When a building needs its own transformer
Most small buildings take their supply from the utility and never think about transformer ratings. Larger buildings, industrial sites and developments with their own substation do: the transformer is the point where the whole connected load has to be delivered at a usable voltage.
Sizing it is a balance. Undersize the unit and voltage drops under load, losses rise and it runs hot. Oversize it and the capital cost and the standing no-load losses grow for no real benefit. The method below works from load data the designer already has, and it moves from the connected load to a standard rating in four steps.
The result is only as good as the load data behind it, so the first and most important task is to gather that data honestly rather than to reach for a familiar number.
Step one: gather the connected load
Start with the connected load in kilowatts: lighting, socket circuits, heating, ventilation, lifts, motors and any process equipment. Every figure should come from the building services design, not from a rule of thumb or from a similar project done years ago.
Keep the loads grouped. The designer needs to know how much is continuous, how much is intermittent and how much is motor load with its high starting current, because these behave differently in the final figure. A load that runs all day shapes the rating in a different way from a load that starts once an hour.
Step two: apply demand and diversity
Not every load is on at once. Demand and diversity factors reduce the connected load to the value the transformer will realistically see. They come from the standards in use and from engineering judgement about the building type.
Two buildings with the same connected load can end up with very different demands. A block of flats has diversity between units, while an industrial hall may run most of its load continuously. One factor applied to every building over-sizes some projects and under-sizes others.
The demand factor should be written down with its source, because it is the assumption a reviewer will question first. A figure that no one can justify is a weakness in the whole sizing calculation.
Step three: convert to apparent power
A transformer is rated in kilovolt-amperes, not kilowatts, so the real power has to be divided by the power factor:
S(kVA) = P(kW) / cos phi
A power factor of 0.9 is a common working value for mixed building loads. If power-factor correction is part of the design, the corrected value should be used, because it lowers the kVA the transformer has to supply and can move the choice down one standard rating.
Working in kVA is not a formality. The current that heats the windings depends on apparent power, so a load with a poor power factor draws more current than its kilowatt figure suggests and puts more heat into the transformer.
Step four: add margin and choose a rating
Add an allowance for growth and for the fact that a transformer should not sit at its limit continuously. Then round up to the next standard rating.
| Common three-phase ratings |
|---|
| 100, 160, 250, 315 kVA |
| 400, 630, 800, 1000 kVA |
| 1250, 1600, 2000, 2500 kVA |
Rounding up rather than down is deliberate. Standard ratings are spaced in steps, and a design that lands just above a step should take the next one, because the cost of the larger unit is usually smaller than the cost of a unit that cannot carry the load.
The margin is not the same as the demand factor, and the two should not be confused. The demand factor estimates what the building will actually draw today; the margin is added afterwards to cover growth and loading. Keeping them separate makes the reasoning visible to anyone who reviews the calculation.
Worked example
A residential building has a connected load of 260 kW. A demand factor of 0.6 brings that to 156 kW. With a power factor of 0.9, the apparent power is:
S = 156 / 0.9 = 173 kVA.
Adding about 20 per cent for growth and loading gives roughly 208 kVA. The next standard rating is 250 kVA, which becomes the working choice. A 200 kVA unit would sit too close to its limit to leave any room for future load.
If the demand factor had been set at 0.8 instead, the load would rise to 208 kW and the apparent power to about 231 kVA, still within the 250 kVA unit but with much less headroom. The example shows how sensitive the final rating is to the demand assumption.
What can change the answer
A handful of effects can move the rating in either direction.
- Large motors, whose starting current can pull the voltage down at the terminals.
- Harmonic-producing loads, which add heating that a simple kVA figure does not show.
- Seasonal peaks, such as heating or cooling, that set the real maximum demand.
- The chance of extending the building, which argues for spare capacity now.
Each of these is a reason to re-examine the assumed factors rather than to add a blanket margin on top of them.
Checks that follow the rating
Once the kVA rating is fixed, the work is not finished. The short-circuit current at the secondary depends on the transformer and has to be known before the downstream protective devices can be selected, because a larger transformer delivers a higher fault current.
Protection settings, the degree of voltage regulation under load, the cooling method if the unit runs near its rating, and the effect of inrush current on the incoming device all follow from the size chosen. Choosing the rating is the start of that chain, not the end of it. It is worth confirming that the devices downstream were selected against the fault current the chosen transformer will actually deliver, not against an earlier assumption made before the rating was fixed.
Sizing transformers in GorkyCAD
GorkyCAD keeps the building's loads, circuits and boards in one electrical model, and includes transformer and short-circuit calculations alongside the rest of the toolset. Because the load data is shared, the transformer rating and the fault levels on the secondary stay tied to the same source.
You can open the browser-based editor and work through a sizing exercise free of charge, with no card required.
Open the editor free — no card needed →
FAQ
Why is a transformer rated in kVA and not kW?
Because the current it carries depends on voltage and apparent power, not on how much of that power does useful work. A kVA rating covers the heating from the current whatever the power factor is.
What power factor should I assume?
For mixed building loads a value around 0.9 is common. If power-factor correction is designed in, use the corrected value, since it reduces the kVA the transformer must supply.
How much spare capacity should I add?
Enough to cover realistic growth over the life of the building. A common approach is a margin of around 20 per cent before rounding up to the next standard rating.
Can two smaller transformers replace one large unit?
Sometimes. Splitting the load can improve security of supply and reduce the effect of a single failure, but it raises cost and complexity, so it is a design decision rather than a default.