EPC contractors and plant owners meet this question at the first design review — and the answer shapes the whole electrical room. There is no universal winner: oil-immersed and cast-resin dry-type transformers are both mature technologies solving different problems. The choice comes down to five variables — where it is installed, what the fire code allows, rating and voltage class, lifetime cost, and who maintains it.

1. The 60-Second Answer

Start from your application:

Your project type	Go with	Deciding reason
Occupied or indoor space — high-rise, hospital, metro, airport, data centre, underground substation	Dry-type	Class F1 fire behaviour, no combustible fluid, sits next to the load
Indoor distribution room in a commercial building	Dry-type	No oil pit or firewall; smaller civil scope
Outdoor industrial substation up to about 2.5 MVA, up to 36 kV	Oil-immersed (dry-type if indoors)	Lower purchase price, stronger overload capability
Utility or grid substation, 10 MVA and above, or 66 kV and above	Oil-immersed	Dry-type reaches its capacity and voltage ceiling
Solar or wind central step-up station (outdoor)	Oil-immersed; dry-type for the LV auxiliary	Weather exposure and cost per MVA
Marine, coastal or high-humidity site	Dry-type class C2, or a sealed oil design	Condensation and salt fog

Then check the fire code against your shortlist — the one constraint you cannot negotiate.

2. What Actually Differs

Oil units immerse the core and windings in insulating liquid (mineral oil per IEC 60296, or ester per IEC 62770 and IEC 61099), which insulates and carries heat away by convection — cooling designations ONAN, ONAF and OFAF per IEC 60076-2. Dry-type units remove the liquid: epoxy vacuum-cast or VPI windings, air-cooled AN or AF, governed by IEC 60076-11 with environmental E2, climatic C2 and fire-behaviour F1 classes. Liquid moves heat far better than air — and that single fact drives everything that follows.

3. Fire Safety and Siting

Cast-resin units reach fire-behaviour class F1 — self-extinguishing, low smoke — and contain no combustible fluid, so they need no bund, oil–water separator, fire-rated wall or clearance distance. They can share a room with the switchgear, close to the load centre.

Oil units contain combustible fluid. Outdoors, in their own compound, that is manageable; indoors it demands a bunded pit, fire-rated construction and separation from escape routes. Pressure-relief devices, Buchholz relays and winding-temperature monitoring reduce the hazard without removing it. Check IEC 61936-1, the local code and your insurer before freezing the layout — many jurisdictions simply ban oil indoors.

4. Rating, Voltage and Overload

• Rating: cast-resin dry-type covers 30 kVA to 20–25 MVA, HV up to 36–38.5 kV; oil-immersed runs to several hundred MVA.
• Overload: oil has greater thermal inertia and better heat removal, so it handles emergency overload and harmonic-rich loads more comfortably.
• Noise: oil is usually quieter at equal rating. State a measurable limit (IEC 60076-10 or JB/T 10088) and have it tested, not catalogued.

5. CapEx vs Lifecycle Cost

Tenders score purchase price, which favours oil. Lifecycle cost often points the other way:

Item	Oil-immersed	Dry-type (cast resin)
Purchase price	Lower — commonly 20–40 % below the same rating	Higher
Civil works	Bund, fire-rated separation, larger plot	No pit or firewall; shares a room with switchgear
Maintenance	Oil sampling, DGA, moisture and BDV testing; later filtration or replacement	Inspection, cleaning, thermography
Net position	Wins large outdoor ratings and CapEx-scored tenders	Wins small–medium indoor ratings and LCC- or ESG-driven projects

For an indoor 1 000–2 500 kVA substation, civil savings and the absence of oil maintenance usually make the dry-type unit cheaper over 20–25 years.

6. What to Write Into Your Enquiry

What to specify	Oil-immersed	Dry-type (cast resin)
Governing standards	IEC 60076-1 / -2 / -3 / -5 / -10	The same, plus IEC 60076-11
Classes and protection	Sealing, pressure-relief device, Buchholz relay, temperature indicators	Environmental E2, climatic C2, fire class F1
Energy efficiency (GB 20052-2024)	S20 = Level 2, S22 = Level 1	SCB14 = Level 2, SCB18 = Level 1
Key type tests	Temperature rise, lightning impulse, short-circuit withstand	The same, plus partial discharge ≤ 10 pC
EU market	Regulation (EU) 548/2014 as amended by (EU) 2019/1783; Tier 2 since 1 July 2021 — check Po / Pk for your rating and Um	The same — verify before committing

Require every bidder to quote losses at the same reference temperature — 120 °C for Class F dry-type — and guarantee sound level and partial discharge with IEC 60076-1 tolerances.

7. Five Questions to Settle First

• Where installed? Indoor or occupied → dry-type; outdoor compound → oil is acceptable.
• Rating and voltage? Up to about 2.5 MVA and 36 kV → both work; 10 MVA+ or 66 kV+ → oil.
• Fire code or insurer? If oil is prohibited → dry-type, full stop.
• Who maintains it? No dedicated O&M team → dry-type.
• How is the tender scored? CapEx-only and large outdoor → oil; LCC, ESG or green building → dry-type.

8. The Bottom Line

Dry-type wins where people are — inside buildings, underground, in hospitals, metros and data centres. Oil wins where power is large and space is not — outdoor substations, grid nodes, renewable step-up stations and CapEx-driven tenders. In the middle, settle the fire code first, then run a like-for-like lifecycle comparison.

Send us your rating, voltage ratio, vector group, installation environment, standard and target loss level: SHENDIAN returns a side-by-side selection with guaranteed loss, sound and temperature-rise figures, plus a quotation, normally within one working day. We supply oil-immersed and cast-resin dry-type units from 30 kVA to 25 MVA at up to 38.5 kV, with IEC / GB type-test reports and OEM / ODM support.