Introduction
Every transformer carries a metal plate that serves as its identity card—the nameplate. For procurement professionals, engineers, and asset managers, understanding this information is essential. The nameplate contains vital statistics that define how the transformer can be operated, connected, and maintained throughout its service life .
This guide decodes the key parameters found on transformer nameplates, explaining what each means and why it matters for specification and procurement.
Part One: Transformer Identification and Basic Information
At the top of every nameplate, you will find the manufacturer’s name and a unique serial number. This serial number is crucial for tracing manufacturing records, test reports, and design documentation—essentially the transformer’s birth certificate.
The manufacturing date (month and year) is also typically provided, enabling age assessment and maintenance planning.
The nameplate will clearly state whether the unit is a “transformer” or “autotransformer,” the number of phases (single or three-phase), and the operating frequency (typically 50 Hz or 60 Hz).
Part Two: Power and Voltage Ratings
Rated Power (kVA or MVA). This indicates the apparent power the transformer can deliver continuously at rated voltage and frequency without exceeding temperature limits. For transformers with multiple cooling stages, different ratings may be shown for each mode.
Voltage Ratings. Voltage information appears in a specific format. For example, “230000GR.Y / 132800-69000GR.Y / 39840-12470″ indicates :
l High-voltage winding: 230,000 V line-to-line, grounded Y, 132,800 V phase-to-neutral
l Low-voltage winding: 69,000 V line-to-line, grounded Y, 39,840 V phase-to-neutral
l Tertiary winding: 12,470 V delta-connected
When a slash appears, it separates line-to-line voltage from phase-to-neutral voltage. GR.Y or Grd.Y indicates the neutral is intended to be grounded -1.
Tapping Information. For transformers with tap changers, a voltage ratio table shows the voltage at each tap position, allowing operators to select the appropriate setting for system conditions.
Part Three: Impedance and Loss Data
Percent Impedance (%Z). This critical parameter, expressed as a percentage, indicates the voltage drop at full load. It affects short-circuit currents and voltage regulation. Typical values range from 4% to 15% depending on transformer size. The percent impedance is specified between each pair of windings, with the kVA base stated.
No-Load Loss (Iron Loss). These losses occur continuously whenever the transformer is energized, regardless of load. They result from magnetizing the core and are closely related to core material quality.
Load Loss (Copper Loss). These losses vary with the square of the load current and occur in the windings due to resistance. Lower load losses mean higher efficiency under loaded conditions.
Part Four: Cooling, Temperature, and Connection Information
Cooling Class. Transformers use standardized codes to indicate cooling methods. For oil-immersed units, designations like ONAN (Oil Natural Air Natural) or ONAF (Oil Natural Air Forced) describe the internal and external cooling media and circulation methods. For dry-type transformers, codes like AN (Air Natural) or AF (Air Forced) are used.
Temperature Rise. This specifies the maximum temperature increase above ambient that the transformer can withstand continuously, typically 55°C or 65°C for oil-immersed units. Insulation class (A, B, F, H) indicates the maximum permissible operating temperature.
Vector Group and Connection Diagram. For polyphase transformers, a vector diagram shows the phase relationship between primary and secondary windings. Common designations like Dyn11 indicate delta primary, wye secondary with neutral, and a 30-degree phase shift. A schematic diagram shows all internal connections and terminal markings.
Part Five: Physical and Safety Information
Weights and Oil Volume. Nameplates typically list the mass of core and coils, tank and fittings, insulating oil, total weight, and heaviest piece for lifting purposes. Oil volume and type of insulating liquid are also specified.
Insulation Levels. Basic Lightning Impulse Insulation Level (BIL) indicates the transformer’s ability to withstand surge voltages, specified for each winding.
Sound Level. Some nameplates include sound level ratings in decibels, measured at a standard distance. This is particularly important for installations in noise-sensitive areas like residential neighborhoods or hospitals.
Conclusion
The transformer nameplate is far more than a simple label—it is a comprehensive technical document that defines the asset’s capabilities and limitations. For procurement professionals, understanding how to read and interpret this information ensures correct specification, prevents operational errors, and enables informed decisions about transformer selection and application.
Post time: Mar-20-2026