Example 1: Three-phase distribution load
Voltage = 415 V, current = 72 A, three phase
kVA = 1.732 x 415 x 72 / 1000 = 51.76 kVA
The next common transformer size is 63 kVA, which gives a practical selection margin above the exact load.
Transformer kVA Calculator
Estimate transformer apparent power from voltage, current, and phase, then compare the result with practical standard transformer sizes.
This transformer kVA calculator estimates apparent load only. Final transformer selection should also consider cooling class, ambient temperature, harmonics, short-circuit duty, and future expansion.
What This Page Calculates
Transformer sizing often starts with apparent power rather than real power. That is because a transformer must carry voltage and current regardless of the exact power factor of the connected load. For that reason, many transformers are selected in kVA instead of kW. If the design current is already known, a quick voltage-and-current calculation gives a fast estimate of the transformer rating needed for the job.
This transformer kVA calculator uses the standard single-phase or three-phase apparent power relationship to estimate transformer demand. It then compares the result against common transformer sizes so that a designer can move more quickly from load current to a practical transformer selection. That makes the page useful for service planning, distribution boards, machinery feeds, building loads, and preliminary transformer checks.
The result is especially useful during early design when the engineer knows the expected line current but still needs to decide whether a 25 kVA, 50 kVA, 75 kVA, or larger transformer is the more realistic direction.
Core Equation
Single-phase transformer kVA: kVA = V x I / 1000
Three-phase transformer kVA: kVA = sqrt(3) x V x I / 1000
Three-phase transformer kVA: kVA = sqrt(3) x V x I / 1000
Key Variables
| Variable | Meaning | Unit |
|---|---|---|
| V | Line voltage or single-phase supply voltage | V |
| I | Load current | A |
| kVA | Apparent transformer load | kVA |
Unit Notes
Voltage is entered in volts and current is entered in amperes. The result is shown in kilovolt-amperes. A second line shows the next common transformer rating above the calculated value, which helps users compare the load with practical market sizes instead of stopping at an exact theoretical number.
Worked Examples
Example 2: Single-phase control transformer duty
Voltage = 240 V, current = 58 A, single phase
kVA = 240 x 58 / 1000 = 13.92 kVA
A practical next size would be 15 kVA, depending on duty and future expansion needs.
Selection Notes
Apparent load is only the first sizing step. A transformer may still need extra capacity because of harmonic content, temperature rise, duty cycle, non-linear loads, or future expansion. Cooling method and insulation class also matter. In industrial work, designers may deliberately choose a larger transformer so that the load remains below the nameplate continuously and future equipment can be added later without a full replacement.
Even with those extra checks, a kVA calculator remains highly useful because it turns raw current and voltage into a clear sizing basis. It also pairs well with feeder-current, load calculation, and short-circuit studies because all of those design checks depend on a realistic understanding of transformer loading.