Choose an oil-immersed transformer for outdoor substations, higher ratings and lower purchase cost; choose a dry-type (cast resin) transformer for indoor installations where fire safety, no liquid and low maintenance matter most. Both types are designed and tested under the IEC 60076 series, so the right choice depends on where the unit will sit and how it will be loaded. This comparison sets out the practical differences for buyers.
How each type works
An oil-immersed transformer has its core and windings inside a steel tank filled with insulating liquid. The liquid insulates the windings and carries heat to the tank walls and radiators. Mineral oil is the usual fluid; natural or synthetic esters are used where higher fire point or biodegradability is wanted. Tanks are either conservator type, with an expansion tank and breather, or hermetically sealed, with corrugated walls that flex as the oil expands.
A dry-type transformer uses air as the cooling medium and solid insulation on the windings. The common form is cast resin: the high-voltage windings are encapsulated in epoxy resin under vacuum. Another form is vacuum pressure impregnated (VPI), where windings are impregnated with varnish rather than cast. Dry-type units are covered by IEC 60076-11.
Side-by-side comparison
| Factor | Oil-immersed | Dry-type (cast resin) |
|---|---|---|
| Insulation and cooling | Mineral oil or ester fluid | Air and epoxy resin |
| Typical location | Outdoor substations, pole-mounted, utility networks, kiosks | Indoor plant rooms, commercial buildings, hospitals, tunnels, offshore and marine |
| Fire risk | Fluid is combustible; may need fire walls, separation distances or suppression | Low; self-extinguishing resin, no flammable liquid |
| Environmental risk | Possible oil leaks; oil pit or bund often needed | No liquid to leak or contain |
| Rating range | Very wide, from small distribution to large power transformers | Mainly distribution and medium ratings |
| Voltage class | Up to transmission voltages | Mainly medium voltage |
| Overload capacity | Good, thanks to the thermal mass of the oil | Lower; forced-air fans can raise output temporarily |
| Losses and noise | Usually lower for the same rating | Often higher, and noise can be higher |
| Maintenance | Oil sampling and testing, gasket and breather checks | Periodic cleaning and inspection of windings and connections |
| Sensitivity to site | Tolerates outdoor dust and humidity in its sealed tank | Needs a clean, dry, ventilated room or a suitable enclosure |
| Purchase price | Generally lower | Generally higher |
| Weight and size | Heavier, needs oil containment | Lighter install, but often larger footprint per kVA |
Fire safety and building rules
Fire safety is the main reason buyers choose dry-type. In many countries, building codes and insurers restrict oil-filled equipment inside occupied buildings, or require fire-rated rooms, oil containment and separation distances. A cast resin transformer can usually be installed close to the load, inside the building, which shortens LV cable runs.
IEC 60076-11 classifies dry-type transformers by environmental class (E0, E1, E2), climatic class (C1, C2) and fire behaviour class (F0, F1). For indoor projects, a common specification is E2, C2, F1. Ask the supplier for test reports proving each class, not just a statement on the nameplate.
If you need an oil-filled unit closer to a building, ester fluids with a higher fire point are an option, but check what your local code accepts.
Loading, losses and life
Oil carries heat well and has a large thermal mass, so oil-immersed units handle short overloads comfortably. Their loading guidance is set out in the IEC 60076 series.
Dry-type windings work at higher temperatures. Cast resin designs commonly use insulation thermal class F or H. Fans (AF cooling) can increase output for limited periods, which is useful for peak loads, but continuous rating should be based on natural air cooling (AN).
For both types, ask for guaranteed no-load loss and load loss in watts. In Chinese catalogues, codes such as S13 (oil-immersed) or SCB13 (cast resin) indicate a loss series, but always compare the guaranteed figures.
Installation and maintenance
Oil-immersed units need a level plinth, an oil pit or bund in many jurisdictions, and access for oil sampling. Dissolved gas analysis and oil quality tests give early warning of faults. Hermetically sealed designs reduce routine work because the oil is not exposed to air.
Dry-type units need a clean, ventilated room. Dust, condensation and conductive pollution are the main enemies, so an IP-rated enclosure may be needed. Maintenance is mostly visual inspection, cleaning and tightening of connections.
Which should you buy?
Use these rules of thumb:
- Outdoor substation, utility feeder or rural network: oil-immersed.
- Inside a building, near people, or where oil containment is impractical: dry-type.
- High ratings or high voltages: oil-immersed is usually the only practical option.
- Prefabricated or kiosk substation: either can be used. Oil-immersed is common for outdoor units; dry-type is used where the station is close to buildings. See ranked prefabricated substation manufacturers.
- Lowest purchase price: oil-immersed. Lowest fire and environmental risk: dry-type.
Ancillary equipment matters too. Specify the MV protection, for example a ring main unit or vacuum circuit breaker, and LV distribution with the transformer so the interfaces match.
What to check with a Chinese supplier
Many Chinese factories build both types, but expertise varies. Check:
- Business registration: the 18-character Unified Social Credit Code, and a business scope that includes transformer manufacturing.
- Factory vs trader: ask for a video tour of the winding area, the resin casting line for dry-type, the tank shop and drying ovens for oil-immersed, and the test bay.
- Standards: confirm design and tests to IEC 60076 (and IEC 60076-11 for dry-type), and any national standard your utility requires.
- Type-test reports: from an accredited independent laboratory, for the same design and voltage class you are buying.
- Certificates: ISO 9001 as a minimum; CCC where relevant for China, and the marks your market requires.
Our scores measure data completeness, not product quality; see the methodology. Then see the ranked list of transformer manufacturers to start your shortlist.
Frequently asked questions
Is a dry-type transformer more efficient than an oil-immersed one?
Not usually. For the same rating, oil-immersed transformers typically have lower losses, though both types can be built to high efficiency levels. Compare guaranteed loss values in each offer.
Can a cast resin transformer be installed outdoors?
Only inside a suitable enclosure that protects it from rain, dust and condensation. Without an enclosure, an oil-immersed unit is the normal choice outdoors.
How long do these transformers last?
Both types can give decades of service when correctly loaded and maintained. Life depends mainly on winding temperature, so avoid sustained overloads and poor ventilation.
Do oil-immersed transformers always need an oil pit?
It depends on local regulations, the oil volume and the site. Many codes require containment for larger units or sites near watercourses, so check before you finalise the civil design.