Everything you need to compare, reorder or safely operate a transformer is stamped on its nameplate. The core fields are the rated power in kVA or MVA, the high- and low-voltage levels, the frequency, the vector group, the impedance voltage and the cooling class. Read those six and you know what the machine is.
What the rating block actually promises
Rated power is the apparent power the transformer carries continuously without exceeding its permitted temperature rise. That promise is tied to the conditions printed alongside it: the cooling class, the reference ambient temperature and, above roughly 1 000 m, the installation altitude. IEC 60076-2 assumes a yearly average ambient of 20 °C and a maximum of 40 °C. A plate reading 1 000 kVA ONAN does not mean 1 000 kVA in a sealed substation room at 45 °C.
The plate also names the standard the unit was built and tested to, normally IEC 60076-1, plus the serial number, the year of manufacture and the manufacturer. Keep those three to hand: they are what a service partner needs before quoting a bushing, a gasket set or a full replacement.
Voltages, taps and rated currents
The voltage line gives the no-load ratio, written as HV / LV — for example 20 000 / 400 V at 50 Hz. Next to it sits the tapping range, usually something like plus or minus 2 x 2.5 %, set by the de-energised tap changer on the HV side. Those taps compensate for a network that sits structurally high or low; they do not regulate voltage under load.
Rated currents are normally printed as well, and if not you can derive them. For a three-phase unit the current equals the rated power divided by the product of the square root of three and the rated voltage. A 630 kVA unit at 400 V therefore draws about 909 A on the LV side — the figure that sizes your cables and your LV breaker.
The fields that decide how the unit behaves in the network
- Vector group, for example Dyn11 — the winding connections and the phase shift, and the first thing to check before paralleling.
- Impedance voltage uk as a percentage, referred to 75 °C — it sets the voltage drop under load and the prospective short-circuit current.
- Cooling class, four letters such as ONAN or ONAF, sometimes with a dual rating like 1 600/2 000 kVA.
- No-load and load losses in watts — the basis for any total-cost comparison between offers.
- Insulation levels: highest voltage for equipment Um, lightning impulse withstand LI and power-frequency withstand AC.
- Temperature-rise limits, typically 65 K average winding rise and 60 K top oil for mineral-oil units.
Masses, fluid and the practical details
Total mass, untanking mass and the mass and volume of insulating liquid matter for transport, crane selection and the containment bund under an oil-filled unit. If the fluid is a synthetic or natural ester rather than mineral oil, the plate says so, and the higher fire point behind the K class follows from it.
On dry-type transformers you will instead find the insulation system class (F or H), the climatic, environmental and fire classes to IEC 60076-11 — for example C2 E2 F1 — and an IP rating if an enclosure is fitted. The cooling code shortens to two letters, AN or AF.
Photograph the nameplate on the day the unit is commissioned and store the image with the asset record. Plates corrode, paint gets over-sprayed, and a legible photo has rescued many an urgent replacement. When a like-for-like unit is needed, send the whole plate rather than the kVA alone: the vector group, impedance and loss figures are what make a replacement actually drop into the existing installation.
