Distinguish watts from volt-amperes
Transformers are commonly rated in VA or kVA. A load expressed only in watts should not simply be treated as the same number of VA. Check its input VA, current or power-factor information from the load manufacturer.
List the loads operating together
List PLCs, indicator lamps, relays, contactors and other control loads, including which operate together. Identify planned expansion. Loads with rectifiers or electronic input circuits may require additional consideration of their input characteristics.
Check starting demand and voltage drop
A contactor or solenoid may require more VA during pickup than while holding. A rating based solely on steady-state demand may not maintain the voltage needed during pickup. Review pickup VA, holding VA, simultaneous starting and the permitted operating voltage range.
Do not rely on a universal margin
There is no single percentage margin that establishes the correct rating for every application. Ambient conditions, duty, starting characteristics and transformer performance all matter. Send the load schedule so the requirements for a manufacturing review can be identified.