Reading a transformer nameplate

Author: Michael Miller


Figure 1: Example nameplate

The nameplate is one of the most important items bolted to your transformer. It contains all of the information you will need to properly transport, install and maintain the unit.


In this example nameplate, the information is grouped in various sections on the plate.

Figure 2: Top section

The upper section lists the basic information including manufacturer, type, class, voltage ratings, vector group, operation frequency, serial number, and year of manufacture.

Figure 3: Middle right section

The middle right section contains the primary, secondary, and tertiary voltages along with its power rating at those voltages. The power ratings are given in MVA (Mega Volt-Amperes). When a tap changer is present the voltage of each tap is given in a table or tables. Also listed in these tables are the internal connections made by the tap changer. The operator or tester can look at the line diagram to get a visual reference to what is happening mechanically inside the transformer in terms of physical connections being made by the tap changer.

Figure 4: Middle left section

In the middle left section of a nameplate, you will see the line diagram mentioned above. In the callout, you will see the physical connections made up by the tap changer. The selectors on terminal five and the reversing switch (K) is in the “-“ position. Looking back at the table in Figure 2, we can determine that the tap changer is in 9L, which gives a secondary voltage of 221,702 volts and a tertiary voltage of 35,642 volts. Shown on the line diagram are the locations of the current transformers. Also indicated below are some of the polarity marks or indications that are shown for the CTs as well as the windings.

Figure 5: Bottom middle section

At the bottom of the middle section, the various ratios and classes are given for the current transformers installed as well as the phasor diagram. The phasor diagram is a visual representation of the IEC vector group given in the upper section, e.g. Yna0d1.

Figure 6: Bottom left section

In the lower left section, the weights and volumes are given as shown. Here you will find the various weights and/or oil volumes for the various components installed.

Figure 7: Bottom left section

In this same section, you will find the terminal arrangement, shown in Figure 7. IEEE dictates that terminal arrangements are to be given with reference to a fixed component of the transformer, e.g. tap changer, control cabinet, etc.

Figure 8: Lower right section

In the lower right section (Figure 8), you will find the power ratings, basic insulation level or BIL rating and Impedances. If you are paralleling or banking the transformer, the nameplate will list the transformer impedance, so you can ensure they are properly matched. The percent impedance is given between each pair of windings.


As shown in Figure 8, a transformer may be specified with more than one MVA rating (also referred to as cooling stages). The transformer nameplate lists the rated MVA and cooling class designation for each rating. The ratings are listed in order of increasing MVA as shown under the Temperature Rise Rating table in Figure 8.

The cooling class designations are normally listed in order with a diagonal slash separating each one. In Figure 2 we see the cooling classes for this transformer are listed as ONAN/ONAF/OFAF. Let’s take a quick look at what those four letters mean.

1st Letter
Internal cooling medium in contact with the windings
Liquid with a flash point ≤ 300 °C
Liquid with a flash point > 300 °C
Liquid with no measureable flash point
2nd Letter
Circulation mechanism for internal cooling medium

Liquid with a flash point ≤ 300 °C

Liquid with a flash point > 300 °C
Liquid with no measureable flash point
3rd Letter
External cooling medium
4th Letter
Circulation mechanism for external cooling medium
Natural convection
Forced circulation [fans (air cooling), pumps (water cooling)]

Cooling class designations have changed from what they were years ago.

Present designations
Previous designations