Transformer Temperature Rise vs Insulation Class

Temperature rise describes how much warmer a winding becomes than its reference cooling air under stated conditions. Insulation class describes the thermal endurance classification of the electrical insulation system. A Class F designation does not mean a 155 K temperature rise, a 155°C room rating or a prescribed controller setting.

For an engineering or procurement review, read the insulation class and guaranteed winding temperature rise as separate fields. Then check the ambient, loading, cooling and measurement basis that makes the rise meaningful.

Parameter	What it describes	How to read the number
Cooling-air ambient temperature	The air temperature used as the thermal reference	An absolute temperature, in °C
Average winding temperature rise	Average winding temperature above that reference	A temperature difference, normally in K
Average winding temperature	Reference ambient plus the corresponding average rise	An absolute temperature, in °C
Winding hot-spot temperature	The highest local temperature within the winding	Not determined by the average alone
Insulation thermal class	The endurance designation of the insulation system	A class designation, not the measured operating temperature

The Sandia/DOE transformer guide, section 3.4.1.2, distinguishes average winding rise and ambient conditions. Its historical rating examples are not current limits for a new project.

Read insulation class as a system designation

IEC 60085:2007 distinguishes thermal classification of an electrical insulation system from classification of an individual insulating material. A resin or wire’s material rating alone therefore does not establish the thermal class of the complete winding insulation system.

For familiar letter designations, the UL electrical-insulation-system brochure, page 4, gives these correspondences:

Letter designation	Numerical thermal class
B	130
F	155
H	180

These are thermal-class designations, associated with temperatures in °C. The table deliberately does not assign a universal winding rise to each class. The applicable transformer standard and offered design must provide that separate value.

UL also describes an electrical insulation system as a compatible combination of materials. Ask for the offered system’s declared class and applicable evidence, rather than accepting one material datasheet as the complete answer. Citing UL’s terminology does not establish UL certification of a CANGO transformer.

Convert average rise into average winding temperature

For corresponding reference conditions:

Average winding temperature (°C) = cooling-air ambient (°C) + average winding rise (K)

A difference of 1 K has the same magnitude as a difference of 1°C, as the BIPM SI Brochure explains. Do not add 273.15 when adding a rise to a Celsius ambient.

Hypothetical temperature difference showing 40 degrees Celsius cooling air plus an 80 kelvin average winding rise giving 120 degrees Celsius average winding temperature — The rise spans from the reference ambient to the average winding temperature. This hypothetical example does not determine a hot spot or an operating limit.

Consider two hypothetical thermal schedules, both specifying numerical class 155 (F), the same rated duty and cooling basis, and a 40°C reference ambient. Their independently specified average rises are different:

Hypothetical schedule	Thermal class	Reference ambient	Average winding rise	Calculated average winding temperature
A	155 (F)	40°C	80 K	40 + 80 = 120°C
B	155 (F)	40°C	100 K	40 + 100 = 140°C

These numbers are chosen to explain the arithmetic. They are not CANGO ratings, test results or a declaration that either schedule satisfies a particular standard. The common class does not tell the buyer which rise was offered.

At the stated comparison point, A has a 20 K lower average winding temperature. That alone does not establish its losses, price, lifetime or permissible overload. Compare those separately; the distribution-transformer losses article explains why loss data and operating profiles need their own basis.

Two hypothetical dry-type transformer schedules share class 155 F and 40 degrees Celsius ambient but have 80 and 100 kelvin average rises, giving average winding temperatures of 120 and 140 degrees Celsius — The same class can appear in different thermal schedules. Thermometers are schematic; the comparison is hypothetical, with no hot-spot value or overload allowance.

If the actual cooling air is hotter than specified, obtain the manufacturer’s assessment for the new conditions. Reusing the same rise without confirming the thermal behaviour is not a site-rating calculation.

An average does not identify the winding hot spot

Temperature is not uniform throughout a winding. The original IEEE paper Hottest spot temperatures in ventilated dry type transformers reports measured thermal gradients in tested ventilated windings. Its 1994 abstract supports the distinction between average and local maximum; it does not validate a hot-spot allowance for a different cast-resin design.

A 120°C average therefore leaves the maximum local temperature unresolved. Subtracting that average from class 155 does not produce a bank of usable overload or a guaranteed protection margin. A controller’s sensor reading also needs its documented location and relationship to the winding temperature; do not assume a displayed value is the global hot spot.

Alarm, fan-control and trip settings need the offered transformer’s instructions, sensor arrangement and protection philosophy. Do not derive them by copying the insulation-class number. Likewise, a higher class alone does not establish longer service life. The scope of IEC 60076-12:2008 links insulation ageing to operating temperature, time and loading; its public abstract supplies no universal life-doubling rule.

Keep the cooling and installation basis with the rating

The Sandia guide’s dry-type discussion describes natural circulation and fan-assisted cooling. When both ratings are offered, require the kVA and guaranteed rise for each mode; do not assume one rise applies to both or invent a fan-assisted capacity increase.

For dry-type power transformers within its stated equipment and voltage scope, IEC 60076-11:2018 addresses dry-type transformer requirements. Its official abstract identifies enclosure performance and altitude-related thermal considerations among the edition’s subjects. It does not make every dry-type transformer suitable for every enclosure or site.

Keep the actual enclosure, cooling-air conditions and ventilation arrangement in the agreed thermal schedule. A bare-unit figure should not silently become an enclosed installation guarantee. Route the wider capacity decision to transformer kVA sizing; compare construction choices in the separate oil-immersed versus dry-type article.

Ask for one complete thermal schedule

Use the following as an offer-review aid, not a substitute for a test standard or an instruction to test energized equipment.

Schedule field	Clarification to request
Offered unit and duty	Exact model, voltage, frequency, tap and kVA associated with the thermal declaration
Insulation system	Numerical thermal class and the evidence applicable to that system
Average winding rise	Guaranteed value for each winding, stated in K, with test method and acceptance basis
Ambient	Maximum and relevant averaging conditions; where the cooling-air reference is defined
Cooling mode	Separate duty and rise for each declared mode, including fan availability assumptions
Installation boundary	Enclosure, altitude and ventilation conditions covered by the declaration
Hot spot and sensing	Applicable assessment or evidence, sensor locations and what the displayed readings represent
Controls and records	Manufacturer-approved control/protection schedule and relevant thermal test documentation

For example, an offer stating only “Class F, 1,000 kVA” leaves the rise, cooling mode and installation basis unresolved. Adding those fields makes the offer assessable; it does not by itself establish compliance or acceptance.

Reading the CANGO catalogue entry

CANGO’s SC(B)12 dry-type transformer page includes separate B-class, F-class and H-class load-loss columns in W. Those headings are not temperature-rise guarantees or controller setpoints. The visible table does not state the complete thermal schedule discussed here; request the offered configuration’s insulation class, guaranteed rise and applicable conditions explicitly.

For a focused review, send the required kVA and voltage combination, load/cooling modes, ambient and altitude, enclosure arrangement, requested insulation class/rise and the required test basis. Use the power-transformer project-information guide for the wider inquiry, then ask CANGO to review the dry-type thermal schedule.