Free transformer sizing calculator & electrical transformer calculator. This post explains through every calculated parameter the IEEE / IEC framework demands: from rated currents and power triangles, through efficiency curves and voltage regulation to thermal hotspot estimation and protection coordination. It is intentionally narrower than. Distribution transformers provide the final voltage step-down, delivering power directly to end users same fundamental principle that governs all transformers. The voltage ratio between primary and secondary is determined by the turns ratio: secondary turns. Transformer capacity is rated in KVA (kilo-volt-amperes).