |
| 100 | 97.34 | 103.08 | 109.51 |
| 150 | 146.02 | 154.62 | 164.27 |
| 200 | 194.69 | 206.16 | 219.02 |
HP to kVA conversion bridges two different ways of measuring power in electrical systems. Horsepower (HP) measures the mechanical output of a motor — the shaft power available to drive pumps, fans, compressors, and other machinery. Kilovolt-amperes (kVA), on the other hand, measures the apparent electrical power that must be supplied to the motor from the power source. Because motors are not 100% efficient and they draw reactive current to maintain their magnetic fields, the kVA input is always greater than the mechanical HP output. Understanding this relationship is essential when sizing transformers, generators, switchgear, and cables that must deliver enough apparent power to keep motors running reliably.
Check the motor nameplate for the horsepower (HP) rating. This is the mechanical output power the motor delivers at full load. Common values range from fractional HP up to several thousand HP for industrial motors.
Find the motor efficiency (η) on the nameplate, typically expressed as a percentage. NEMA Premium motors run at 90-96% efficiency, while standard motors are 85-92%. Convert the percentage to a decimal for calculation.
The power factor (PF) indicates how much of the current is doing useful work. Motor power factors range from 0.70 to 0.95 depending on size and load. If unknown, use 0.85 as a reasonable default for most induction motors at full load.
Plug the values into kVA = (HP × 0.746) ÷ (η × PF). For example, a 50 HP motor at 90% efficiency and 0.85 PF requires (50 × 0.746) ÷ (0.90 × 0.85) = 48.76 kVA from the power supply.
Transformers are rated in kVA, not HP. Undersizing a transformer for motor loads causes overheating, voltage drops, and premature failure. Accurate HP to kVA conversion ensures the transformer can handle the full apparent power demand.
Generators must supply both real and reactive power to motor loads. Selecting a generator based on HP alone leads to undersized equipment that stalls under load. The kVA rating accounts for the total power the generator must produce.
Oversizing electrical equipment wastes capital, while undersizing risks equipment damage. Precise HP to kVA calculations help engineers select right-sized equipment, reducing both upfront costs and ongoing energy expenses.
| Motor HP | Running kVA | Starting kVA (DOL) | Starting kVA (VFD) |
|---|---|---|---|
| 5 | 5.15 | 30-40 | 5-8 |
| 10 | 10.31 | 60-80 | 10-15 |
| 25 | 25.77 | 150-200 | 26-38 |
| 50 | 51.54 | 300-400 | 52-77 |
| 75 | 77.31 | 450-600 | 77-116 |
| 100 | 103.08 | 600-800 | 103-155 |
| 150 | 154.62 | 900-1200 | 155-232 |
| 200 | 206.16 | 1200-1600 | 206-310 |
DOL = Direct On-Line starting. VFD = Variable Frequency Drive. Running kVA assumes PF=0.85, η=0.90.
During startup, motors draw 5-8 times their full-load current (called inrush current), which dramatically increases the kVA demand on the power source. This is why generators and transformers must be sized to handle starting kVA, not just running kVA. Soft starters and variable frequency drives (VFDs) reduce starting kVA to 1.0-1.5 times the running value, making them invaluable for large motor applications.
You should convert each motor individually because different motors may have different efficiencies and power factors. After converting each motor's HP to kVA, sum the individual kVA values. Then apply a demand factor (typically 0.6-0.8) since not all motors usually run at full load simultaneously, which prevents oversizing the power supply equipment.
kW represents the real power consumed by the motor, while kVA represents the apparent power drawn from the supply. The ratio of kW to kVA equals the power factor (PF). For motors, kVA is always higher than kW because the motor requires reactive power to maintain the magnetic field that creates torque. A motor drawing 50 kVA at 0.85 PF consumes only 42.5 kW of real power.
VFDs improve the power factor on the supply side to near unity (0.95-0.99), which significantly reduces the kVA drawn from the power source for the same HP output. This means smaller transformers and generators can be used when VFDs are installed. However, VFDs also introduce harmonic distortion, which may require derating of transformers by 10-15% depending on the harmonic content.
For transformers supplying motor loads, add a 25% safety margin above the calculated running kVA to account for voltage fluctuations, future expansion, and transient loads. For generators, the margin should be even larger (30-50%) because generators are more sensitive to overloading. Always verify the largest motor can start without causing excessive voltage drop on the system.