🔁 Transformer Sizing

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

Rating a transformer for a known load, and finding the resulting primary and secondary currents.

A transformer's rating (in kVA, not kW) reflects its apparent power capacity, independent of the load's power factor — this is because the transformer's windings and core have to handle the actual current and voltage regardless of how much of that power is real versus reactive. The rating is the transformer's OUTPUT capacity, so the load sets it directly: pick the next standard size at or above the load kVA.

Once sized, primary and secondary current follow directly from the voltage ratio and the power — this is what determines the wire size and protective devices on each side. Efficiency does not change the rating or the secondary current; it only means the primary has to supply slightly more than the load draws, because a few percent is lost as heat in the windings and core.

Key formula

Rating = the next standard kVA size at or above the load
Secondary current = (load kVA × 1000) / V_secondary
Primary current = (load kVA / efficiency × 1000) / V_primary

Variables

Efficiency

fraction of input power delivered to the load, rest lost as heat in the windings/core

Notes & limitations

• This treats the load as single-phase; three-phase transformer sizing uses a √3 factor in the current calculation, the same way the Power Calculation article does.
• Transformers are usually loaded to 80% or less of nameplate rating in continuous service to manage heating and extend insulation life — treat the rated kVA as a ceiling, not a target.