How the calculation works
The calculator estimates full-load current: I = kVA·1000 / (1.732·V) (balanced 3Φ) or kVA·1000 / V (1Φ). It does not select primary or secondary protective devices. Actual protection depends on transformer type, protection arrangement, current thresholds, conductors, terminals, code edition and applicable exceptions. Verify with the adopted NEC/CEC and a qualified designer.
Worked example
75 kVA, 3Φ 480→208 V: primary 90.2 A, secondary 208.2 A. These are full-load current estimates only, not breaker or conductor selections.
Common mistakes to avoid
- Using the single-phase formula on a three-phase transformer.
- Confusing primary-only protection with primary-plus-secondary protection.
- Ignoring inrush current when choosing the primary device.
Frequently asked questions
Conductors?
Conductor sizing and protective-device selection require the adopted code, transformer configuration, terminal ratings and applicable rules; this calculator does not select them.
What kVA do I need for a 100 A load at 208 V three-phase?
kVA = 1.732 × 208 × 100 ÷ 1000 ≈ 36 kVA, so select the next standard size (37.5 kVA or larger).
Do I add margin for future load?
Often yes. Size for expected growth and check the conductors on both sides.
Related calculators
For informational and educational purposes only. Always consult a licensed electrician and local code authority. See our Electrical & Legal Disclaimer. Found an error? Tell us.