Class F Insulation with Class B Temperature Rise: What the Thermal Margin Is Really Buying You

Class F insulation with Class B temperature rise means a motor is built with a 155 degree Celsius insulation system but designed to rise only 80 K at rated load in a 40 degree ambient. On the hot-spot basis used by ABB and WEG, that puts the winding at 130 degrees against a 155 degree limit, a 25 K thermal margin. That margin buys you one of three things: roughly five to six times the insulation life by the 10 degree rule, tolerance for a hotter, higher or dirtier-supplied site, or short-term overload capacity. It buys each of them only once, and a tender that says “Class F insulation” without a rise class may buy none of them.

TL;DR

• Insulation class and temperature rise class are two different numbers. The first says what the insulation can survive; the second says how hot the design actually runs. A specification needs both.

• At 40 degrees ambient, a Class F machine designed to Class B rise sits 25 K below its hot-spot limit. A Class F machine designed to Class F rise sits at zero margin on day one.

• The 10 degree rule says every 10 K cooler roughly doubles insulation life, so 25 K is worth about 5.7 times the thermal life of the same insulation run at its limit.

• The same 25 K can instead absorb high ambient, altitude, voltage unbalance, harmonics or short overloads. Spend it on site conditions and nothing is left for life.

• Catalogue ratings quote rise at a stated ambient. A range rated for 70 K at 50 degrees ambient keeps its full reserve on a hot site; a frame in the same range rated 80 K at 50 degrees has only 15 K left.

• Class H insulation with Class F rise carries the same margin as F/B in a more compact machine. Neither protects a winding from inverter voltage spikes, which need inverter-duty insulation.

What does “Class F insulation, Class B rise” actually specify?

“Class F insulation with Class B temperature rise” specifies two separate limits: the thermal class of the insulation system, and the temperature rise the machine is designed not to exceed at rated output. The thermal class comes from IEC 60085:2007, which establishes how thermal classes are assigned to electrical insulating materials and systems. The rise limits sit in IEC 60034-1, whose fifteenth edition was published on 13 March 2026 and replaced the 2022 edition.

The thermal class is a ceiling. The rise class is a design target. A motor nameplate that reads only “Ins. Class F” tells you the ceiling and nothing about how close to it the machine runs.

WEG’s Specification Guide for Electric Motors builds the hot-spot temperature from three parts: a 40 degree ambient, the permitted rise by resistance method, and a fixed allowance for the difference between average winding temperature and the hottest spot. ABB’s Low Voltage Motors Motor Guide (July 2019) gives the same allowances.

An F/B machine is wound with materials rated for the F row and loaded to the temperatures of the B row. The gap between the rows is the thermal margin.

Where does the 25 K come from, and why do some sources quote 35 K?

The 25 K figure is the Class F hot-spot limit minus the Class B hot-spot temperature: 155 minus (40 plus 80 plus 10) equals 25. ABB states it directly: “the use of class F insulation with class B temperature rise gives ABB products a 25 degree safety margin.” WEG’s composition table produces the same result.

Leroy-Somer’s technical note Insulation Class / Temperature Rise Class calculates the margin without the hot-spot allowance, as 155 minus (80 plus 40), and arrives at 35 K. Both figures describe the same machine. The difference is whether you measure against the average winding temperature or the hottest spot.

For a purchase specification, use 25 K. The insulation fails at its hottest spot, not at its average, and WEG notes that “a single weak spot in the insulation is enough to damage the winding completely.” Planning on 35 K overstates what you can spend by 10 K, which by the 10 degree rule is half the life you think you are protecting.

What is 25 K of margin worth in insulation life?

Twenty-five kelvin of thermal margin is worth roughly five to six times the thermal life of the same insulation operated at its class limit. The arithmetic comes from the rule of 10: Leroy-Somer’s note states that reducing operating temperature by 10 degrees doubles insulation system lifetime, an approximation derived from the Arrhenius equation. Two and a half doublings is a factor of about 5.7.

Leroy-Somer’s own table, for its generators, puts an F/F design at 4.6 years of insulation life and an F/B design at 26 years. The same note cites IEC 60085 for a life expectancy of 20,000 hours for an insulation system held permanently at its class temperature. Those are thermal-ageing figures only. Moisture, contamination, vibration and voltage stress are not in them.

The rule works in the other direction too, and that is the direction plants actually experience. NIEL’s field rule for rerolling mill duty motors is blunt: every 10 K of sustained operation above the insulation class limit roughly halves winding insulation life. The rewind simply arrives two years early and gets blamed on quality.

What else can the thermal margin be spent on?

ABB lists four uses for the F/B margin: increasing motor loading for limited periods, operating at higher ambient temperatures or altitudes, tolerating greater voltage and frequency variation, and extending insulation life. Each use consumes the same 25 K.

The margin is not a licence to skip derating. ABB builds its standard motors as F/B and its motor guide still tells users to derate above 40 degrees ambient: permitted output falls to 96.5% at 45 degrees, 93% at 50 degrees, 90% at 55 degrees and 86.5% at 60 degrees. The same table takes output to 96% at 1,500 m and 92% at 2,000 m above sea level. A manufacturer that publishes those factors for F/B machines is protecting the reserve, not assuming you will quietly consume it.

What happens when the margin is spent twice?

A motor that spends its thermal margin on two things at once runs above its insulation limit under what everyone on site believes is normal duty. This is the most expensive error in motor specification because every individual calculation looks correct.

NIEL’s worked example for an African copper mine shows how it happens. A slurry pump absorbs 78 kW at the duty point on a 550 V, VFD-fed system. The naive selection is 90 kW. The site is at 1,350 m, the sheet-roofed pump house runs at 50 degrees, measured unbalance at the MCC is 2%, and the motor is a standard machine on a PWM drive. Indicative factors of 0.98, 0.92, 0.95 and 0.95 multiply to 0.81, so the 90 kW motor delivers about 73 kW on that site.

The motor is running at 107% of what it can deliver, continuously. Losses rise by roughly 8%, the winding runs about 8 K over its limit, and by the 10 degree rule that is close to half the winding life. Nobody sees a trip or an alarm.

The fix is two lines in the order, not one. Step up to 110 kW, which gives about 89 kW usable against a 78 kW load. Then specify Class F insulation with Class B rise, so the residual margin goes to life instead of being consumed by the site. The derating method behind those factors is set out in our guide to derating electrical equipment for African installations.

How do you read a catalogue to find out how much margin you are really getting?

You find the real margin by reading the rise figure, the ambient it is quoted at, and whether it changes by frame. Two ranges can both be “Class F insulated” and deliver margins 25 K apart.

The Kirloskar Electric Primo series (frames 63 to 160) and Spectrum series (frames 180 to 315) brochure in NIEL’s motor library shows why. Its features list says “Class F insulation with class B temperature rise limit.” Its performance charts are headed “50 ambient, Class F insulation, IP 55/IC 411, temperature rise = 70K.” At 50 degrees ambient plus 70 K, the average winding sits at 120 degrees, exactly where an F/B machine sits at 40 degrees. That range has been designed so the full 25 K survives a hot Indian plant floor, on a supply the brochure defines as 415 V plus or minus 10%, 50 Hz plus or minus 5%, to IEC 34-1 and IS 325 tolerances.

The same Spectrum chart carries a footnote against its largest 315L rating: “Temperature Rise = 80K.” At 50 degrees ambient, that frame averages 130 degrees and keeps 15 K of reserve rather than 25. Same brochure, same insulation class, 10 K less margin, which by the 10 degree rule is half the thermal life of its neighbours.

Across NIEL’s live product pages the margin is stated in several forms:

NIEL’s high voltage motor brochure puts the last case plainly: Class F insulation using Class H material “results in an automatic and generous reserve over the ratings declared” and “ensures that the motor can take higher momentary loads.” That is the overload use of the margin, stated by design.

Is Class H insulation with Class F rise better than F/B?

Class H insulation with Class F rise carries the same thermal margin as Class F insulation with Class B rise, so neither is inherently longer-lived. Leroy-Somer shows the arithmetic: 180 minus (105 plus 40) and 155 minus (80 plus 40) both give 35 K on its average-temperature basis, and its note concludes that “generators designed as F/B or H/F have the same thermal margin, and hence the same insulation system lifetime.”

What H/F buys is size. Leroy-Somer states that H/F designs allow more compact and economic machines with the same design life. That is why the H/F pattern appears on NIEL’s mill duty DC motors, where a steel mill wants maximum torque from a constrained frame.

If you want more life rather than a smaller machine, the combination to ask for is higher insulation with a lower rise. Leroy-Somer’s generator table puts H/B at 146 years of thermal life, at least five times F/B for the same active part size and mass. For a critical, hard-to-access drive, ask whether H insulation with B rise is available on the line offered, and price it.

What does F/B not protect you from?

The F/B margin protects against heat. It does nothing for three failure routes that often arrive together on VFD-fed plant.

1. Voltage stress from drives. At 550 V and 690 V, PWM reflected-wave peaks at the motor terminals can approach twice the DC bus voltage on long cable runs. That is a dielectric problem, not a thermal one, and a higher thermal class does not solve it. The 2022 edition of IEC 60034-1 lists IEC 60034-18-41, covering insulation systems for machines fed from voltage converters, among its normative references. Specify inverter-duty insulation and check cable length against the dV/dt limits, as covered in our guide to VFD panels and what drives do to your supply.

2. Cooling collapse at low speed. A totally enclosed fan cooled motor carries its fan on its own shaft, so airflow falls with speed. On a constant-torque load such as a conveyor or crusher, current stays where it was while cooling drops away. A 25 K reserve disappears fast under those conditions; forced ventilation is the answer, not a better insulation class.

3. Continuous overload dressed up as margin. WEG defines service factor as the allowable continuous load under specified conditions, and notes that a service factor of 1.0 means the motor was not designed for continuous operation above rated output. The F/B reserve is not a service factor. ABB allows the margin to cover higher loading “for limited periods,” which is a different thing from a permanent 10% overload.

How should you specify and verify F/B in a tender?

Each line of this checklist closes a gap the cheapest compliant bid would otherwise exploit.

1. State both classes on separate lines. Write “Insulation: thermal class 155 (F). Temperature rise: not to exceed 80 K (Class B) by resistance method at rated output.” “Class F insulation” on its own permits a zero-margin design.

2. Tie the rise to your real ambient and altitude. Quote the air temperature entering the fan cowl in the worst month, with ventilation assumed failed, and the site altitude. A rise guaranteed at 40 degrees says nothing about a 50 degree pump house.

3. Decide what the margin is for. Write it down: life, site conditions, or momentary overload. If it is already spent on site conditions, do not also count it against voltage unbalance or harmonics.

4. Read the rise per frame, not per range. Check the performance chart footnotes for exceptions, as the Spectrum 315L 80 K footnote shows. The largest rating in a range is the likeliest to carry less margin.

5. Ask for the temperature rise test result for the frame offered. IEC 60034-1 defines rise by the resistance method. A bidder who claims F/B should be able to show the measured rise, not just the class letter.

6. Specify winding temperature detectors on critical drives. Thermistors or RTDs in the winding heads show how much of the margin the site is using.

7. Add inverter-duty insulation for VFD service. At 550 V and above, name it explicitly and check cable length against dV/dt limits.

8. Protect the margin at rewind. Require the rewinder to restore the original insulation class and copy the winding data. IEC 60034-23:2019 covers repair, overhaul and reclamation; the burnout-temperature questions are in our piece on why an ageing motor costs more than a new one.

9. Re-size the downstream circuit if the frame steps up. Cable, contactor, overload relay, feeder and busbar follow the installed motor, not the driven load.

   Frequently asked questions

   Which is better, Class B or Class F insulation?

   Class F insulation withstands a higher temperature, 155 degrees against 130 degrees for Class B, so on its own it is the more capable system. But a Class F machine designed to Class F rise runs at its limit, with no more margin than a Class B machine run to Class B rise. The useful comparison is F/F against F/B, and F/B carries 25 K more reserve. ABB describes that combination as the most commonly required insulation system for industrial motors.

   Is Class F insulation better than Class H?

   Class H insulation has a higher thermal class, 180 degrees, and a larger permitted rise, 125 K. Whether it gives a longer life depends again on the rise. H/F carries the same margin as F/B and is chosen mainly because it allows a more compact machine for the same design life. H/B carries substantially more margin than either. For heavy, constrained-frame duties such as NIEL’s mill duty DC motors, H/F is the standard pattern; for a critical drive where you want maximum life, ask for H/B.

   What is the temperature rise limit for Class F insulation?

   Manufacturers applying IEC 60034-1, including ABB and WEG, give the permitted rise for a Class F system as 105 K by the resistance method, with a 10 K allowance for the hottest spot, at a 40 degree ambient. That gives a 145 degree average winding temperature and a 155 degree hot spot. A machine described as “Class F with Class B rise” is held to 80 K instead, which is where the 25 K margin comes from. Always check the ambient the rise is quoted at, because some ranges quote it at 45 or 50 degrees.

   Can an F/B motor run at 50 degrees ambient without derating?

   Thermally, a machine designed for 80 K rise at 40 degrees will still sit inside its Class F limit at 50 degrees, but only by spending 10 of its 25 K. ABB still publishes a factor of 93% of rated output at 50 degrees for its F/B standard motors. You can run it without derating, but you have then used the margin on ambient, and none of it remains for voltage unbalance, harmonics or life. If the site also has a weak supply or a VFD, derate or buy a range rated for 50 degrees.

   Does F/B mean the motor can be overloaded continuously?

   No. The F/B margin allows higher loading for limited periods, which is how ABB describes it, not a permanent overload. WEG is explicit that a service factor of 1.0 means the motor was not designed for continuous operation above rated output. A permanent overload raises losses and winding temperature, uses up the 25 K reserve, and then starts ageing the insulation at the rate the 10 degree rule predicts. If a duty genuinely needs more continuous power, buy the next frame and re-size the cable, contactor and overload relay with it.

   Does rewinding keep the Class F, Class B margin?

   Only if the rewinder restores it. A rewind with lower-class materials, fewer or thinner conductors, or an uncontrolled burnout that damages the core can raise losses and temperature and quietly remove the margin you paid for. Name the insulation class, require copy rewinding of the winding data, and reference IEC 60034-23:2019 in the repair order. A motor that comes back from rewind with the same nameplate but a hotter winding has lost the margin without anyone recording it.

New India Electricals Ltd specifies and supplies AC motors with the insulation class, rise class and site ambient written into the order, not left to the bidder. Talk to our team with your site conditions and duty, and we will tell you how much margin the machine on offer really carries.

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