Transformer Cooling Classes Explained: ONAN, ONAF, OFAF, and More

What Are Cooling Classes?

Transformer cooling class designations describe how heat is removed from the transformer during operation. The cooling method directly affects the transformer's capacity rating and determines where it can be installed.

Reading Cooling Class Codes

Cooling class codes use four letters (or two pairs of two letters):

Position 1-2: Internal cooling medium and circulation

Position 3-4: External cooling medium and circulation

Letter Meanings

Letter	Position 1	Position 2	Positions 3-4
O	Oil (mineral)	-	-
K	Insulating liquid (other)	-	-
N	-	Natural circulation	Natural circulation
F	-	Forced circulation	Forced circulation
D	-	Directed flow	-
A	-	-	Air
W	-	-	Water

Common Cooling Classes

ONAN (Oil Natural Air Natural)

The most common cooling class for distribution transformers.

**How it works**: Oil circulates naturally by convection; air flows naturally over radiators
**Typical application**: Distribution transformers up to ~10 MVA
**Advantages**: Simple, reliable, no auxiliary power needed
**Limitations**: Limited capacity; requires adequate ambient airflow

ONAF (Oil Natural Air Forced)

Adds fans to increase capacity.

**How it works**: Oil circulates naturally; fans force air over radiators
**Typical application**: Distribution and small power transformers, 2.5-30 MVA
**Advantages**: Higher capacity from same tank; fans only run when needed
**Limitations**: Requires auxiliary power; fan maintenance

Dual Rating: ONAN/ONAF transformers have two ratings:

ONAN rating (fans off): ~70-80% of ONAF rating
ONAF rating (fans on): Full rating

OFAF (Oil Forced Air Forced)

Both oil and air are pumped for maximum cooling.

**How it works**: Pumps circulate oil; fans force air over radiators
**Typical application**: Large power transformers, 30+ MVA
**Advantages**: Maximum heat removal; highest power density
**Limitations**: Depends on pumps and fans; higher maintenance

ODAF (Oil Directed Air Forced)

Oil is directed through windings for targeted cooling.

**How it works**: Oil pumped through specific paths in windings; fans cool radiators
**Typical application**: Large power transformers, especially generator step-ups
**Advantages**: Most effective winding cooling; highest capacity
**Limitations**: Complex design; pump/fan dependent

OFWF (Oil Forced Water Forced)

Water cooling for special applications.

**How it works**: Oil pumped through oil-to-water heat exchangers
**Typical application**: Underground substations, industrial plants with cooling water
**Advantages**: Compact; no air heat rejection needed
**Limitations**: Requires cooling water supply; heat exchanger maintenance

Dry-Type Cooling Classes

AN (Air Natural)

Self-cooled by natural convection
Most common for dry-type transformers
Typical up to ~5 MVA

AF (Air Forced)

Fans assist cooling
Higher ratings than AN
Common for larger dry-type units

ANS/AFS

Some dry-type transformers have sealed enclosures with internal fans for dirty or hazardous environments.

Selecting the Right Cooling Class

For Distribution Transformers (< 10 MVA)

ONAN is the default choice unless:

Space is very limited → Consider ONAF for more capacity per footprint
Indoor installation → Consider dry-type AN or AF
High ambient temperature → Consider ONAF for derating margin

For Power Transformers (10-100 MVA)

ONAN/ONAF dual-rated is typical for:

Normal utility and industrial applications
Moderate load cycling
Adequate installation space

OFAF is preferred when:

Maximum capacity from given footprint
Continuous high loading
Hot ambient conditions

For Large Power Transformers (100+ MVA)

ODAF or OFAF are standard due to:

High heat density in large windings
Need for reliable forced cooling
Often redundant cooling systems specified

Temperature Rise and Cooling

Transformer ratings assume specific temperature rise limits:

Cooling Class	Typical Winding Rise	Typical Top Oil Rise
ONAN	65°C	55°C
ONAF	65°C	55°C
OFAF	65°C	55°C

55°C rise transformers (older standard) can carry more load in cooler weather but have lower nominal ratings.

65°C rise is current standard and provides good balance of capacity and life.

Altitude and Ambient Derating

Standard ratings assume:

1,000 meters altitude
30°C average ambient, 40°C maximum

Higher altitude or temperature requires derating or specifying enhanced cooling.

Questions?

FluxCo's engineering team can help you select the right cooling class for your application.