Transformer sizing
Transformer Sizing Calculator
Size a single- or three-phase transformer in kVA or MVA — with spare capacity, IEC standard ratings, full-load current and fault current.
Sizing to IEC 60076 preferred ratings. An indicative design aid — verify the final rating, impedance and vector group with the supplier.
Result
Enter your load — the recommended transformer rating appears here.
How the size is chosen
A transformer is sized on apparent power (kVA), not real power (kW), because its windings and core carry the full current regardless of power factor.
1. Find the load in kVA — directly, from kW ÷ power factor, from √3 × V × I (three-phase), or by summing a load schedule with demand and diversity factors. 2. Add spare capacity — typically 20–30% for future growth and motor inrush. 3. Round up to the next IEC 60076 standard rating. The full-load current then follows from the rating and winding voltage, and the secondary fault current from the impedance (Isc ≈ IFL ÷ Z%).
The course explains why each margin matters and how to defend the rating in a design review.
Frequently asked questions
Why is a transformer sized in kVA and not kW?
A transformer is rated in kVA (apparent power) because its windings and core carry the full current whatever the load power factor. Real power in kW only tells part of the story — convert it with kVA = kW ÷ power factor before sizing.
How do I calculate transformer kVA from voltage and current?
For a single-phase supply, kVA = (V × I) ÷ 1000. For three-phase, kVA = (√3 × V × I) ÷ 1000, where V is the line-to-line voltage and √3 ≈ 1.732. Use the tool's 'Voltage & current' mode to do this automatically.
How much spare capacity should I add to a transformer?
A margin of about 20–30% is common, to cover future load growth and motor inrush without running the transformer near its limit. The tool defaults to 25%; adjust it to suit your project.
What if I don't know my total load?
Use the 'Load schedule' mode. List each load with its quantity, a demand factor (how much of it runs at peak), then apply an overall diversity factor. The tool adds them into a diversified peak demand and sizes the transformer from that.
What transformer impedance percentage should I enter?
Enter the nameplate short-circuit impedance (%Z, also called Uₖ or impedance voltage) from the transformer's rating plate — typically 4–6% for distribution transformers. Leave it blank and the tool applies a typical value for the size, used to estimate fault current.
How is the secondary fault current estimated?
The prospective fault current is roughly the full-load current divided by the per-unit impedance: I_fault ≈ I_FL ÷ (%Z ÷ 100). It is an indicative figure that assumes a stiff upstream supply; the real value is a little lower once the network impedance ahead of the transformer is included.
When should I use more than one transformer or an N+1 spare?
Split the load across parallel units when it exceeds a single practical unit or a transport limit, or choose N+1 redundancy so one unit can be taken out for maintenance or fault without dropping the load. Parallel units must share the same voltage ratio, vector group and impedance to divide load correctly.
Which standard does this calculator use?
Ratings follow the IEC 60076 preferred-number series, and the tool rounds the required kVA up to the next standard size. It is an indicative design aid — the final rating, impedance, vector group and cooling must be verified against the standard by a competent engineer.