What is the three-phase kVA formula?

For a balanced three-phase load, apparent power equals the square root of three times the line-to-line voltage times the line current, divided by 1000 to convert VA to kVA. The square-root-of-three factor accounts for the phase relationship between line currents and line-to-line voltages.

Why size in kVA instead of kW?

Transformers are rated in kVA, apparent power, because their thermal limit depends on current and voltage regardless of the load power factor. Sizing from kW would understate the requirement for inductive loads where current runs higher than the real power implies.

What margin should I add?

A 20 to 25 percent spare-capacity margin is common to allow for load growth, harmonics, and to keep the transformer below full load for efficiency and longevity. Critical or rapidly growing loads may justify a larger margin.

Why round up to a standard size?

Transformers are manufactured only in standard kVA ratings such as 15, 30, 45, 75, 112.5, 150, and 225 kVA. The tool selects the smallest standard rating that meets or exceeds your margined load so you can specify a real unit.

Do I use line-to-line or line-to-neutral voltage?

For the three-phase formula always use the line-to-line voltage, such as 480 V or 208 V. Using line-to-neutral voltage would give an incorrect, undersized result because the square-root-of-three formula is built around line-to-line values.

Transformer kVA Calculator

Sizing a transformer means matching its kVA rating to the connected load with room to spare, and the calculation differs between single and three-phase systems. This tool computes apparent power from current and voltage and rounds up to a real standard transformer rating.

How it works

Apparent power comes directly from voltage and current, with a phase factor for three-phase, then a margin and a standard-size round-up:

single phase:  kVA = V × I / 1000
three phase:   kVA = √3 × V_LL × I / 1000
margined  = kVA × (1 + margin%)
size      = smallest standard rating ≥ margined

Standard ratings include 15, 30, 45, 75, 112.5, 150, and 225 kVA and larger. Always use the line-to-line voltage in the three-phase formula.

Example and notes

A three-phase 480 V load drawing 60 A computes to about 49.9 kVA. Adding a 25 percent margin gives 62.4 kVA, which rounds up to a standard 75 kVA transformer. Because the rating is apparent power, you do not need the power factor for this sizing, but if you only know the real power in kW you must divide by the power factor first to recover the apparent power. Keep the loaded transformer below about 80 percent of its rating for efficiency and thermal headroom.