EngineerHub.orgDrawing No. EH–EE–022 // Electrical Engineering
Transformer Cooling Calculator
Reviewed August 2026
Estimate transformer cooling airflow, oil flow, water flow, heat-transfer area, fan and pump power, cooling margin and simplified temperature rise for AN, AF, ONAN, ONAF, OFAF and OFWF systems.
Start with the physical question
How many kilowatts of loss must be removed, which cooling class is used, and how much temperature increase is acceptable in the cooling medium?
Simple cooling inputs
Simple cooling results
| Installed-system screen | Result |
|---|---|
| Air-flow margin | — |
| Oil-flow margin | — |
| Water-flow margin | — |
| Cooling-area margin | — |
| Entered coolant transport temperature rises | — |
Advanced thermal-network inputs
Advanced thermal results
| Advanced check | Result |
|---|---|
| Air-flow margin | — |
| Oil-flow margin | — |
| Water-flow margin | — |
| Fan power / oil-pump power / water-pump power | — |
| Heat-transfer UA | — |
| Required temperature difference at installed UA | — |
Cooling class explained
The modern IEC-style liquid cooling code uses four letters. Dry-type cooling is commonly described with two letters.
| Code | Plain-language meaning | What moves the heat? |
|---|---|---|
| AN / AA | Dry transformer, natural air | Natural convection through winding ducts and enclosure openings |
| AF / FA | Dry transformer, forced air | Fans increase airflow through the windings or enclosure |
| ONAN / OA | Oil natural, air natural | Buoyancy circulates oil; tank or radiators reject heat by natural air convection |
| ONAF / OA-FA | Oil natural, air forced | Oil circulation remains natural; fans move air over radiators |
| OFAF / FOA | Oil forced, air forced | Oil pumps and radiator fans are both active |
| OFWF / FOW | Oil forced, water forced | Oil pump sends hot oil through an oil-to-water heat exchanger |
What changes between air and liquid cooling?
Air cooling
Air is simple and readily available, but its density and heat capacity are low. Removing several kilowatts with a small air-temperature rise can require a large airflow and carefully designed ventilation paths.
Oil or ester cooling
A liquid carries much more heat per unit volume and also provides insulation. Natural circulation can be effective, while pumps are used when higher loading or controlled flow is required.
Water-cooled exchanger
Water has high heat capacity and can make the external cooler compact, but the system adds pumps, water quality control, leak barriers, maintenance and consequences of loss of cooling.
Worked example
A transformer dissipating 8 kW with a permitted 15 K air rise requires approximately:
For oil with density 860 kg/m³, heat capacity 1.9 kJ/kg·K and a 10 K rise, the equivalent heat-transport flow is about:
Natural circulation does not use a pump, but the fluid still must circulate internally at a rate sufficient to transport the heat.
Background
Engineering background, equations and assumptions used by this calculator.
For air, oil or water, a first heat-balance estimate is based on mass flow, heat capacity and coolant temperature increase.
For a dry enclosure or naturally cooled surface, a simple screening estimate uses P = hAΔT. The real coefficient varies with orientation, duct geometry, chimney effect, enclosure openings and radiation.
Required area depends on both the heat-transfer coefficient and the temperature difference driving heat through the exchanger. Advanced mode asks for an effective mean driving temperature difference rather than inventing hot- and cold-side terminal temperatures. A detailed exchanger design would instead use actual terminal temperatures and an LMTD or ε-NTU method.
The estimated hot spot is the external reference temperature plus the temperature difference required across the installed cooling conductance, plus a winding-to-fluid rise based on load loss and an entered thermal resistance. In Simple mode, where the no-load/load-loss split is unknown, 80% of total loss is used as an illustrative winding-related share.
A one-node thermal model uses τ = Cth/UA. It describes a smooth exponential warm-up and is useful for intuition, not detailed loading-guide compliance.
- IEC 60076-2 identifies liquid-immersed transformers by cooling method and defines temperature-rise test requirements.
- IEC 60076-7 addresses loading and thermal ageing of mineral-oil-immersed transformers.
- IEC 60076-11 applies to dry-type transformers.
- IEC 60076-12 provides loading guidance for dry-type transformers.
Frequently Asked Questions
Practical questions about assumptions, inputs and limitations.
Enter total transformer loss at the operating load, including no-load loss and load-dependent winding and stray loss. Do not enter transformer output power.
ONAN means oil natural, air natural. ONAF means oil natural, air forced. The letters identify internal coolant, circulation method, external coolant and external circulation method.
No. It is a preliminary heat-balance and thermal-resistance screening model. Final compliance requires manufacturer design calculations and applicable temperature-rise testing.
Fans or pumps increase heat-transfer capacity, allowing more loss to be removed for a similar temperature rise. The permissible rating increase must still come from the manufacturer.
No. Oil still circulates by buoyancy. The calculator reports an equivalent heat-transport flow, but actual natural circulation depends on internal hydraulic resistance and density differences.
The screening equation is Q̇ = P_loss/(ρcₚΔT). A larger allowed coolant temperature rise reduces the required volumetric flow. Natural-circulation classes still have internal fluid motion even when no pump is installed.