Transformer Temperature Rise (AA/FA/FOA Cooling Classes Explained)
Introduction #
Transformer temperature rise is the allowed average winding (and often oil) temperature increase above a defined ambient under rated load—printed on the nameplate (e.g. 55 °C / 65 °C rise). This guide is for engineers selecting dry-type or liquid-filled units who must interpret rise ratings and cooling class designations such as AA, FA, FOA / OA, FA, FOA.
Best for: reading nameplates, comparing AA vs FA vs FOA capacity steps, and linking rise to insulation class and site ambient.
Not ideal for: laboratory temperature-rise type tests, thermal network FEM, or replacing manufacturer heat-run reports.
Pair with Transformer Derating Factors (ambient / altitude) and the Transformer Size Calculator.
Temperature rise vs ambient vs hotspot #
| Term | Meaning |
|---|---|
| Ambient | Surrounding air (or cooling medium) temperature used as the rating basis (often 40 °C for many standards) |
| Average winding rise | Average winding temperature minus ambient at rated load (e.g. 80 °C / 115 °C dry-type classes, 55 °C / 65 °C liquid) |
| Hotspot | Local hottest winding region—higher than average; drives insulation aging |
| Total hottest-spot temperature | Roughly ambient + average rise + hotspot allowance (OEM / IEEE tables) |
Example: A liquid-filled unit rated 65 °C rise at 40 °C ambient implies average winding ≈ 105 °C at rated load before hotspot adder. If the room runs hotter, usable kVA falls—apply ambient derating.
What do 55 °C and 65 °C rise mean? #
Historically, 55 °C rise liquid-filled designs used older insulation systems; 65 °C rise is common for modern units and often allows more compact designs for the same kVA. Dual-rated 55/65 °C nameplates mean the transformer can be applied at either thermal basis with corresponding capacity—follow the OEM nameplate, do not invent a third rating.
Dry-type units often list rises such as 80 °C / 115 °C or 150 °C insulation systems—again, treat the nameplate as authoritative.
Cooling classes: OA / AA / FA / FOA (and AA/FA) #
Cooling class letters describe how heat is removed. Exact letter strings vary by standard era (ANSI/IEEE vs IEC), but the planning idea is the same: forced cooling raises allowable continuous load versus self-cooled.
| Class (common US shorthand) | Cooling idea | Planning note |
|---|---|---|
| OA / AA | Oil natural / air natural (self-cooled) | Base continuous rating |
| FA / AF | Forced air (fans) | Higher continuous kVA when fans run |
| FOA / OFAF etc. | Forced oil + forced air | Further capacity for large power transformers |
| AA/FA (dual) | Self-cooled and fan-cooled ratings on one nameplate | Two kVA numbers—do not assume fans always available in fire / maintenance modes |
Worked example: A pad-mount lists ONAN 2000 kVA / ONAF 2500 kVA (IEC-style) or OA/FA dual US ratings. For continuous plant load of 2200 kVA, you need the fan-cooled rating in service—and a plan if fans fail (alarm, load shed, or larger OA base).
Mike Holt / Schneider-style FAQ angle: “AA” or “AA/FA” on dry-type nameplates denotes self-cooled vs self-cooled / forced-air capacity steps—confirm with the manufacturer’s cooling class table for that product line.
How temperature rise affects sizing #
- Start from load kVA (Transformer Size Calculator / Sizing Guide).
- Confirm the rise rating matches site insulation expectations and OEM tables.
- If ambient > basis or altitude is high, apply derating factors—rise class does not replace ambient derating.
- If you rely on FA/FOA capacity, document fan / pump availability in operations.
Assumption / disclaimer: Tables here are educational. Final thermal capability is OEM nameplate + IEEE/IEC product standards.
Common mistakes #
- Confusing temperature rise with ambient derating
- Buying on the FA rating but installing without working fans
- Ignoring hotspot / aging when loading near nameplate in hot rooms
- Mixing IEC letter codes with ANSI shorthand without OEM mapping
FAQ #
What is the temperature rise of a transformer at 55 / 65? #
Those numbers are average winding rise ratings (°C above rated ambient), not the absolute winding temperature. A 65 °C rise unit at 40 °C ambient has roughly 105 °C average winding at full load before hotspot adder.
What is the acceptable temperature for a transformer? #
Follow the nameplate rise + ambient basis + hotspot limits for that insulation system. “Acceptable” absolute °C varies by liquid vs dry-type and insulation class—use OEM and IEEE/IEC tables.
What does AA or AA/FA mean on a transformer? #
Typically self-cooled (AA) versus self-cooled / forced-air (AA/FA) dual ratings. The higher kVA applies only when forced cooling is operating per the manufacturer.
What is the temperature rise test for transformers? #
A factory heat-run / temperature-rise test verifies the unit meets its rated rise at specified load and cooling. Field operators do not recreate this; they monitor oil/winding temperatures against alarm setpoints.
How is this different from derating? #
Rise is a nameplate thermal rating. Derating reduces usable kVA when site ambient, altitude, or harmonics exceed the rating basis—see Transformer Derating Factors.
Next step #
- Size continuous kVA with the Transformer Size Calculator.
- Read the nameplate rise and cooling class; if you need FA/FOA capacity, confirm fans/pumps in the O&M plan.
- Apply site derating and harmonic notes in Sizing for Harmonic Loads.
- For energization peaks (separate from thermal rise), read Transformer Inrush Current.
Hub: Power Calculators · Authority: Transformer Sizing Guide.