Calculation reviewed: 4 October 2026 · Engineering Desk Tools
How to use the Transformer size calculator
Estimate the required transformer kVA, the next standard rating and the rated secondary current. Start with measurements or documented values for the equipment and reporting period you actually want to assess. The calculator does not fill in a typical installation for you. Its blank fields help keep a demonstration value from being mistaken for your own operating data.
Choose the required options, enter the values in the labelled units and select Calculate. If a field is rejected, check its range and unit before changing it. The result is accompanied by the equation and assumptions so you can decide whether it answers your engineering question.
Inputs and measurement basis
- Supply. Use three-phase for a balanced three-phase supply and single-phase for a two-wire circuit.
- Maximum load (kW). Maximum demand: the realistic peak of all loads running together, in kW. The value must be greater than zero.
- Load power factor (decimal, up to 1). Weighted average power factor of the load at peak, as a decimal. The allowed maximum is 1. The value must be greater than zero.
- Spare capacity for growth (%). Allowance for future load and motor starting. Set it to your project policy; 10 to 25 percent is common. The allowed maximum is 300.
- Secondary voltage (V). Rated secondary line voltage, for example 400 V. The value must be greater than zero.
Record the measurement source and operating condition. Retain unrounded values for verification.
Method and interpretation
A transformer is limited by current, and so by kVA rather than kW. The load in kW is therefore converted to apparent power with the power factor. A growth margin is added and the next standard rating is chosen. The rated secondary current follows from the kVA and the secondary voltage.
Load kVA = kW / PF. Required kVA = load kVA × (1 + margin / 100). Rated secondary current = standard kVA × 1000 / (√3 × V) for three-phase, or kVA × 1000 / V for single-phase.
Use maximum demand, not the sum of all nameplates, unless every load can run together. The standard ratings listed (25 to 3150 kVA) are common values; availability varies by region and manufacturer. Motor starting, harmonics, ambient temperature, cooling class, impedance and loading policy can change the right choice.
Worked example
This example does not populate the form. Enter your own installation data.
Calculated result: 300 kVA required. Confirm impedance, starting duty, harmonics, temperature and availability with the supplier before purchase.
Check the arithmetic independently using the displayed equation. Retain intermediate precision and compare the final value with your equipment documentation.
Quick-reference table
These are illustrative values, not certified ratings or manufacturer data. Use the form for your own conditions.
Common mistakes
Adding every motor nameplate when the loads never run together; leaving out the margin for growth and motor starting; mixing line-to-neutral with line-to-line voltage; forgetting that a larger transformer has a higher fault level and different losses. Before relying on the output, check the decimal format, unit scale and source of each value. Recheck unusual results against the formula.
Frequently asked questions
Are the example values used automatically?
No. Inputs remain blank when the tool opens or when you clear them. The worked example and table are reading material below the calculator. Enter the actual values you want to check each time, and label saved reports so their context is not lost.
What does the result mean?
The result is the kVA the transformer must supply with margin, the next standard rating above it and that rating’s secondary current. A loading close to 100 percent leaves little room for growth or motor starting.
Can I use a zero or a negative value?
The allowed range follows the meaning of each input. A divisor needs a positive value; other fields may allow zero. Check the form labels and error message rather than forcing an invalid value. A rejected input must be corrected before a result can be shown.
Is this a final design or equipment approval?
No. Use maximum demand, not the sum of all nameplates, unless every load can run together. The standard ratings listed (25 to 3150 kVA) are common values; availability varies by region and manufacturer. Motor starting, harmonics, ambient temperature, cooling class, impedance and loading policy can change the right choice. Use the calculation as support for a checked decision, and review equipment documentation and applicable requirements before changing an installation or operating setting.
How can I save or share a calculation?
Calculate first, then use Copy result or Print report. Some maintenance tools also provide a CSV summary. Reports contain your own entered data, so review them before sharing. You can share the public page URL without including plant records or confidential measurements.
How do I report an unexpected result?
Use the Contact page and include the tool URL, input values, units, expected output and the reference you used. Explain the measurement basis. Exclude confidential information.
Related tools and records
Use related calculations with consistent units, periods and equipment scope.
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