How to size a switchboard and service transformer
A switchboard is never sized on connected load alone, because everything plugged into it does not run flat out at the same moment. Applying a demand factor gives the real maximum demand in kW, dividing by power factor converts it to kVA, and the next standard transformer rating above that number is the one to buy.
Written out: maximum demand (kW) = connected load × demand factor and apparent power (kVA) = demand ÷ power factor. Three phase full load current is kVA × 1,000 ÷ (√3 × volts), and the bus is picked from the UL 891 standard ratings at or above 125% of that current, the continuous load allowance in NEC 215.2 and 408.36.
With 500 kW connected, a 0.7 demand factor, 0.9 power factor and a 480Y/277 V service, demand is 350 kW and 388.9 kVA, so a 500 kVA transformer fits. Full load current is about 468 A, and 125% of that is 585 A, so a 600 A bus works. Transformer loading lands near 77.8%, which leaves little room for future growth.
Ratings shown follow ANSI/NEMA transformer sizes and UL 891 switchboard bus ratings current as of 2026. Diversity between feeders, harmonic and inrush behaviour, available short circuit current and the NEMA temperature rise and ventilation limits inside the enclosure all need separate checks, so confirm the final specification with a licensed electrical engineer and your serving utility.
Frequently asked questions
It reflects the fact that connected equipment rarely all runs at once. Office buildings commonly fall between 0.6 and 0.8 and industrial plants between 0.5 and 0.9, while data halls and cold storage with near continuous load sit close to 1.0. NEC Article 220 also gives calculated load rules that may govern.
Rounding up to a standard ANSI kVA rating usually leaves 10 to 30% headroom. Above about 80% loading there is little room to grow, and below roughly 40% no load losses and poor power factor make a smaller unit worth considering.