Transformer sizing is a common detailed subject and it's having different conditions / philosophy based the different type of the transformer to be sized. Generator step up transformer of an ac power plant is having different concept from that is applicable for distribution transformer. Station or startup transformer is having it's own philosophy to size the optimum rating. In general, the distribution transformers are widely sized and the parameters to be considered as following points:
- Gather kW/KVA or the current rating of all the loads connected to the transformer.
- Convert all loads into a common unit, preferably on kVA by converting kW and current value to corresponding KVA (you need to know the power factor and the rated voltage for this conversion).
- Apply load factor for each load and come to the value of actual kVA demand of individual load. Load factor varies for continuous load, intermittent loads etc.
- Sum up all the demands to a total kVA rating.
- Apply diversity factor (depending upon how many connected loads will be running together at a particular time).
- Apply required margin to take care of future growth and some case to take care of the voltage transformer factor (i.e 433/415 for 415 V system).
- Select the next higher standard size of transformer. Mainly, for LV, it is generally, 250, 500, 750, 1000, 1250, 1600, 2000 & 2500 kVA.