Transformer kVA calculator
Transformer kVA calculator
The kVA a load demands, the rating that carries it inside its continuous loading limit, and the margin for what comes later.
How this is calculated
kVA = kW / cos φ. The rating that carries it at your loading limit is kVA / (loading / 100), and the margin multiplies that. Full-load current is kVA x 1000 / V on single phase, or / (√3 x V) on three phase.
- Transformers are wound and billed in kVA rather than kW because the core and the copper see apparent power, not the part of it that does work — which is exactly why the power factor changes the size you need.
- Standard ratings come in a fixed series, so the number to order is the next published rating above the computed figure. The series differs between distribution transformers and dry-type units for a building, and the manufacturer's list governs.
A transformer is rated in kVA because its core and windings are heated by the whole of the current, whether or not the load turns any of it into work. The kW it can supply therefore depends on the load’s power factor.
Loading limit, margin and what they are for
A transformer will carry its rated kVA; the loading limit is the share of it you choose to draw continuously, and it is a design decision rather than a physical constant. Some utilities state one for a connection, some manufacturers state one for a dry-type unit in a switchroom, and the two need not agree.
The future margin is a different thing again — an allowance for machines that are not yet installed. Both are entered here because both change the size you order, and neither belongs as a hidden default inside a formula.
Worked through
A 120 kW load at a power factor of 0.85 demands 120 / 0.85 = 141.18 kVA. Designing to 80 per cent continuous loading needs 141.18 / 0.8 = 176.47 kVA, and a 15 per cent allowance for future load takes that to 202.94 kVA.
At 415 V three-phase, that rating would carry 282.3 A when fully loaded — the figure a switchgear and a cable on the secondary are chosen against. The number to order is the next published rating above 202.94 kVA, and the series is the manufacturer’s.
Questions readers ask
Why is a transformer rated in kVA rather than kW?
Its losses and its temperature rise come from voltage-dependent core loss and current-dependent copper loss, neither of which knows how much of the current is doing work. The load’s power factor decides how many kilowatts a given kVA can carry, so the machine is rated in kVA and the kW is the customer’s to work out.
Can I run a transformer above its rating?
Transformers carry short-term overload by design, with a stated rise in hot-spot temperature and a corresponding cost in insulation life. Continuous operation above rating is a different matter and is governed by the cooling class, the ambient and the manufacturer’s loading guide — none of which this page can see.
Does improving the power factor change the size?
Directly. The same 120 kW at 0.95 asks for 126.3 kVA instead of 141.2, which is a smaller transformer, a lower loss and often a different tariff category. That is why capacitor banks are sized against this figure rather than against the load.
What about the inrush current when it is energised?
A transformer magnetises with a surge well above its full-load current for a fraction of a second, and protective devices and generators upstream have to live with it. It is on the manufacturer’s data sheet and it is not derived from these numbers.
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