Transformer Temperature Rise: Decoding 55/65 on Nameplates

If you’ve ever seen a transformer nameplate, you may have noticed a value for “temperature rise” with a rating of “55/65.”

But what does this value actually mean, and why is it important for transformers? In this article, we’ll explain the meaning of transformer temperature rise and explore the significance of 55/65 on the transformer nameplate.

We’ll also discuss the factors affecting transformer temperature rise, the effects of excessive temperature rise, and how to maintain safe temperature rise levels for your transformers.

Understanding Temperature Rise 55/65 on Transformer Nameplate

First, let’s define what we mean by transformer temperature rise. Temperature rise refers to the maximum temperature that the transformer’s insulation can reach above the ambient temperature during normal operation.

The temperature rise rating on a transformer nameplate indicates the maximum allowable temperature rise for the transformer.

The values “55/65” on a transformer nameplate indicate that the transformer has a temperature rise of 55°C above the ambient temperature when it operates at full load, and a temperature rise of 65°C above ambient when it operates at 50% of full load.

To understand this rating, it’s important to know how transformer load affects temperature rise.

When a transformer is under load, it generates heat due to the resistance of the windings and the core.

The amount of heat generated is proportional to the amount of current flowing through the transformer.

The higher the current, the more heat is generated, and the higher the temperature rise.

The ambient temperature also affects the temperature rise of a transformer. If the ambient temperature is high, the transformer will naturally be hotter, and the temperature rise will be higher.

Conversely, if the ambient temperature is low, the transformer will be cooler, and the temperature rise will be lower.

Factors Affecting Transformer Temperature Rise

Several factors can affect the temperature rise of a transformer, including the insulation class, transformer design, type of cooling, and load capacity.

Insulation Class:

The insulation class of a transformer determines the maximum allowable temperature of the transformer’s insulation system.

The insulation class is defined by the temperature at which the insulation system can operate for an extended period of time without degrading.

The most common insulation classes for transformers are Class A (105°C), Class B (130°C), Class F (155°C), and Class H (180°C).

Read my comprehensive article on transformer insulation failure: The Shocking Truth About Transformer Insulation Failure: Causes and Prevention

Transformer Design:

The design of a transformer can also affect its temperature rise. Factors such as the type of core material, the number of windings, and the configuration of the windings can impact the amount of heat generated by the transformer.

Type of Cooling:

Transformers can be cooled by air, oil, or a combination of both. The type of cooling used can affect the temperature rise of the transformer.

For example, oil-cooled transformers can handle higher loads and have lower temperature rises than air-cooled transformers.

Load Capacity:

The load capacity of a transformer refers to the amount of power that the transformer can handle. Transformers that are overloaded can generate more heat and have higher temperature rises.

Effects of Excessive Temperature Rise on Transformers

Excessive temperature rise can have several negative effects on transformers, including degradation of insulation, reduced transformer life, risk of transformer failure, and safety concerns.

Read also my comprehensive article: Hot Topics: Common Signs of Transformer Temperature Rise.

Degradation of Insulation:

When a transformer’s insulation is exposed to high temperatures for an extended period of time, it can degrade and lose its dielectric strength. This can result in insulation failure and ultimately lead to transformer failure.

Reduced Transformer Life:

Excessive temperature rise can also shorten the lifespan of a transformer. When a transformer operates at high temperatures for prolonged periods, it can cause premature aging and breakdown of the insulation system, reducing the overall lifespan of the transformer.

Risk of Transformer Failure:

As mentioned earlier, excessive temperature rise can lead to insulation failure, which can cause a transformer to fail.

Transformer failure can be costly and result in extended downtime and lost productivity.

Safety Concerns:

Overheating transformers can pose safety risks to personnel working in the vicinity of the transformer.

High temperatures can cause the insulation to break down, leading to potential electrical arcing and fires.

Maintaining Safe Temperature Rise Levels

To maintain safe temperature rise levels for your transformers, it’s essential to ensure that the transformer is designed and installed correctly and that it’s operated within its rated load capacity. Here are some tips for maintaining safe temperature rise levels:

Proper Installation:

Transformers should be installed in a well-ventilated area to allow for adequate cooling. The installation should also comply with all applicable codes and standards.

Proper Load Management:

Transformers should be loaded in accordance with their rated capacity. Overloading a transformer can cause excessive temperature rise, leading to potential problems.

Regular Maintenance:

Transformers should be regularly inspected and maintained to ensure they are operating correctly.

This includes checking for signs of overheating, such as discolored insulation or hot spots, and addressing any issues promptly.


Transformer temperature rise is an important parameter to consider when designing, installing, and operating transformers.

The rating of 55/65 on the transformer nameplate indicates the maximum allowable temperature rise of the transformer at full load and 50% load, respectively.

Several factors can affect the temperature rise of a transformer, including insulation class, transformer design, cooling method, and load capacity.

Excessive temperature rise can cause degradation of insulation, reduce transformer life, increase the risk of transformer failure, and pose safety risks.

To maintain safe temperature rise levels, it’s important to ensure that transformers are installed correctly, loaded correctly, and regularly maintained.

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