Transformer Sizing Calculator

Size transformers based on connected load and demand factors.

Calculation Method

Total Connected Load

Load Unit

Power Factor

Secondary Voltage

Demand Factor (%)

Future Growth (%) Recommended: 20-25% for future expansion


                            kVA = kW / Power Factor


                            kVA = (V x I x 1.732) / 1000


                            kVA = (V x I) / 1000

Transformer Requirements (NEC Article 450)

Transformers over 1000V: 125% for fuses, 300% for circuit breakers. Under 1000V with primary protection only: 125%.

Secondary conductors sized at 125% of secondary FLA. OCPD sized per 240.4 based on conductor ampacity.

Dry-type transformers require adequate ventilation. Liquid-filled must meet NEC 450.27 vault requirements.