Estimate the transformer size you need from your load. Enter the load in kW and power factor, in kVA, or from voltage and measured current. The calculator adds a safety margin, suggests the next standard size and shows full-load current on both sides.
Estimate only. Check against your utility rules, the applicable standard and a qualified engineer before ordering or installing equipment. Standard sizes shown are common ones (kVA): 5, 10, 16, 25, 50, 63, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500. Your utility list may differ.
Formulas used
- kVA from kW: kVA = kW divided by power factor
- kVA from current (three-phase): kVA = 1.732 times line voltage times line current, divided by 1000
- Full-load current: I = kVA times 1000, divided by (1.732 times line voltage)
- Loading: actual load kVA divided by rated kVA, times 100
Example
A load of 80 kW at a power factor of 0.8 is 100 kVA. With a 20% margin you need 120 kVA, so the next standard size is 125 kVA. At 415 V the LV full-load current of a 125 kVA transformer is about 174 A.
Related: our guide to AI for electrical engineers and the AI Prompts for Electrical Engineers toolkit.
Frequently asked questions
How do I calculate transformer size?
Find the total load in kVA (kW divided by power factor), add a safety margin for growth and starting currents, then choose the next standard transformer rating above it.
What safety margin should I use?
Many designers use 10 to 25 percent for growth. The right figure depends on your load type, motor starting and local rules, so follow your utility or standard.
Why use kVA and not kW?
Transformers are limited by current and voltage, not by how much of that current does useful work. kVA includes both, so it is the correct rating unit.
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