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Astro’s Primer on Determining Thermal Class

Thermal classification, Class F, Class H, insulation, generator, transformer, motor, ASTM, IEEE, IEC, NEMA

Thermal classification, Class F, Class H, insulation, generator, transformer, motor, ASTM, IEEE, IEC, NEMAThis article explores the various ways the electro-mechanical industry approaches thermal classification, with a specific focus on insulation materials. We examine relevant industry standards from organizations such as NEMA, IEEE, IEC, UL, and ASTM. Additionally, we compare the methods by which these standards define how materials fall into specific thermal classes (refer to Table I and Table II). Lastly, we touch on common workflows used to assess the thermal classification of an insulation system; and highlight the methods that Astro Chemical employs to characterize our products.


The thermal classification of insulating materials used in large electrical machines is critical due to the significant stresses these machines endure throughout their operational lifespan. This rating plays a pivotal role in assessing the performance capabilities and potential service life of a material under varying operating temperatures. Given that large capital equipment, such as generators, motors, and transformers, is particularly vulnerable to mechanical, thermal, and electrical stresses, a thorough understanding of thermal classification becomes essential.

In the electrical insulation industry, materials are typically categorized into two main groups: electrical insulation materials (EIM) and electrical insulation systems (EIS). EIM refers to the base constituent or material, while EIS encompasses the overall system. Various governing bodies, including IEEE, IEC, and ASTM, publish test standards, thermal classification rating systems, and test procedures to regulate different aspects and applications within the industry.

Despite the seemingly straightforward nature of this classification, challenges arise due to overlapping internal methods and specific test criteria adopted by certain original equipment manufacturers (OEMs) and material suppliers. While these internal criteria often align with published standards, discrepancies can occur. Additionally, different standards and governing bodies may emphasize varying aspects and additional test requirements, leading to confusion regarding which thermal classification system to apply and which test criteria to follow. The situation becomes even less clear when dealing with specialty applications where no single method directly applies.

Understanding the methodology and criteria used to establish a thermal classification rating is crucial. With a comprehensive understanding, end-users can make informed decisions when selecting a product or system that aligns with their functional requirements. While it is not feasible to cover every criterion exhaustively, recognizing the significance of understanding how a product or system’s advertised thermal classification is determined remains essential.


The standards, procedures, and methods developed by major governing bodies play a primary role in defining and standardizing criteria for applications throughout the electrical insulation industry. Accelerated aging techniques are commonly used to assess and compare the performance properties of EIMs and to extrapolate their expected service life at specific temperatures.

Key standards and their purposes:

  • NEMA RE-2 (National Electrical Manufacturers Association – Electrical Insulating Varnish): Defines criteria for organic binder systems used in NEMA-defined rotating machines. It is intended to be used as part of NEMA MG-1, which outlines the classification of machines based on various criteria such as application, electrical type, environmental protection, and cooling.
  • IEEE Std 117: Defines standard test procedures for thermal evaluation of insulating materials for random wound AC electrical machines.
  • IEEE Std 1776: Recommends practices for thermal evaluation of sealed or unsealed insulation systems for AC electrical machinery employing form-wound pre-insulated stator coils for machines rated 15,000 V and below.
  • IEC 60216: Provides accelerated aging test procedures to standardize the thermal capabilities of electrical insulating materials. It aims to replace historical listings of thermal capabilities based on service experience, considering the advancements in polymer technology.
  • IEC 60085: Distinguishes between thermal classifications of EIM and EIS, establishes criteria for evaluating their thermal endurance, and outlines procedures for assigning thermal classifications to both materials and systems. It is utilized in multiple IEC standards, including IEC 60216 and IEC 61857.
  • UL 1446: Used for evaluating Class E (120°C) or higher electrical insulation systems where thermal aging is the predominant factor. It does not cover EIS where overvoltage or partial discharge is present. UL 746 outlines requirements for polymeric materials used in electrical equipment.
  • ASTM E1877: Describes a method for determining thermal endurance, thermal index, and relative thermal endurance using Arrhenius activation energy generated by thermogravimetric analysis (TGA). This method allows extrapolation of thermal index to given weight loss endpoints at specific time intervals, providing a means to compare projected thermal endurance across different test criteria.

Understanding, property selecting, and adhering to these standards and test procedures enable manufacturers, suppliers, and end-users to ensure the reliability and safety of electrical insulation materials and systems used in large electrical machines.


Determining the endpoint for accelerated aging testing and assigning a thermal classification to an insulating material requires careful consideration of various factors, including property measurement, test method, and data analysis. Different standards and specifications may employ varying criteria for determining this endpoint, adding complexity to the process.

Common criteria for determining endpoints in accelerated aging testing include:

  • Weight Loss: Measuring the weight loss of the material over a period of time(s) at a specified temperature(s);
  • Loss of Mechanical or Electrical Insulating Strength: Assessing changes in mechanical or electrical properties of the material after thermal aging;
  • Loss of Adhesive Properties: Measuring the loss of adhesion strength after exposure to a specific temperature after a specified period of time.

These criteria are often provided in standards such as IEC 60216 and ASTM E1877, among others.

Internal specifications from OEMs and EIM suppliers may go beyond standardized thermal classifications, adding another layer of complexity. These internal specifications may be proprietary and unclear to end-users or owners of electrical machines.  This only further emphasizes the importance of clear communication between suppliers and users.

One effective method for assigning thermal endurance ratings is via a side-by-side accelerated aging test using an unknown material alongside a known, reference material with a proven service experience. This approach allows for the determination of a relative thermal index (RTI) for the unknown material based on the actual temperature index (ATI) of the reference material, provided that the aging mechanisms and rates of both materials are similar and relevant to the application.

The process of determining the endpoint for accelerated aging testing and assigning thermal classifications involves considering various factors and criteria specified by standards and internal OEM specifications. It is important to note that standard tests are not intended to predict the actual service life at a given service temperature; rather, they serve as a guide based on the known service life performance of a reference material. For example, ASTM E1877 states:

“This practice shall not be used for product lifetime predications unless a correlation between test results and actual lifetime has been demonstrated. In many cases, multiple mechanisms occur during the decomposition of a material, with one mechanism dominating over one temperature range, and a different mechanism dominating in a different temperature range. Users of this practice are cautioned to demonstrate for their system that any temperature extrapolations are technically sound.”

In other words, it is incumbent on the supplier to communicate the test standard, the criteria, and the test conditions for determining a stated thermal classification so the end user may determine if an EIM is suitable for their application.

Table I provides an overview of the different thermal classifications by governing body.

Table 1. Overview of Thermal Classification by Organization

117 1776 Thermal Letter Thermal Letter 1446
90 Y
105(A) 105 105 A 105 A
120 E 120(E)
130(B) 130 130 B 130 B 130(B)
155(F) 155 155 F 155 F 155(F)
180(H) 180 180 H 180 H 180(H)
200(N) 200 200 N 200(N)
220(R) 220 220 220 R 220(R)
240(S) 240 S 240(S)
250 >240(C)

Due in large part to the advent and expanded use of Class F (155°C) insulation chemistries in the 1990s, we now have a number of insulating materials available for use as a reference to determine RTI values for new materials. By utilizing a specific standardized accelerated aging test and applying the proper data analysis, known materials may be used as an effective benchmark in predicting the thermal endurance of new materials, provided the applications under consideration are the same.

Whether via internationally recognized standards, or internal OEM-specific methods, a variety of potential criteria exist for defining thermal classification of an EIM or EIS. Table II below summarizes some of methods used to achieve a Class F (155°C) rating.

Table 2. Examples of selected Class F (155°C) Definition

Defining Document Criteria Maximum Allowed Hours
ASTM D3377 Weight loss 5% 20,000
ASTM E1641 Weight loss 5% 20,000
ASTM E1877 Weight loss 10% 20,000
NEMA RE2 Numerous Numerous 20,000
IEC60216-2 Weight loss 10% 20,000
IEC60216-2 Flexural strength 50% 20,000
OEM X Weight loss 5% 60,000
OEM Y Weight loss 10% 100,000

In general, the steps to evaluate the thermal classification performance of an organic insulation system include:

  • Determination of required temperature classification: This step involves identifying the specific temperature classification needed for the organic insulation system based on its intended use. This could be Class F (155°C), Class H (180°C), or any other required temperature rating based on the application’s needs;

  • Selection of standard test methods: Once the required temperature classification is determined, appropriate standard test methods must be selected. These methods may include internationally recognized standards such as IEC 60216, ASTM E1877, or internal test methods specific to the organization, industry, or OEM;

  • Identification of failure mechanisms to be measured: Numerous failure mechanisms can occur under thermal stress, including weight loss, mechanical property degradation, adhesion loss, or dielectric breakdown. It is essential to identify which specific failure mechanism(s) are critical for the given application;

  • Determination of measurement endpoints: For each selected failure mechanism, specific endpoints need to be defined. For instance, these could include the time required to achieve a certain percentage of weight loss, the percentage of mechanical property loss, or the occurrence of dielectric breakthrough after thermal exposure at a specific temperature;

  • Data analysis and extrapolation: Analysis of the data obtained to allow for a reasonable extrapolation and the determination of a scientifically achievable thermal classification;
  • Justification and reporting: During the evaluation process, it is crucial for researchers or evaluators to provide clear justifications for their choices and decisions at each step. This practice ensures transparency and reliability in the reported results. The determined thermal classification should include a summary of the methods employed, the results obtained, and the rationale behind the selected classification.

While various methodologies and criteria may be employed, the ultimate goal is to achieve standardization and instill confidence in the final product or system. There is no universally “right” or “wrong” approach; as long as the chosen methods yield reliable and consistent results that meet the requirements of the application, the approach is valid.

Astro Chemical employs the standard test methods outlined in ASTM E1641 and E1877 to determine the thermal index of a cured epoxy system. Additionally, we adhere to the criteria specified in IEC 60216-2, which involves measuring the temperature at which a 10% weight loss occurs after 20,000 hours for thermal classification. However, we acknowledge that different end-users have varying criteria for their thermal classifications.  Consequently, we publish the methods used, along with the thermal index for 10% weight loss at both 20,000 and 60,000 hours.


We recognize the significance of offering comprehensive data and transparent methodologies for determining the thermal classification of our products. This classification plays a critical role in ensuring the safety and reliability of electrical equipment, particularly given the escalating thermal demands in large electrical machines. At Astro Chemical, we strive to provide extensive thermal property information to alleviate any potential confusion and empower end-users to make well-informed decisions. Our commitment is to provide clarity and support, enabling customers to select Astro products that best align with their intended applications

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