Transformer calculator
Enter the transformer rating and voltages to get full-load current on both sides and the turns ratio, or enter your load to find the next standard kVA size.
Size a transformer for a load
25% headroom is a common design margin, not a code rule; the actual size should come from a load calculation (NEC Article 220) and the manufacturer's data. Overcurrent protection per NEC 450.3(B) and secondary conductor rules per 240.21(C) need a qualified designer.
Transformer formulas
Single-phase: I = kVA × 1000 ÷ VThree-phase: I = kVA × 1000 ÷ (1.732 × VLL)Turns ratio = Vprimary ÷ Vsecondary = Isecondary ÷ IprimaryExample: a 75 kVA, 480 V delta to 208Y/120 V transformer carries 75,000 ÷ (1.732 × 480) = 90.2 A on the primary and 208.2 A on the secondary. The turns ratio is 2.31 : 1.
Standard kVA sizes
Single-phase: 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250 kVA. Three-phase: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000 kVA. Availability varies by manufacturer.
Questions people ask
How do I calculate transformer full-load current?
Single-phase: I = kVA × 1000 ÷ V. Three-phase: I = kVA × 1000 ÷ (1.732 × V). A 45 kVA, 480–208Y/120 V transformer carries 54.1 A primary and 124.9 A secondary.
What are standard transformer sizes?
Common dry-type sizes: single-phase 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167 kVA; three-phase 15, 30, 45, 75, 112.5, 150, 225, 300, 500 kVA. Check the manufacturer’s catalog.
How are transformer overcurrent devices sized?
NEC 450.3(B) sets maximum primary and secondary protection as a percentage of rated current, depending on voltage and whether secondary protection is provided. Have the design checked by a qualified person.