Size a transformer on apparent power, not on the kW printed on your equipment. Add up the peak simultaneous demand, divide by the power factor to get kVA, then correct for the conditions the unit will actually work in and add a margin for growth. The answer is rarely the number you started with.
Why kVA and not kW
A transformer heats up from current, and current follows apparent power. A load of 500 kW at a power factor of 0.85 draws roughly 590 kVA — nearly 100 kVA more than the kW figure suggests. Sizing on kW alone is the most common route to a transformer that runs hot from the first day.
So establish the power factor before anything else. Motor-heavy plant sits around 0.80 – 0.85 uncorrected; a site with modern drives and capacitor banks may reach 0.95 or better. If reactive compensation is planned, size for the situation before it is switched in, because it will not always be available.
Build a load profile, not a single number
The peak sets the thermal rating, but the shape of the day sets the losses and the ageing. Note the maximum simultaneous demand, the daytime plateau and the overnight base load. A distribution transformer averaging 40 % loading with a two-hour evening peak is a different design problem from a data-centre feeder sitting at 70 % around the clock.
Diversity is real: not every machine, charger or oven runs at once. Applying a defensible diversity factor to the connected load, instead of summing every rating, usually removes a whole frame size. For EV charging be conservative, though — chargers increasingly do run simultaneously and at full output.
Corrections that reduce the rating on the plate
- Ambient temperature: the IEC 60076-2 reference is a 20 °C yearly average and a 40 °C maximum; a hot, poorly ventilated substation room derates the unit.
- Altitude: above roughly 1 000 m thinner air cools less effectively, and the rating falls by around 0.4 % per additional 100 m for ONAN units.
- Harmonics: rectifiers, drives and LED lighting add eddy-current heating, so a K-factor rated design or a deliberate oversize is needed.
- Load unbalance and inrush from motor starting, capacitor switching or a large single-phase load.
How much headroom is sensible
A workable rule of thumb is to run a distribution transformer at 70 – 80 % of rating at peak. That leaves room for growth and keeps the unit inside its efficient band. Loss-optimised designs often reach peak efficiency around 40 – 50 % loading, so a very lightly loaded transformer is not automatically an economical one.
Oversizing is not free. No-load losses are paid every hour of every year regardless of load, so one frame size too many costs energy for three decades. Undersizing costs more: sustained overload accelerates insulation ageing, and roughly every 6 K above the design hot-spot temperature halves the remaining life of the paper insulation.
Work through it in that order — real peak in kVA, load profile, ambient and altitude corrections, then growth — and pick the standard rating just above the result. Where future growth is genuinely uncertain, an ONAN/ONAF design gives you a second rating with fans later instead of a second transformer. Send us a load profile rather than a connected-load list and we will size against what the installation actually does.
