Calculation of KVA or MVA capacity of transformer is just a matter of multiplication of current and voltage. This quantity is defined as Apparent power (denoted by S in common) of a transformer. We will define what are the terms KVA and MVA stand for.
KVA stands for Kilo-Volt Ampere and MVA stands for Mega-Volt Ampere. Transformers are usually rated with both these quantities KVA or MVA.
In common Apparent power (KVA or MVA) is calculated by multiplying line maximum current (kA) and maximum phase voltage(kV).
As stated above, line maximum current (kA) and maximum phase voltage(kV) for single phase transformer is directly measurable. Transformer output maximum current and output maximum voltage are obtained for this calculation.
Apparent \, power (S) = Current (I) * Voltage (V) = VI