Transformer sizing helps ensure that a transformer can supply an electrical load without overheating or operating beyond its rated capacity. This calculator estimates the minimum apparent power requirement in kilovolt-amperes (kVA) from the load current, voltage, and whether the system is single-phase or three-phase.
How to Calculate Transformer Size (step by step)
Step One: Identify the transformer phase
The phase configuration determines the multiplier used in the apparent power calculation. Single-phase systems use a factor of 1. Balanced three-phase systems use the square root of 3, approximately 1.732.
Phase factor = 1 for single-phase
Phase factor = √3 = 1.7320508075688772 for three-phase
With the default selection, the transformer is single-phase:
Transformer type = Single-phase
Phase factor = 1
Step Two: Multiply load current by voltage
First calculate the apparent power in volt-amperes by multiplying the load current by the load voltage.
Apparent power = Load current × Load voltage × Phase factor
Using the default inputs:
Apparent power = 25 A × 240 V × 1
Apparent power = 6,000 VA
Step Three: Convert volt-amperes to kVA
Divide the apparent power in volt-amperes by 1,000 to express the result in kilovolt-amperes.
Minimum transformer size = Apparent power ÷ 1,000
Using the default values:
Minimum transformer size = 6,000 VA ÷ 1,000
Minimum transformer size = 6 kVA
What your results mean
The result is the minimum kVA required for the entered load. With the default values of 25 A, 240 V, and single-phase operation, the calculator returns 6 kVA.
This value describes apparent power, not the transformer’s physical dimensions or efficiency. A transformer rated at 6 kVA may technically match this calculated load, but selecting a standard rating above 6 kVA is usually more practical. For example, a 7.5 kVA or 10 kVA transformer may provide useful operating margin, depending on available ratings and the installation requirements.
Important sizing considerations
For continuous loads, avoid sizing the transformer right at its limit. Motors, compressors, welders, and other inductive equipment can draw substantially more current during startup than during normal operation. Power factor, voltage drop, harmonics, ambient temperature, enclosure type, and expected future load can also affect the final selection.
The calculator does not apply a power factor correction or a design margin. It gives the basic apparent power requirement from the entered voltage and current, so use the result as a starting point and verify the final transformer rating with applicable electrical codes and equipment specifications.