There is no single preferred type, but for most indoor installations in occupied buildings a cast resin (dry-type) transformer is the sensible default: the fire load is low and no oil containment is needed. Oil-immersed units are still used indoors — in dedicated, fire-rated transformer rooms — where the rating, the efficiency or the overload capability justifies the extra civil works.
Why cast resin dominates indoors
The decisive argument is fire behaviour. A cast resin unit classified F1 to IEC 60076-11 is self-extinguishing and produces little smoke, so it can sit close to people in offices, hospitals, car parks and retail buildings. Because there is no liquid, there is no bund, no leak path to the drains and no oil sampling programme — the room becomes markedly simpler and cheaper to build.
The compact footprint has a second effect that is easy to overlook. A dry-type unit can often be placed closer to the load, which shortens the low-voltage cable runs. At several thousand amps, a few metres of copper saved is a real number on the project budget, and the cable losses saved run for the life of the installation.
Be honest about the downsides. Cast resin is not maintenance-free, despite the claim being repeated everywhere: the windings collect dust, dust reduces cooling and holds moisture, so periodic de-dusting, visual inspection and thermographic checks belong in the schedule. Air is also a poorer coolant than oil, so a dry-type unit derates faster in a hot or badly ventilated room, and many designs are audibly louder per kVA.
When an oil-immersed unit is still the right indoor choice
Oil is a better coolant and a better dielectric than air, so an oil-immersed transformer reaches a given rating in a smaller, cheaper package and absorbs short-term overloads more comfortably. That advantage grows with rating and voltage, which is why substations and heavy industry inside buildings still specify oil above roughly a few MVA and beyond 36 kV.
The conditions are strict: a separate room with a defined fire resistance rating, a bund sized for the full oil volume, ventilation or cooling sized for the losses, and access for oil sampling. Where fire risk is the only obstacle, ester fluid is a genuine middle path — biodegradable, with a fire point above 300 °C, and accepted in many indoor locations where mineral oil is not.
What the comparison really comes down to
Five criteria decide almost every indoor case. Work through them in order and the type usually selects itself.
- Fire load and occupancy — the closer people are, the stronger the case for cast resin or ester fluid.
- Room cost — a fire-rated room with a bund can cost more than the price difference between the two transformers.
- Rating and voltage — above roughly a few MVA and beyond 36 kV, oil-immersed becomes markedly more economical.
- Ambient and ventilation — cast resin derates faster in hot, dusty or poorly ventilated rooms.
- Overload and duty — oil handles cyclic and short-term overloads better; both types can meet EcoDesign Tier 2.
Two claims you will read elsewhere deserve correcting. Cast resin is not inherently inefficient: Tier 2 applies to both technologies and a well-specified dry-type meets it. And its life is not automatically shorter — it is shorter when the unit runs hot and dusty, which is a room problem more than a transformer problem.
Specifying the indoor unit
Once the type is settled, fix the transformer and the room together. A factory-fitted IP21 enclosure with ventilation louvres makes a cast resin unit touch-safe in accessible spaces while preserving natural air cooling. Size the ventilation openings for the actual losses, state the maximum ambient temperature you expect rather than assuming 40 °C, and confirm the environmental and climatic classes (E2 and C2 for condensation and pollution) if the room is humid or dirty.
Add winding temperature sensors and, where the load is cyclic, a fan control step (AN/AF) that buys extra rating without a larger transformer. Then check that the unit can be brought in and taken out again through the doors, lift and floor loading you actually have.
Rule of thumb: if people share the building, start with cast resin and only move to oil when the rating, the efficiency target or the overload duty makes the extra room cost worth paying. Compare the two on installed cost — transformer plus room plus containment plus ventilation — rather than transformer price alone, and the decision usually makes itself.
