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A transformer's rating is its apparent power in volt-amperes (VA) or kilovolt-amperes (kVA) — the product of voltage and current, regardless of power factor. For a single load, VA = V×I (single-phase) or VA = √3·VL·IL (three-phase). Real installations have several loads that rarely all peak together, so the total connected load is scaled by a diversity factor (typically 0.7–0.9) to reflect the realistic simultaneous demand, and then a safety margin (15–25%) is added for growth and inrush.
| Quantity | Formula |
|---|---|
| Single-phase VA | VA = V × I |
| Three-phase VA | VA = √3 × VL × IL |
| Total (diversified) load | Σ(Load kVA) × Diversity Factor |
| Transformer size | Total × (1 + Margin/100), rounded up to a standard size |
After computing the required kVA, round up to the nearest standard transformer rating (e.g. 10, 16, 25, 40, 63, 100, 160, 250 kVA) rather than an exact custom size.
It is the apparent power the transformer can supply, the product of voltage and current regardless of power factor: VA = V×I (single-phase) or √3·VL·IL (three-phase). It is expressed in volt-amperes or kilovolt-amperes.
A transformer's windings and core must handle the full current and voltage regardless of the load's power factor, so its heating and rating depend on VA (apparent power), not the real power (W) that depends on power factor.
It is a factor (usually 0.7–0.9) applied to the sum of individual loads to account for the fact that not all loads reach their peak demand at the same time. It gives a more realistic total demand than simply adding nameplate ratings.
A margin (commonly 15–25%) covers future load growth, inrush currents, and keeps the transformer from running at its absolute limit continuously, which improves lifespan and reliability.
Add up each load's kVA, multiply by the diversity factor to get the realistic simultaneous demand, then add the safety margin. Round the result up to the next standard transformer size.
Common distribution transformer ratings include 10, 16, 25, 40, 63, 100, 160, 250, 400, 630 kVA and larger. Always size up to the next standard rating rather than specifying an exact custom value.
Use VA = √3 × line voltage × line current. The √3 factor (about 1.732) accounts for the phase relationship in a balanced three-phase system.
Not directly — the transformer is rated in VA regardless of power factor. However, poor power factor means more current is needed for the same real power, which can push the VA (and hence the required transformer size) higher.
It will overheat under sustained load, accelerating insulation ageing and risking failure. Voltage regulation also worsens as the transformer is pushed toward or beyond its rated current.
It costs more, takes more space, and runs less efficiently at very light load (core losses are constant regardless of load), though it has more headroom for growth and better voltage regulation.
Yes, if growth is expected. Either increase the safety margin or add an explicit allowance for planned future loads before rounding to the next standard transformer size.
This calculator gives a simple, practical estimate suitable for most projects. A full load flow or demand study for a large facility would use metered or more detailed load data and possibly different diversity factors per load type.
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