Transformer Sizing Calculator — Get the Right kVA Rating Every Time

Transformer sizing is the process of selecting a transformer with the correct kilovolt-ampere (kVA) rating to supply an electrical installation. The transformer must be large enough to supply the maximum demand of the connected loads without overheating or exceeding its rated capacity, but not so oversized that it operates inefficiently at a fraction of its rating.

In the UK, transformers are used in a wide range of applications. Distribution transformers step down the 11kV or 33kV supply to 400V three-phase for commercial and industrial premises. Isolation transformers provide separated supplies for special locations under BS 7671. Control transformers provide reduced voltages for building management systems, lighting controls, and industrial control circuits.

Selecting the correct transformer size requires calculating the total connected load, applying diversity factors, accounting for power factor, and adding an appropriate oversize allowance for future growth and transient loads. An undersized transformer will overheat, trip on overload, or fail prematurely. An oversized transformer wastes capital and operates less efficiently due to higher iron losses at light load.

For single-phase transformers, the kVA rating is calculated from the total load in kilowatts and the power factor of the load:

The current drawn from a single-phase transformer is:

For example, a single-phase load of 15kW at a power factor of 0.85 requires a transformer rated at: 15 / 0.85 = 17.6 kVA. The next standard rating above this is 25 kVA.

For three-phase transformers, the kVA rating calculation uses the same fundamental relationship between real power and apparent power:

The line current drawn from a three-phase transformer is:

For a three-phase load of 80kW at a power factor of 0.9, the required transformer rating is: 80 / 0.9 = 88.9 kVA. The next standard rating is 100 kVA. The line current on the secondary side would be: 100,000 / (1.732 x 400) = 144.3A. This determines the cable size and switchgear ratings needed on the secondary side.

When sizing three-phase transformers, it is important to consider the phase balance. If the load is significantly unbalanced, the transformer must be sized for the highest loaded phase, not the average. Use the three-phase power calculator to assess phase balance before sizing the transformer.

The connected load of an installation is rarely the same as the maximum demand. Diversity factors account for the fact that not all loads operate simultaneously. After applying diversity, an oversize factor is added to allow for future load growth and transient conditions.

The Elec-Mate diversity factor calculator can help determine the diversified maximum demand before feeding the result into the transformer sizing calculation.

Transformers are manufactured in standard kVA ratings. After calculating the required kVA, you must select the next standard rating above the calculated value. The standard ratings for oil-filled distribution transformers commonly available in the UK are:

Dry-type transformers (cast resin) are available in similar ratings and are used where oil-filled transformers are not permitted — for example, inside buildings, in basements, or in areas where fire risk from transformer oil is unacceptable. Dry-type transformers are typically more expensive but require less maintenance and have no oil leakage risk.

For site supply transformers (110V centre-tapped earth for construction sites), common ratings are 3 kVA, 5 kVA, 10 kVA, and 15 kVA single-phase, and 10 kVA, 15 kVA, and 25 kVA three-phase.