HP to kVA
Calculator
Convert motor horsepower to transformer kVA. Accounts for motor efficiency and power factor to give you the true electrical demand.
From Motor Nameplate to Transformer kVA
Locate the motor nameplate for horsepower rating, efficiency (%), and power factor. If unlisted, use NEMA Premium values: 90% efficiency and 0.85 PF are typical for motors 5–100 HP.
One mechanical horsepower equals 0.7457 kW of output. Divide by motor efficiency to get the electrical input: kW = HP × 0.7457 ÷ η. This accounts for heat losses in the motor windings.
kVA = kW ÷ Power Factor. The difference between kW and kVA is reactive power drawn by the motor's magnetic field — real work the transformer must supply but that does no mechanical work.
The calculated kVA is for steady-state running. For direct-on-line starts, multiply by 1.25–1.5 to handle inrush. For VFD-driven motors the drive limits inrush, so a tighter margin is acceptable.
HP is a unit of mechanical output power. To find the electrical power drawn, you must account for motor efficiency (heat loss in the motor) and power factor (reactive vs real power). A typical 90% efficient motor at 0.85 PF requires about 0.976 kVA per horsepower.
For NEMA Premium Efficiency motors, efficiency ranges from 88% (small motors) to 96% (large motors). Power factor is typically 0.82–0.90 at full load and drops significantly at partial loads. Check the motor nameplate or use 90% efficiency and 0.85 PF as conservative estimates.
Calculate the motor kVA, then size the transformer to at least 125% of that value (divide kVA by 0.8) to allow for motor starting inrush current, which can be 6–8× full-load current for direct-on-line starts. For VFD applications, the starting surge is eliminated and a smaller transformer may be acceptable.
Yes — add the kVA of all running motors, plus typically 1–1.5× the largest motor kVA for starting the largest motor at the same time as the others are running. This is called the "largest motor starting" scenario. Use our kVA Calculator for the final transformer sizing.
Need a Transformer for Your Motor Load?
Our team can specify a transformer matched to your exact motor configuration — single motor, multi-motor bus, or VFD-driven loads.