Transformer size from the demand
Size a transformer from the demand, with growth and losses.
Try a job 4
Each job fills in example values. Replace them with your values.
- Size a transformer for a new siteThe rating, loading and losses for a site demand with room to grow.
- Size a transformer from a kW demandA demand in kilowatts turned into the transformer rating at its power factor.
- Check a rural single-phase transformerThe size and voltage drop of a single-phase transformer for a farm supply.
- Check a plant transformer regulationThe voltage a large transformer drops at a low power factor.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Maximum demand
- 220
- Demand in
- kVA
- Power factor
- 0.9
- Growth allowance
- 20 %
- Most loading at the design load
- 100 %
- Phases
- Three phase
- Primary voltage
- 11,000 V
- Secondary voltage
- 400 V
- Impedance
- 4.5 %
- X/R ratio
- 4
- No-load loss, of the rating
- 0.2 %
- Load loss at full load, of the rating
- 1.1 %
Transformer size
300kVA
- Design load with growth
- 264kVA
- Loading today
- 73.3%
- Loading with growth
- 88%
- Voltage regulation with growth
- 2.59%
- Losses with growth
- 3.16kW
- Secondary full load current
- 433.01A
- Secondary fault current
- 9,622.5A
This result is a design aid. Verify it against the current standard.
Calculation steps
- Design load with 20 % growth264kVA
- Smallest typical size at 100 % loading300 kVA
- Loading today73.3%
- Loading with growth88%
- Resistance, of the rating1.09%
- Reactance, of the rating4.37%
- Voltage regulation with growth2.59%
- Secondary voltage at the design load389.7V
- Losses with growth3.16kW
- Efficiency with growth98.69%
- Secondary full load current433.01A
- Secondary fault current9,622.5A
About this calculator
How this is calculated
A transformer is sized from the maximum demand it will carry, with room for the load to grow. Too small and it runs hot and drops too much voltage; too large and it costs more and wastes energy in its core every hour of the year. This tool finds the size and shows what it does at the load.
You enter the maximum demand, in kVA or in kW with a power factor, and a growth allowance. The design load is the demand plus that growth. The tool picks the smallest rating from a list of typical sizes that carries the design load within the loading limit you set. The list is common distribution and site ratings, not a standard's list.
At that size it works the loading today and with growth, then the voltage regulation: the drop through the transformer's own resistance and reactance at the design load and power factor. The impedance on the rating plate splits into the two by the X/R ratio. The losses are the core loss, which is there whenever the transformer is energised, plus the winding loss, which rises with the square of the loading. The full load and fault currents at the size come from the transformer tool.
The impedance, X/R ratio and losses are maker's data. The defaults are typical figures, and the result names them as assumptions.
| Step | Table or clause | What it decides |
|---|---|---|
| Design load | Demand plus the growth allowance | The load the size must carry |
| Size | Smallest typical rating within the loading limit | The transformer rating |
| Resistance and reactance | Impedance split by the X/R ratio | The two parts of the drop |
| Voltage regulation | Loading times the drop at the power factor | The voltage lost through the transformer |
| Losses | Core loss plus winding loss times loading squared | The heat and the running cost |
| Currents | The transformer tool | The full load and fault current at the size |
Worked examples
- A 220 kVA site with 20 % growth needs a 300 kVA transformer. It runs at 88 % with growth and loses 2.59 % of its voltage.
- A 180 kW load at 0.85 power factor with 25 % growth also needs 300 kVA, at 88.2 % with growth.
- A 420 kVA factory with 30 % growth, loaded to 80 % at most needs 750 kVA, at 72.8 % with growth.
- A 60 kVA single-phase rural supply with no growth needs 75 kVA, at 80 %.
- A 900 kVA plant at 0.8 power factor, 6 % impedance and X/R of 7 needs 1,250 kVA. Its regulation is 3.57 % at 82.8 % with growth.
Limits
The regulation is the drop through the transformer alone. The network upstream and the cables downstream add their own drop; the total voltage drop tool adds the cable parts. A tap changer or off-load taps can raise the secondary voltage to make up for it.
The tool does not cover transformers in parallel, harmonic loads, the temperature rise of the windings or the network company's own sizing rules for a supply transformer.
See also: Transformer, Maximum demand and Fault level at a board.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
How do I size a transformer?
Start from the maximum demand, add an allowance for growth, and pick the smallest standard rating that carries the result. Then check the voltage the transformer drops at that load and what it loses as heat.
How much growth should I allow for?
It depends on the site. 20 to 30 % is common where more load is likely. A transformer lasts decades, and changing it later costs more than one size up now.
What is transformer voltage regulation?
The drop in secondary voltage from no load to the load, as a percent. It comes from the transformer's resistance and reactance, and it is larger at a low power factor.
Why not just pick a much bigger transformer?
A larger unit costs more and its core loss runs all day, every day, even at light load. Sized close to the load, a transformer runs where its losses are lowest for the energy it delivers.
Where do the impedance and losses come from?
The impedance is on the rating plate. The losses and the X/R ratio are in the maker's test report. The tool's defaults are typical figures until you have the real ones.
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