What the cooling does
Every transformer loses a little energy as heat in its core and windings. That heat has to leave, or the insulation on the windings breaks down early. The cooling method is simply how the heat gets from the windings to the outside air.
Oil-type
The core and windings sit in a tank filled with insulating mineral oil. The oil does two jobs: it insulates, and it circulates, carrying heat to the tank walls and radiator fins, where the air takes it away. Oil-type units are the familiar grey tanks with fins seen on poles and in yards.
- Suited to outdoor installation and to larger ratings.
- Tolerant of overloads for short periods, because the oil stores heat.
- Needs periodic checks: oil level, leaks, and the silica-gel breather that keeps moisture out.
Dry-type
There is no oil. The windings are insulated with solid materials and cooled by air moving over them, naturally or with fans. With no oil to leak or burn, dry-type units are often chosen for indoor rooms.
- Suited to indoor spaces: shops, offices, clinics, basements.
- No oil to check, top up or dispose of.
- Needs clean, dry air and clear ventilation; dust and damp are its enemies.
How people usually decide
If the unit will stand outside or in a yard, oil-type is the usual answer. If it will sit inside a building near people or stock, dry-type is often preferred. Rating, budget and local rules then narrow it further — ask your supplier to compare like for like at the rating you need.
General guidance only — every installation is different. For work on live equipment, use a licensed electrician. Last reviewed 28 September 2026.