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How to read a transformer nameplate

A transformer nameplate provides essential information about the unit’s capabilities and operational parameters to professionals, technicians, and users.

June 23, 2023

A padmount transformer nameplate

Transformers play a crucial role in the generation, transmission, and distribution of electrical power. While transformers all perform the same task of converting a given input voltage to a different output voltage they can vary greatly from each other in terms of size, shape, and specification. These specifications are outlined on the transformer’s nameplate.

What is a transformer nameplate?

A transformer nameplate is a metal or plastic placard that provides the essential information to electrical professionals, technicians, and users who interact with the transformer. It helps ensure proper installation, maintenance, and operation of the transformer by conveying key details about the unit’s capabilities and operational parameters.

Nameplates can be difficult to read if you’re not familiar with them, so in this article, we’ll guide you through the most important information found on a transformer’s nameplate.

Where Is a Transformer’s Nameplate?

First, where do you find a transformer’s nameplate? Depending on the transformer, the nameplate will be found in different locations. Generally, the nameplate is located in a visible and accessible location for easy reference. For padmount transformers the nameplate will usually be inside the cabinet door, while substations and dry types will have the nameplate attached to the outside of the transformer or enclosure.

What Information Does a Transformer Nameplate Contain?

Rating Information

The most important information on a nameplate is the transformer’s rating information, the essential data about the transformer's capacity and operating parameters. This includes kVA Rating, Voltage Rating, and Frequency Rating.

Rating information on transformer nameplate

kVA Rating

The kVA rating denotes the size of the transformer and indicates the load that the transformer is designed to serve. This rating is the most important factor in determining the proper transformer size for the equipment it will serve.

Voltage Rating

The primary voltage and secondary voltages are typically labeled together near the top of the nameplate. For medium voltage applications, the primary will always be designated by “HV” and the secondary by “LV”. It is important to ensure that the HV and LV values match the voltage being fed into the transformer and the voltage rating of the equipment the transformer is feeding.

Voltage Taps

Some transformers will come with adjustment taps above and below the nominal rated primary voltage. These taps make small allowances for a higher or lower incoming voltage from the utility, in order to ensure the output voltage remains the same as the rated nameplate value.

Voltage taps on transformer nameplate

Frequency Rating

This rating indicates the frequency at which the transformer operates, usually specified in Hertz (Hz). Standard frequencies include 50 Hz or 60 Hz. Virtually all transformers in the US will be rated at 60 Hz.

Phases

Transformers are built as either single-phase or three-phase. This information is usually found near the top of the nameplate: “1-Ph” for single-phase and “3-Ph” for three-phase. Whether you need a single-phase or a three-phase transformer will depend on your application.

Phasor Diagram

The phasor diagram depicts the relationship between the primary and secondary windings in the transformer, showing whether each winding is connected as delta or wye. It also shows the angular displacement of voltages between the windings–whether there is any sort of phase shift between the windings.

Nameplate phasor diagram with Delta and Wye configurations

Wiring Diagram

Transformer Windings

The winding diagram details the internal connections of the coils for both windings. This sketch shows the markings of the leads for the high and low voltage terminals as shown below (H1, H2, and H3 for the high voltage; X1, X2, X3, and X0 for the low voltage).

Transformer windings on nameplate

Fuses

Fuses and other protection devices help protect transformers from overcurrents and short circuits and prevent damage to the electrical system from failed transformers. The transformer’s fusing will be labeled in the wiring diagram: the bayonet fuses may be labeled “BAY” and the isolation links are labeled “ISO”. Special current limiting fuses may be labeled “CLF”.

Bayonet fuse and isolation link on transformer nameplate

Loop Feed and Radial Feed

The wiring diagram also indicates whether the transformer is a loop feed or radial feed transformer. A loop feed transformer has a bushing configuration specifically designed for a loop distribution system, although it can also be used in other systems. A loop feed transformer will be labeled like this with six different primary bushings (H1A, H1B, H2A, H2B, H3A, and H3B):

Loop feed designation on transformer nameplate

Radial Feed

The bushing layout of a radial feed transformer is typically suited toward radial systems, and looks like this with only three different primary bushings (H1, H2, H3):

Radial feed designation on transformer nameplate

Load break switches

Load break switches, or LBOR switches, will also be labeled in the wiring diagram. A load break switch is designed to switch the power “on or off” or change the position when the transformer is energized (meaning it has a load on it)—the switch will break this load. The two most common types of load break switches are 2-position and 4-position switches.

2-position load break switch on transformer nameplate

Electrostatic Shield or E-Shield

Some transformers are fitted with an electrostatic shield, or “E-Shield”. This will be indicated by a line labeled “ES” between the transformer’s windings. An E-shield is a copper winding that is placed between the primary and secondary windings of a transformer. It is used to reduce electromagnetic interference (EMI) between the two windings.

E-shield on transformer nameplate

Other Transformer Specifications:

Bushing Location

On a substation transformer, the physical layout of the bushings will be labeled on the nameplate by their ANSI Side designations. ANSI Side 1 is the “front” of the transformer—the side of the unit that hosts the drain valve and nameplate. Moving clockwise, Side 2 is the left side, Side 3 is the back side, and Side 4 is the right side.

ANSI Side designations

Impedance

Impedance is the resistance to the flow of electrical current. Impedance influences voltage regulation and fault current capacity: lower impedance allows more fault current to flow, higher impedance allows less. Every transformer has a particular impedance value which is stamped on the nameplate. The value stamped on the nameplate is calculated for the full load (the total rated kVA) of the transformer. This value is often important when replacing existing transformers, as the system will typically be designed around the original transformer’s impedance.

Temperature Rise

The temperature rise value on a nameplate indicates the average temperature of the windings (above ambient temperature) when the transformer is at full load (max kVA). This rating is expressed in degrees C. The most common temperature rise ratings in oil filled transformers are 65°C, 55°C, and 75°C.

Cooling Class

Transformers employ different cooling methods. The nameplate specifies the cooling type, which is crucial for determining proper maintenance, installation conditions, and operational considerations.

(Learn more about transformer coolings classes)

The most common cooling class ratings in oil filled transformers are ONAN and KNAN. The first letter designates the cooling medium (in this case liquid), with”O” denoting a fluid with a flashpoint below 300 degrees C and “K” designating a less flammable fluid with a flashpoint above 300 degrees C such as FR3 and silicone oil. Less flammable fluid is often required for installations very close to or inside buildings. The second letter covers the cooling mechanism. In this case, the “N” means the transformer is cooled with natural convection.

These ratings can also be extended with ONAN/ONAF, or KNAN/KNAF which means additional kVA can be achieved with the assistance of forced air cooling. Read more about cooling classes here.

Winding Conductor

The nameplate will typically indicate the conductor material for the primary and secondary windings: “Cu” for copper, and “Al” for aluminum.

Weight:

The weight of the transformer is usually given in pounds (lb) or kilograms (kg). The nameplate typically lists the untanking weight (transformer core and coil), fluid weight, and the weight of the tank and fittings. The weight of your transformer may influence installation considerations. For example, if you need to install a liquid-filled unit on a roof, be sure you know the weight of the transformer when it’s filled with oil.

Transformer Fluid

The transformer oil type and oil capacity are also included on the transformer nameplate for liquid-filled units. The oil capacity is usually given in gallons (gal) or liters (L).

Manufacturer's Information

Towards the top of a transformer’s nameplate, you’ll find details about the manufacturer and the unit.

Manufacturer's information on transformer nameplate

Manufacturer's Name

The manufacturer's name, logo, and contact info are usually located at the top of the nameplate. It’s important to identify reputable manufacturers—their expertise and quality standards can greatly impact the performance and reliability of the transformer.

Serial number

The serial number is a unique identifier for the transformer. It can be used to track the transformer's history and to identify it in case of a problem.

Model number

The model number is a generic identifier that denotes a certain type of design (typically pre-engineered). Some transformers may have model numbers, but many are custom-built to customer specs. Where applicable, it can provide a quick and easy way to order certain transformer designs. The model number can be used to order replacement parts or to find technical information about the transformer.

Conclusion

Understanding the information presented on a nameplate empowers you to make informed decisions about transformer selection, installation, and maintenance. If you have any questions about what transformer would be best for your application, drop us a line! We’d love to answer any questions you may have.

Maddox padmount transformer loaded on truck

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