The transformer’s MVA rating is usually determined based on the system expected load growth, available generation and expected rate of return on the investment. The business plan is based on capital cost recovery, depreciation rate and the profit to be made each year. Transformer applications such as two winding, autotransformer, step-up or step-down etc. influence the transformer rating. Other factors such as expected power demand growth and load factor have a large influence in determining the transformer rating.
The supplied equipment as induction motor starting current and maximum voltage drop permitted could also influence the transformer rating [As a rule of thumb the transformer power should be 3 times the biggest induction motor starting D.O.L.]
