Transformer Insulation (Thermal) Classes and Temperature Limits

Thermal

Transformer Insulation Thermal Classes (IEC Reference)

The thermal class defines the maximum permissible temperature for the insulation system. It should not be confused with temperature rise, which is specified separately by the relevant transformer standard and test method.

Thermal Class
(°C)		 Letter Class Operating Limits
(summary values commonly used in transformer practice)
Max Insulation System Temperature
(°C)		 Dry-type: Max Average Winding Temperature Rise
(K, at rated current)
90 Y 90 —
105 A 105 60
120 E 120 75
130 B 130 80
155 F 155 100
180 H 180 125
200 N 200 135
220 R 220 150
250 — 250 —
Note: The “temperature rise” values above apply to dry-type transformers and refer to the average winding temperature rise (typically determined by resistance method) during the temperature-rise test. Hot-spot temperature can be higher than the average and depends on design and cooling method.
Short-Circuit Thermal Withstand Maximum Permissible Average Winding Temperature After Short Circuit
(values depend on transformer type and conductor material)
Transformer Type Copper (°C) Aluminium (°C)
Reference values Oil-immersed (105 / Class A) 250 200
Reference values Dry-type (105 / Class A) 180 180
Reference values Dry-type (120 / Class E) 250 200
Reference values Dry-type (130 / Class B) 350 200
Reference values Dry-type (155 / Class F) 350 200
Reference values Dry-type (180 / Class H) 350 200
Reference values Dry-type (200 / Class N) 350 200
Reference values Dry-type (220 / Class R) 350 200
Note: Short-circuit temperatures are the maximum permissible average winding temperatures after a symmetrical short circuit. For oil-immersed transformers using insulation systems other than thermal class A, different values may be agreed between manufacturer and purchaser.

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Transformer Manufacturer

DATSAN Transformer manufactures oil-immersed distribution and power transformers for industrial, utility, and infrastructure projects worldwide.

Established in 1992, DATSAN combines over 30 years of engineering experience with modern manufacturing and testing capabilities. Our products are designed in accordance with IEC 60076, ANSI C57 and relevant international standards, with Ecodesign-compliant options available upon request.

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Datsan Elk. İnş. San. ve Tic. Ltd. Şti.

Address: Kızıltepe Mardin Karayolu 6.km
Kızıltepe, 47400 Mardin / TÜRKİYE

Phone : +90 (482) 215 3188
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