How a transformer rating is decided

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 forty machines at once, and a rating built on the assumption that it does will be larger, more expensive and less efficient than the one the site needs.

The result of that step is a demand in kilovolt amperes. It is the starting point rather than the answer.

The two factors that change the answer

Motor starting is the first. A direct on line motor draws several times its rated current while it accelerates, and the resulting voltage dip is what the rest of the installation notices. Where a large motor starts against a small transformer, the rating is decided by the dip rather than by the running load. Soft starters and variable speed drives change this materially, so how a motor starts belongs in the calculation.

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

A working sequence

From kW to kVA, worked 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. The arithmetic only: diversity, growth, harmonics, derating and motor starting come before a rating is chosen. A request for a "200 kW transformer" is a request for a transformer that can carry a 200 kW load, and transformers are rated in kVA.

Where this ends

This method gets a project to a rating that is defensible and to a specification that a supplier can price. It does not replace the design a qualified engineer signs, and the final rating on a project with unusual load behaviour, harmonics or a critical supply belongs to that engineer.