Best Engineer Tools
enptes
Electrical

Transformer Size Calculator

Calculate the required kVA capacity for a single-phase or three-phase transformer based on secondary voltage and load current, and find the nearest standard transformer size.

Inputs

Result

Enter values above to calculate

These calculators provide estimates and educational results. Always verify critical engineering calculations against applicable standards and qualified professional review.

Related Calculators

What Is It

Transformer sizing determines the kVA (apparent power) rating a transformer must have to serve a given load without overheating or excessive voltage regulation. Undersizing causes overheating and premature failure; oversizing wastes capital and increases no-load losses.

How to Calculate

Select single or three-phase, enter the secondary voltage and the load current the transformer must supply. The calculator computes the required kVA, rounds up to the nearest standard manufactured size, and shows the resulting full-load current at that standard size.

Example

A three-phase 208V panel draws 100A. Required kVA = √3 × 208 × 100 / 1000, about 36 kVA, rounding up to the standard 37.5 kVA size, which supplies up to about 104A at 208V three-phase.

Recommendations

Size the transformer for the calculated connected load plus a reasonable growth margin (typically 15-25%), not the bare minimum - transformers run cooler and last longer when not operated at their nameplate limit continuously.

Frequently Asked Questions

Why round up to a standard size instead of the exact calculated kVA?

Transformers are manufactured in standard kVA sizes; a custom size isn't available off the shelf, so you always specify the next standard size at or above your calculated requirement.

Does transformer sizing account for inrush current?

No - this covers steady-state load current. Transformers themselves also have their own magnetizing inrush at energization, a separate consideration for upstream protection sizing, not for the transformer's continuous kVA rating.

How does three-phase differ from single-phase sizing?

Three-phase power includes a √3 factor because power is distributed across three conductors with a phase relationship - a three-phase transformer serving the same current at the same voltage delivers about 1.73x more power than a single-phase unit.

Last reviewed: 2026-09-12