Dry Type Transformer: How It Works, Where It Fits, and How to Specify One

Not every project can put an oil-filled transformer in the plan. Hospitals, high-rise buildings, metro stations, and indoor substations all come with fire codes and space constraints that push specifiers toward a dry type transformer instead. It cools with air rather than oil, removes the leak and disposal concerns that come with liquid insulation, and — done right — holds up for decades in demanding indoor environments.

This guide covers what a dry type transformer actually is, how it compares with oil-filled designs, the standards it’s tested against, and what to check before you specify one for a project.

What Is a Dry Type Transformer?

A dry type transformer is a power transformer that uses solid insulation and air cooling instead of mineral oil or another liquid dielectric. The windings are typically encapsulated in cast epoxy resin, wrapped and vacuum-pressure-impregnated (VPI) with varnish, or otherwise protected with solid insulating material, then cooled by natural convection or forced air from fans.

Because there’s no oil to leak, sample, or dispose of, dry type transformers are widely specified for:

• Indoor substations and switchrooms
• Hospitals, schools, airports, and high-rise commercial buildings
• Metro and railway stations
• Renewable energy sites where a compact, low-maintenance footprint matters
• Any application where fire codes restrict liquid-filled equipment

dry-type-transformer-with-cast-epoxy-resin-windings-for-indoor-power-distribution

Dry Type vs. Oil-Filled Transformers

The core trade-off comes down to cooling medium and everything that follows from it.

Fire safety — Dry type units contain no flammable liquid, which is why building codes frequently allow or require them inside occupied structures. Oil-filled transformers carry a higher fire risk and are usually placed outdoors or in dedicated fire-rated vaults.

Maintenance — Oil-filled transformers require periodic oil sampling, dissolved gas analysis, and leak monitoring. Dry type transformers are largely maintenance-free by comparison, with routine inspection being the main requirement.

Efficiency and capacity — Oil generally provides more effective cooling under sustained heavy load, which is part of why oil-filled transformers still dominate at higher power ratings and utility-scale applications.

Environmental exposure — Dry type transformers handle dust, humidity, and moderate contamination well, particularly cast-resin designs, but they’re more sensitive to extreme outdoor conditions than a sealed oil-filled tank.

Footprint and noise — Dry type units are typically more compact for a given indoor installation but tend to run louder than oil-filled equivalents due to cooling fan operation.

Neither type is universally “better” — the right choice depends on the installation environment, fire code requirements, load profile, and voltage class.

Standards and Testing

Dry type transformers intended for utility, industrial, or commercial use should be designed and tested to IEC 60076-11, the international standard covering design, ratings, and test requirements for dry-type power transformers up to 72.5 kV (see the current edition on the IEC webstore). The current edition also introduced climatic, environmental, and fire behavior classes, which matter more for dry type units than for oil-filled ones since the winding insulation is directly exposed to the surrounding air rather than sealed inside an oil-filled tank.

Typical tests and classifications to confirm with a supplier include:

• Temperature rise test — verifying winding and core temperature stay within rated limits under load
• Dielectric tests — power-frequency and lightning impulse withstand voltage
• Climatic class (C1/C2) — condensation and icing resistance for the installation environment
• Environmental class (E0/E1/E2) — resistance to dust and water pollution
• Fire behavior class (F0/F1) — self-extinguishing performance, particularly relevant below 1,000 kVA
• Sound level testing — since cooling fan noise is a common constraint in indoor installations

A good practical comparison of dry-type and oil-filled performance trade-offs — covering maintenance load, lifespan, and noise — is available from Larson Electronics’ transformer selection guide, which is a useful second opinion alongside the formal IEC requirements.

How to Specify a Dry Type Transformer

Work through these points with your supplier before placing an order:

1. Rated power (kVA) and voltage ratio matched to the load and system design
2. Insulation system — cast resin vs. VPI/VPD varnish-impregnated windings, depending on environmental exposure and budget
3. Climatic, environmental, and fire behavior class appropriate to the installation site
4. Cooling mode — natural air (AN) or forced air (AF), and whether the load profile justifies the added complexity of forced cooling
5. Sound level requirements, especially for installations near occupied spaces
6. Altitude and ambient temperature at the installation site, since both affect insulation and thermal ratings
7. Test certification — request IEC 60076-11 type test reports and routine test reports for the specific unit, not just a general catalog compliance statement

Where Dry Type Transformers Fit in Degatech’s Product Line

Dry type transformers sit alongside the rest of Degatech’s medium-voltage equipment as part of a complete distribution package rather than a standalone commodity item. Our transformer range is designed to pair with our switchgear products, so utilities, contractors, and EPC customers can source a matched substation package — transformer, switchgear, and protection — from a single technical point of contact rather than coordinating multiple suppliers.

Because winding insulation performance in a dry type transformer depends on the same epoxy resin engineering used elsewhere in our product range, it’s worth reviewing our guide to switchgear insulation parts if you’re specifying a full substation package rather than a transformer alone.

Established in 2007 and based in Zhejiang with logistics support through Ningbo and Shanghai ports, our team supports global utility, industrial, and EPC customers with both standard and custom OEM/ODM transformer configurations. More on our background is available on our company profile page.

Frequently Asked Questions

What voltage range do dry type transformers cover? Dry type transformers are commonly used from low voltage up through 72.5 kV under IEC 60076-11, though most commercial and industrial installations fall in the medium-voltage range up to around 36 kV.

Are dry type transformers suitable for outdoor installation? Yes, with the correct environmental and climatic class rating, though oil-filled units remain more common for harsh outdoor or high-capacity utility applications.

How long do dry type transformers last? With correct sizing, insulation class, and installation conditions, dry type transformers commonly achieve service lives of 25-30 years or more, comparable to oil-filled equivalents in the same duty cycle.

Do dry type transformers need regular maintenance? They require far less than oil-filled units — mainly periodic visual inspection, cleaning of cooling paths, and infrared thermal checks rather than oil sampling or leak monitoring.

Planning a substation or indoor distribution project that needs a dry type transformer? Contact the Degatech team with your kVA rating, voltage class, and installation environment for a tailored quotation.