Calculators
Transformer Sizing
Three-phase transformer sizing from connected motor and lighting/misc loads with demand, future-growth, and ambient/altitude corrections. Selects the next standard NEMA kVA rating and reports primary and secondary full-load current plus an infinite-bus secondary short-circuit estimate. Per IEEE C57.12.00, NEC Article 450, NEMA TP-1.
Method: Motor input kVA = Pmotor / (PF · η). Lighting/misc kVA = P / PF. Total connected = sum, diversified by demand factor, scaled by (1 + growth), divided by (kamb · kalt). Standard NEMA ratings: 300, 500, 750, 1000, 1500, 2000, 2500, 3000, 3750, 5000, 7500, 10000 kVA. Full-load amps = kVA · 1000 / (V · √3). Secondary bolted short-circuit (infinite bus) = IFL,sec / (Z/100). References: IEEE C57.12.00; NEC Article 450 (2023); NEMA TP-1 (dry-type efficiency) / DOE 10 CFR 431 Subpart K (liquid-filled).
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