Transformer Sizing Calculator

Enter your voltage, amperage, and phase type to calculate the required transformer kVA size. The Transformer Sizing Calculator supports both single-phase and three-phase systems — fill in any two of the three fields (Volts, Amps, kVA) and the third is calculated automatically. Results include required kVA, full load amps, and a recommended standard transformer size.

Select whether your system is single-phase or three-phase.

V

Enter the system voltage in volts. Leave blank if solving for voltage.

A

Enter the full load current in amps. Leave blank if solving for amps.

kVA

Enter the kVA if known. Leave blank to calculate kVA from volts and amps.

Results

Required kVA

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Recommended Standard Transformer Size

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Full Load Amps

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Voltage (Solved)

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Required vs. Headroom kVA

Results Table

Frequently Asked Questions

How do I size a transformer?

To size a transformer, you need to know the kVA (kilovolt-amps) your load requires. Use the formula: kVA = (Amps × Volts) / 1000 for single-phase, or kVA = (Amps × Volts × 1.732) / 1000 for three-phase. Always select the next standard transformer size above your calculated kVA to ensure safe operation.

What is kVA and how is it different from kW?

kVA (kilovolt-amps) is apparent power — the total power a transformer must supply, including both real (kW) and reactive power. kW is real (working) power. For transformer sizing, kVA is the relevant measure because transformers are rated in kVA, not kW. The ratio of kW to kVA is the power factor.

What is the formula for transformer sizing?

For single-phase: kVA = (Amps × Volts) / 1000. For three-phase: kVA = (Amps × Volts × 1.732) / 1000. You can rearrange these to solve for amps or volts if kVA is known. Always round up to the nearest standard transformer size (e.g. 15, 25, 37.5, 50, 75, 100 kVA).

What is voltage?

Voltage (measured in volts, V) is the electrical potential difference that drives current through a circuit. Common system voltages include 120V, 208V, 240V, 277V, 480V, and 600V. When sizing a transformer, you need both the primary (input) voltage and secondary (output) voltage.

What are amps?

Amps (amperes, A) measure the rate of electrical current flow in a circuit. The full load amps of a transformer is the maximum current it can continuously supply at its rated kVA. Exceeding this rating causes overheating and can damage the transformer.

What is a transformer?

A transformer is an electrical device that transfers energy between two circuits by changing voltage levels while keeping power (kVA) approximately constant. Step-up transformers increase voltage; step-down transformers decrease it. They are essential for distributing electrical power safely and efficiently.

What standard transformer sizes are available?

Common standard single-phase transformer sizes include: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250 kVA. Three-phase standard sizes typically include: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA. Always select the next size above your calculated requirement.

Can I use two of the three values (Volts, Amps, kVA) to find the third?

Yes. This calculator lets you enter any two of the three values — Volts, Amps, or kVA — and it will solve for the missing one. For example, enter Volts and kVA to find the full load amps, or enter Amps and Volts to find the required kVA.

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