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). The letter codes and what each position means are defined by IEC 60076-2, the standard governing temperature rise and cooling designations for oil-immersed transformers.

Position 1-2: Internal cooling medium and circulation Position 3-4: External cooling medium and circulation

Letter Meanings

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): 75-80% of ONAF rating

• ONAF rating (fans on): Full rating

A 10/12.5 MVA ONAN/ONAF transformer carries 10 MVA on natural convection alone and 12.5 MVA with the fans running, a 25 percent increase over the ONAN base rating. A 12/16/20 MVA ONAN/ONAF/ONAF transformer is a three-stage unit rather than a dual rating, with a three-rating nameplate and two stages of fans: 12 MVA ONAN base, 16 MVA on the first fan stage, and 20 MVA on the second. That 12/16/20 nameplate follows the 100/133/167 rating progression defined in IEEE C57.12.00.

As a general rule, a single stage of fans commonly adds 25 to 33 percent capacity over the ONAN base rating. The 25 percent figure is common practice for single-stage units rather than a value fixed by any standard.

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

What does ONAN mean?

ONAN (Oil Natural Air Natural) means the transformer is cooled entirely by convection: heat moves into the oil, the oil rises and circulates through the radiators on its own, and ambient air carries the heat away with no pumps or fans. ONAN units are the default for distribution transformers and for outdoor pad-mounted and pole-mounted installations, where no auxiliary power is available for fans. They are also common on lightly loaded substation transformers that never need the extra capacity forced cooling would provide.

What is the difference between ONAF and ODAF?

In an ONAF transformer, the oil circulates by natural convection and fans force air over the radiators. In an ODAF transformer, pumps circulate the oil and that oil is directed through the windings by ducts, so the flow is aimed at the hottest parts of the winding rather than left to find its own path. OFAF sits between the two: pumps circulate the oil through the radiators, but inside the windings the oil still moves by convection rather than being directed.

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

ANV and GA

Non-ventilated (ANV) and sealed gas-filled (GA) dry-type designs enclose the core and coils for dirty, wet, 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, or ONAN/ONAF/ONAF triple-rated above about 12 MVA, 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:

Temperature rise limits for oil-immersed transformers are set by IEEE C57.12.00: 65°C average winding rise, 65°C top oil rise, and 80°C hottest-spot rise over ambient. IEC 60076-2 uses slightly different limits, including a 60 K top oil rise.

• 65°C rise: the current IEEE standard, balancing capacity and insulation life.

• 55/65°C dual rise: an older convention still specified by some utilities. The unit is rated at 55°C rise and can carry about 12 percent more load at 65°C rise.

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.