Transformer Sizing Calculator

How it’s calculated. Full-load amps: single-phase FLA = (kVA × 1000) ÷ V; three-phase FLA = (kVA × 1000) ÷ (√3 × V). The tool picks the next standard transformer kVA at or above your load, then sizes overcurrent protection from NEC Table 450.3(B) (transformers 1000 V or less): primary-only protection is 125% of primary FLA (≥9 A), 167% (2–9 A), or 300% (<2 A); with secondary protection the primary may go to 250% and the secondary to 125% (≥9 A) or 167% (<9 A). The 125% values round up to the next standard device (240.6); the 167/250/300% values are ceilings that round down. Suggested conductors are the smallest 75°C copper carrying 125% of FLA.

Buck-boost. A buck-boost is a small insulating transformer field-connected as an autotransformer, so it only transforms the voltage change — buck-boost kVA = (load amps × ΔV) ÷ 1000. That’s why a ~1.3 kVA unit can serve a 10 kVA load when boosting 208 V to 240 V. Three-phase uses a bank of single-phase units — two in open-delta on a 3-wire source, three in wye on a 4-wire source. Always confirm the selected unit’s winding-current rating on the manufacturer’s selection chart.

Low-voltage / control (12–24 V). Control transformers are sized to the sealed (holding) VA of everything energized at once, and must also supply the inrush VA when coils pick up without the secondary voltage sagging below pickup — the tool combines them as √(sealed² + inrush²) and flags the manufacturer regulation check. Lighting / resistive 12–24 V loads are sized at the connected VA plus 25% spare; watch secondary voltage drop on long low-voltage runs.

This is a planning aid, not a stamped design. For commercial transformer, feeder, and service sizing — with the studies and the install — our engineering team handles it as part of design-build. Verify every result against the current NEC and your local amendments.