Transformer kVA Sizing
Distribution-transformer sizing from connected load. Applies a demand factor (load is rarely at 100% simultaneously) and a future-growth allowance, then rounds up to the next standard kVA size. Reports primary and secondary full-load amps for switchgear sizing.
Defaults: 500 kW connected, PF = 0.85, DF = 0.80, 25% growth, 13.8 kV primary to 480V secondary — typical industrial substation. Standard kVA sizes: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500, 3000, 5000.
Demand factors — NEC 220
Demand factor accounts for the fact that not every load operates simultaneously at full output. NEC Article 220 specifies factors by occupancy:
- Residential dwelling units: 0.40–0.65 for first 10 kVA, 0.40 for remainder (NEC 220.83).
- Commercial general lighting: 1.0 for first 12.5 kVA, 0.5 thereafter (NEC 220.42).
- Office: 0.7–0.8
- School: 0.5
- Industrial: 0.85–1.0 for process loads (often run continuously)
- Hospital: 0.4–0.6 (load varies by department)
Use Table 220.42, .54, .55, .56 for specific occupancies. Industrial loads often run at 1.0 because the connected load IS the operating load.
Growth allowance
Standard practice: 20–30% future growth on commercial/industrial transformers. For data centers or rapidly growing campuses, 50–100%. The cost difference between a 1500 kVA and 2000 kVA transformer is small, and adding capacity later is expensive. Specify utility-coordination at the highest growth tier you can financially justify.
Standard kVA sizes (ANSI C57.12.10)
- Pad-mount, 3-phase, 15 kV class: 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500, 3000 kVA
- Substation, 3-phase: 1500, 2000, 2500, 3000, 5000, 7500, 10,000, 12,500, 15,000 kVA
- Single-phase residential: 10, 15, 25, 37.5, 50, 75, 100, 167 kVA
Always round up to the next standard size. Custom sizes are available but cost 30–50% more for the manufacturer to build.
Loading guidelines
Continuous loading at > 100% rated kVA shortens transformer life. ANSI C57.91 allows brief overloading per the loading guide:
- 110% continuous: 50% normal life
- 120% for 4 hr/day: 50% normal life (cyclical)
- 140% for 30 min: emergency only
Standard practice: design at 80% of nameplate rated kVA. The 25% growth allowance + 80% loading gives 25% × 1.25 = 56% spare capacity at design point.
Primary and secondary current
I = kVA × 1000 / (√3 × V) for three-phase. The primary current at 13.8 kV is small (1500 kVA / (1.73 × 13800) = 63 A) — easy to switchgear. The secondary at 480V is much larger (1804 A) and requires substantial bus, breakers, and ground-fault protection.
Reference: National Electrical Code (NFPA 70-2023), Article 220 and 450. ANSI C57.12.00 / C57.12.10. IEEE Std 141 (Red Book).