The alternator is the part of a diesel generator that fails first when it runs hot. The two figures on the
nameplate that predict this — insulation class and temperature rise — are often
confused. This guide explains what they mean and what to check before you buy.
What buyers should know first
Insulation class is the maximum temperature the winding insulation can survive. Temperature rise is the
actual increase above ambient the design allows in service. A robust alternator is built with a higher
insulation class than its operating rise — giving headroom.
Insulation classes
| Class | Max winding temp | Typical use |
|---|---|---|
| B | 130°C | Older / lighter duty |
| F | 155°C | Common modern alternators |
| H | 180°C | Heavy-duty, tropical, continuous |
A very common and sound combination is class H insulation with a class F temperature rise (written
“F/H”). The windings can take 180°C, but the design only allows a rise to the F limit, so the machine runs cooler
than its insulation rating — extending life.
Temperature-rise limits
Rise is measured by the resistance method over the winding. For class F, the permitted rise is around
105 K above a 40°C ambient. So a well-built F/H alternator settles near 145°C winding
temperature in a 40°C room — well under its 180°C insulation ceiling.
Standards such as IEC 60034 and the rating code in ISO 8528 define how rise is
tested and stated. Ask for the test certificate, not just the printed nameplate.
Altitude derating
Cooling air gets thinner with altitude. Above 1000 m, alternator (and engine) output typically
drops about 3–4% per 500 m unless the machine is specially compensated. For a high-altitude
site, size up the kVA or specify a compensated alternator.
Common configuration mistakes
- Reading “class H” and assuming the machine runs at 180°C — it is the insulation limit, not the service temperature.
- Ignoring altitude: a set sized at sea level loses output on a mountain site.
- No winding temperature detectors (RTDs/PT100) for remote alarm on large sets.
Procurement checklist
| Item | Confirm |
|---|---|
| Insulation / rise | e.g. “Class H insulation, Class F rise (F/H)” |
| Ambient rating | 40°C or higher for tropical sites |
| Altitude | Derated output stated for site elevation |
| Detectors | RTD/PT100 on large alternators |
| Certificate | IEC 60034 rise test available |
Related equipment to compare
Larger alternators pair with MTU-powered sets and the
1000–1500 kW range. Browse the generator category or
read about our build standards.
Frequently asked questions
What does “F/H” mean?
Class H insulation (180°C limit) with a class F temperature rise (about 105 K). It is a conservative, long-life build.
Why derate above 1000 m?
Thinner air cools less and the engine draws less mass flow, so both alternator and engine lose output.
Is class H better than class F?
For the same rise, class H simply gives more thermal margin. The rise limit is what protects the winding in service.
Do I need RTDs?
On small sets, no. On large or continuously-run sets, winding temperature detectors give early warning of overload or cooling loss.
What standard covers the rise test?
IEC 60034 for the machine, with rating referenced in ISO 8528 for generating sets.