Transformer Sizing Calculator

About the Transformer Sizing Calculator

Calculates the transformer kVA rating needed for a three-phase (or single-phase) load from voltage and current, then suggests the next standard size.

How to use this calculator

1. Enter the load voltage in V .
2. Enter the load current in A .
3. If the load is three phase, use the three phase formula shown by the calculator.
4. Read the calculated transformer size in kVA and round up to a suitable standard rating.

The formula explained

The calculator uses kVA = \( \frac{\sqrt{3}\,VI}{1000} \) to compute apparent power for a three phase load. It tells you how much transformer capacity the load needs before any safety margin or future growth is added.

• kVA = the apparent power rating needed for the transformer
• V = the line voltage in volts
• I = the line current in amperes
• \(\sqrt{3}\) = the three phase conversion factor

Step by step method

1. Write down the load voltage V and current I .
2. Multiply \sqrt{3} by V and then by I .
3. Divide the result by 1000 to convert from volt amperes to kilovolt amperes.

Worked example

Problem. Find the transformer size for a three phase load with V = 415 and I = 32 .

1. Substitute the values into the formula, kVA = \frac{\sqrt{3}\cdot 415 \cdot 32}{1000} .
2. Compute the numerator, \sqrt{3}\cdot 415 \cdot 32 \approx 23013.4 .
3. Divide by 1000 , so kVA \approx 23.0 .

Tips and common mistakes

• Make sure you use the line voltage and line current that match the three phase formula, not phase values unless the calculator asks for them.
• If your result is not a standard transformer size, round up to the next available rating.

Frequently asked questions