What this calculator does
Transformer capacity is bought in standard sizes, so the real question is not "what is the load" but "which standard unit covers the load after diversity and growth". This calculator answers that directly and reports the resulting utilisation.
How to use it
- Enter the total connected load in kVA — the sum of every load that could run at once.
- Set the demand factor, typically 60–80 % for a mixed-use building.
- Add a future-growth allowance if the installation is expected to expand.
- The result is the standard rating to specify and the utilisation you can expect to run at.
Governing formula
S_unit ≥ S_connected · DF · (1 + g) Where:
-
S_c= Connected load [kVA] -
DF= Demand factor as a fraction [–] -
g= Future growth fraction [–]
Worked example — 400 kVA connected, 70 % demand, 15 % growth
The figures below were produced by running this page's own engine with the example inputs (Connected load (kVA), Demand factor (%), Future growth allowance (%)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| 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 400 × 0.70 = 280 kVA.
- Adding 15 % growth gives 322 kVA, so the next standard rating (400 kVA) is selected.
- Utilisation of 322 / 400 ≈ 81 % leaves sensible headroom for load steps.
Assumptions & limitations
Engineering calculation. Applies a demand factor and a future-growth allowance to the connected load, then selects the next standard rating. Load scheduling, harmonics and standby capacity are outside the calculation.
- No load profile, coincidence or diversity study — the demand factor is an assumption you supply.
- Does not consider motor starting, inrush, harmonics, ambient derating or the transformer impedance/vector group.
- Ratings above 3150 kVA are not in the table; split the load across parallel units.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- IEC 60076-1 — power transformers, general requirements and rating.
- BS 7671 — selection of the source for an installation.