Every motor nameplate, transformer datasheet, and cable spec you read leans on one classification system. Class 155 (F). Class 180 (H). Most engineers use these labels daily without knowing what the numbers really promise — or that the letters are optional.
This guide covers the three ways insulating materials are classified: by thermal class, by physical state and chemistry, and by tracking resistance. The thermal class system is defined in IEC 60085, Electrical insulation – Thermal evaluation and designation, and that’s where we’ll spend most of our time, because it’s the classification that decides how long your equipment lives.
Table of Contents
The short answer
Insulating materials are classified three ways:
- By thermal class — a number equal to the recommended maximum continuous use temperature in °C: 90, 105, 120, 130, 155, 180, 200, 220, 250. Letters (Y, A, E, B, F, H, N, R) are optional shorthand.
- By physical state and chemistry — gaseous, liquid, or solid; organic, inorganic, or hybrid.
- By tracking resistance — material groups I, II, IIIa, IIIb, ranked by Comparative Tracking Index (CTI).
Now the details, because the details are where specs get misread.
Axis 1: Thermal classification — the one on the nameplate
The thermal class table
A thermal class is a designation equal to the numerical value of the recommended maximum continuous use temperature, in degrees Celsius. The class is assigned from the material’s or system’s thermal endurance index — its ATE or RTE (more on those below).
| ATE or RTE (°C) | Thermal class | Letter designation |
|---|---|---|
| ≥90 and <105 | 90 | Y |
| ≥105 and <120 | 105 | A |
| ≥120 and <130 | 120 | E |
| ≥130 and <155 | 130 | B |
| ≥155 and <180 | 155 | F |
| ≥180 and <200 | 180 | H |
| ≥200 and <220 | 200 | N |
| ≥220 and <250 | 220 | R |
| ≥250 and <275 | 250 | — |
Above 250, classes continue in increments of 25 — Class 275, Class 300, and so on. No letters are assigned up there.
Three things the table doesn’t tell you
The number is the class. The letter is decoration. The correct designation is “Class 155” or “Class 155 (F)”. The letter alone is permitted where space is tight — a nameplate, for example — but the number carries the meaning. If you’ve been writing “Class F insulation rated 155°C,” you had it backwards: it’s 155°C insulation, optionally tagged F.
Note the asymmetric bands. A material with a thermal endurance index of 154°C is Class 130, not Class 155. The index has to reach the class threshold — you round down, always. The band for Class 130 runs all the way from 130 to just under 155, which is why so many general-purpose materials land there.
A class is a recommendation, not a cliff. Running an insulation system above its class doesn’t cause instant failure. It shortens expected life. The thermal endurance relationship is roughly exponential with temperature, so the penalty compounds fast — but the class temperature itself is a design recommendation for normal continuous operation, set by the relevant product committee.
Material class vs system class — the trap in every datasheet
IEC 60085 draws a hard line between two things people constantly mix up:
- EIM — electrical insulating material. The enamel on a wire. A polyester film. An impregnating resin.
- EIS — electrical insulation system. The complete insulating structure in the device: materials plus the conductors they touch, working together.
A motor’s Class 155 rating belongs to the system, not to each material inside it. And the relationship runs in both directions:
- Other materials in the system can protect a weaker component — a varnish sealing a wire enamel from oxygen, for instance — so the system class can be higher than the endurance of an individual material inside it.
- Incompatibility between materials (chemical interaction, differing expansion) can drag the system class below the endurance of its best components.
The practical rule from the standard: never deduce a material’s thermal capability from the class of a system it belongs to, and never assume a material rated at some index is automatically fit for a system of that class. The two are evaluated separately — materials per the IEC 60216 series, systems per IEC 60505 and the IEC 61857 series.
Where the numbers come from: TI, ATE, and RTE
A thermal class isn’t measured with a thermometer. It comes out of accelerated ageing programs, and three indices matter:
Temperature index (TI). The temperature in °C at which a material takes 20,000 hours to reach a defined end-point — a set loss of mechanical strength, breakdown voltage, or another chosen property. 20,000 hours is roughly 2.3 years of continuous exposure, so nobody tests at the rated temperature directly. Specimens are aged at three or four higher temperatures, times-to-failure are plotted on a log-time versus reciprocal-absolute-temperature graph (the Arrhenius plot), and the line is extrapolated down. The rules are strict: the lowest ageing temperature must give more than 5,000 hours to end-point, and the extrapolation can’t stretch more than 25 K below it. That keeps the guesswork honest.
Assessed thermal endurance (ATE). The temperature up to which a reference material has known, satisfactory service experience in a given application. This is field history, not oven data.
Relative thermal endurance (RTE). The workhorse for new materials. A candidate material is aged side by side with a reference material of known ATE. The RTE is the temperature at which the candidate’s estimated life matches the reference’s life at its ATE. Comparative testing cancels out a lot of the systematic error in absolute TI determinations, which is why class assignment uses ATE or RTE rather than raw TI.
One more index worth knowing: the halving interval (HIC) — the temperature step, in kelvin, that cuts the time to end-point in half at the TI. For many organic materials it sits somewhere near 8–12 K. It’s the standardized version of the old “10 degrees halves the life” rule of thumb, except now it’s measured per material instead of assumed. A result is reported as, for example, TI (HIC): 152 (9,0).
A material can legitimately carry more than one TI. Different properties degrade at different rates — flexural strength may collapse before dielectric strength does — so the index is only meaningful together with the property and end-point it was derived from. When two datasheets disagree on a material’s index, check what property each one tracked before assuming one is wrong.
Typical materials by thermal class
The standard assigns classes; it doesn’t list materials. But established practice puts common materials roughly here:
| Class | Typical materials |
|---|---|
| 90 (Y) | Untreated cellulose paper, cotton, natural silk, some thermoplastics |
| 105 (A) | Oil-impregnated paper and cotton, PVC, polyamide films |
| 120 (E) | Polyester enamels, phenolic laminates, cellulose triacetate |
| 130 (B) | Mica and glass fiber with organic binders, polyester resins |
| 155 (F) | Mica and glass fiber with epoxy/alkyd binders, polyester-imide enamels |
| 180 (H) | Silicone resins and elastomers, mica-glass with silicone binder, aramid paper |
| 200 (N) / 220 (R) | Polyimide films and enamels, PTFE, aramid systems |
| 250+ | Pure mica, ceramics, glass, polyimide systems for special duty |
Treat this table as orientation, not gospel. The same base polymer can land in different classes depending on formulation, and the class that matters for equipment is the tested class of the complete system.
Axis 2: Physical state and chemistry
The second classification axis is simpler and cuts across the first.
By physical state:
- Gaseous — air, nitrogen, SF₆, hydrogen (in large generators). Gases self-heal after a discharge, which is their great advantage, but their dielectric strength depends on pressure and gap geometry.
- Liquid — mineral oil, synthetic and natural esters, silicone fluids. Liquids insulate and cool at the same time, and like gases they largely recover after a disturbance.
- Solid — everything from kraft paper to ceramics. Highest dielectric strength per millimeter, but damage is permanent: once a solid tracks or punctures, it never heals. That’s why nearly all insulation testing concentrates on solids and on oil condition.
By chemistry:
- Organic — carbon-based: paper, cotton, most polymers, natural resins. Generally lower thermal classes; heat breaks their molecular chains.
- Inorganic — mica, glass, ceramics, asbestos historically. Thermally robust, often brittle, found at the top of the class table.
- Hybrid/composite — the practical middle: mica paper on a glass backing with an epoxy binder is the classic high-voltage machine insulation. The organic binder is usually the thermally weakest link, and it’s what sets the composite’s class.
Notice the pattern: climb the thermal class table and materials shift from purely organic (Class 90–120), to organic-bonded inorganic composites (130–180), to nearly pure inorganic (250+). The chemistry axis and the thermal axis are two views of the same physics.
Axis 3: Tracking resistance — the CTI material groups
The third axis matters for creepage distances in equipment design. Insulating surfaces exposed to pollution and moisture can develop conductive carbon tracks under electrical stress. The Comparative Tracking Index (CTI) measures a material’s resistance to this, in volts, and materials are sorted into four groups used throughout insulation coordination practice:
| Material group | CTI (V) |
|---|---|
| I | ≥ 600 |
| II | ≥ 400 and < 600 |
| IIIa | ≥ 175 and < 400 |
| IIIb | ≥ 100 and < 175 |
A material can sit high on the thermal table and low on the tracking table, or the reverse. Thermal class tells you nothing about tracking performance — they’re independent classifications answering different design questions. Thermal class sets how hot the insulation can run; the material group sets how much creepage distance you need across its surface.
What this classification means for testing
Classification and testing meet in the field. A few connections worth keeping in mind:
- The class assumes thermal ageing is the dominant factor. IEC 60085 says so explicitly. Moisture, contamination, vibration, electrical stress, and chemicals all attack insulation too, and none of them respect the thermal class. A Class 180 winding soaked in moisture fails an insulation resistance test just as quickly as a Class 105 one.
- Class sets your temperature correction context. Insulation resistance readings are strongly temperature-dependent, and knowing the insulation system class tells you what materials you’re dealing with when you normalize readings.
- Operating above class shows up in your trends first. A machine run hot doesn’t fail on day one — it ages fast. Falling polarization index and rising tan delta over successive tests are how accelerated thermal ageing announces itself long before failure.
Frequently asked questions
Is Class F the same as Class 155?
Yes — 155 is the class, F is its optional letter designation. Write it as Class 155 (F).
Why is there no letter for Class 250?
The letter series stops at R (Class 220). Class 250 and everything above it (275, 300, in steps of 25) are designated by number only.
Can a Class 130 material be used in a Class 155 machine?
Sometimes, yes. If the rest of the system protects it — sealing it from oxygen or moisture, for example — the complete system can test to Class 155 even with that material inside. But this must be proven by system-level thermal evaluation, never assumed.
What does the 20,000-hour figure mean for real service life?
It’s the standardized reference time for the temperature index, not a promised lifetime. Real equipment at its class temperature typically lasts far longer, because it rarely runs at maximum temperature continuously, and the class includes design margin.
Where do Class 200, N, and R come from? I never see them.
They exist for high-temperature systems — traction motors, hermetic compressors, aerospace. Most industrial equipment stops at Class 180 (H), which is why the upper classes are unfamiliar.
The bottom line
The classification of insulating materials runs on three independent axes. Thermal class — the number on the nameplate — is a recommended maximum continuous use temperature derived from accelerated ageing and assigned per IEC 60085. Physical state and chemistry explain why materials land where they do on that table. And CTI material groups govern surface tracking, a completely separate failure mode.
The single most useful thing to remember: the class belongs to the number, the number belongs to the system, and the system is more than the sum of its materials.