Transformer sizing guide, from load to rating

Start from the connected load, then apply diversity, because connected load and simultaneous load are rarely the same number. A workshop with forty machines does not run all forty at once. Size for forty and the transformer comes out larger and more expensive than the site needs, and less efficient at the load it carries.

Divided by the power factor, the simultaneous load gives a demand in kilovolt amperes. The rating is built up from there.

The two factors that change the answer

Motor starting is the first. A motor started direct on line draws several times its rated current while it accelerates, and the rest of the installation feels the voltage dip.

Where a large motor starts on a small transformer, that dip sets the rating. Soft starters and variable speed drives reduce it considerably, so the starting method belongs in the calculation.

Power factor is the second. Ratings are stated in kilovolt amperes, loads are often given in kilowatts, and the two differ by the power factor. Converting a kilowatt figure without it understates the rating.

Six steps from load to rating

kW to kVA for four loads

Demand = kW ÷ power factor, rounded, then the next step of the common series 100, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500 kVA. These rows are the arithmetic alone; diversity, growth, harmonics, derating and motor starting all come before a rating is chosen. A "200 kW transformer" is a transformer that can carry a 200 kW load, and its rating is in kVA.

Where the engineer decides

The method gives a rating you can defend and a specification a manufacturer can quote. Where the load is unusual, carries harmonics or feeds a critical supply, the final rating belongs to the engineer who signs the design.