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:
- Dwelling units, NEC optional method: first 10 kVA of general load at 100%, remainder at 40% (NEC 220.82). Existing dwellings under 220.83 use a different threshold.
- Warehouse general lighting: first 12.5 kVA at 100%, remainder at 50% (NEC lighting demand-factor table). Most other non-dwelling occupancies take general lighting at 100%.
- 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)
The office, school, industrial and hospital figures above are planning rules of thumb for whole-building diversity, not NEC demand factors. For service and feeder calculations, use the NEC Article 220 demand-factor tables for each load type (lighting, receptacles, appliances, dryers, cooking equipment). 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 above nameplate shortens insulation life. IEEE C57.91 calculates loss of life from the hottest-spot winding temperature, which depends on the load, the ambient temperature and the daily load cycle. There is no single overload percentage that is always safe: a planned overload on a cool day may cost little life, and the same overload in summer heat may cost a lot.
A common practice is to load to about 80% of nameplate at the design point. With a 25% growth allowance on top, nameplate = 1.25 / 0.80 = 1.56 times today’s design load.
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).