Transformer sizing looks like a simple calculation, but there are enough variables to get it wrong. The basic math: total load in VA divided by 1000 = kVA. For three-phase: kVA = (V x I x 1.732) / 1000. But the real question is always: what’s your actual demand, not your connected load?
Connected load is the sum of all equipment nameplate ratings. Demand load is what’s actually running at any given time - and it’s almost always less than connected load. NEC Article 220 gives you the framework for calculating demand loads, but for commercial and industrial work you often need to understand the operational profile of the facility.
Size for 80% continuous loading per NEC 450.3 transformer protection requirements. A 75 kVA transformer should not be continuously loaded above 60 kVA. Also consider future load growth - upgrading a transformer later costs more than slightly oversizing now. A rule of thumb: size to 60-70% of transformer capacity at expected peak demand, which leaves room for continuous loading rules and growth.