Understanding Power Transformer Ratings: kVA, Voltage, and Impedance Explained

A power transformer's nameplate carries all the critical specifications needed to safely install and operate it. Understanding these ratings is essential for any electrical engineer or procurement manager sourcing equipment for a substation or industrial facility.

kVA Rating

The kilovolt-ampere (kVA) rating indicates the transformer's maximum output capacity. Unlike kilowatts, kVA is the apparent power — it includes both the real power (kW) consumed by the load and the reactive power (kVAR) required by inductive loads like motors.

When sizing a transformer, calculate your connected load in kVA (not kW), add a 20–25% safety margin for future expansion, and then select the next standard rating above that value. Standard IEC ratings include 25, 50, 100, 160, 250, 315, 400, 500, 630, 800, 1000, and 1250 kVA.

Voltage Ratio

The primary voltage is what the transformer receives from the supply (e.g., 33 kV or 11 kV). The secondary voltage is what it delivers to your load (e.g., 433V for LT distribution). The turns ratio between primary and secondary windings determines this conversion.

Most LT distribution transformers in Bangladesh are rated 11/0.433 kV or 33/0.433 kV, stepping down from the utility grid to the standard three-phase 415V supply used in industrial facilities.

Percentage Impedance (%Z)

Impedance (typically 4–6% for distribution transformers) determines the fault current the transformer can supply during a short circuit. Lower impedance means the transformer can feed more fault current, which affects your downstream protection relay settings and circuit breaker ratings.

Cooling Classification

ONAN (Oil Natural Air Natural) is the standard cooling method for most distribution transformers.