Choosing a transformer: why standard sizes matter more than the load
Applying diversity and growth to connected load, then selecting the next standard rating and understanding the utilisation.
The engineering question
What transformer do I actually specify? The connected load and the equipment ratings never seem to match.
Worked example
The figures below were produced by running the Transformer Sizing Calculator with the same inputs — during the build, from its authoritative engine.
| Standard transformer rating | 400 kVA |
|---|---|
| Maximum demand | 280 kVA |
| Demand after growth | 322 kVA |
| Resulting utilisation | 80.5 % |
| Assessment | A 400 kVA transformer runs at ~81% of its rating after the 15% growth allowance. |
- Maximum demand is the connected load after the demand factor.
- Future growth is applied to the demand, not to the connected load.
- Select the next standard rating above that figure and check the resulting utilisation.
Sanity check
Running continuously above about 80 % of rating shortens transformer life and leaves no headroom for load steps.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
Engineering review required
This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.
References & standards
- IEC 60076-1 — power transformers
- BS 7671 — selection of the source
Related problems
Why a low power factor costs money, and how much correction is needed
Converting kW and power factor into the reactive power to remove, and choosing a defensible target.
Sizing a cable for both current and voltage drop
Two independent constraints that must both be satisfied, and recognising which one actually chose the size.