Comparison

Dry-Type vs Liquid-Filled Transformers

Dry-type units suit indoor locations with fewer fire constraints. Liquid-filled units are more compact, efficient at larger sizes and common outdoors. Compare them side by side.

Electrical & Power · Last reviewed October 15, 2026

Short answer: dry-type transformers are the usual choice indoors, in buildings and near occupants. Liquid-filled transformers are the usual choice outdoors and for larger ratings, where cooling and cost favor them and fire separation is available.

	Dry-type	Liquid-filled
Insulation and cooling	Air with solid insulation (ventilated, cast resin or VPI)	Mineral oil, silicone or natural ester fluid
Fire behavior	No liquid fuel load. Fewer fire-separation rules indoors	Fluid can burn. Less-flammable fluids (higher fire point) reduce but do not remove the requirement for separation or containment
Size and weight	Larger and heavier for the same rating	More compact
Cooling and overload	Lower thermal capacity, sensitive to ambient and ventilation	Good heat removal and overload capacity
Losses and cost	Often higher cost per kVA at large ratings	Usually lower cost and losses at medium and large ratings
Maintenance	Keep clean and dry	Check fluid level and condition, sample the fluid, and manage leaks
Typical use	Indoor and commercial buildings, up to a few MVA	Utility, industrial and outdoor installations, large ratings

How to choose

Where will it sit? Indoors near occupants favors dry-type. Outdoors with space favors liquid-filled.
Check the code. Liquid-filled transformers indoors or near buildings have vault, containment or fire-separation requirements that depend on the fluid type and the rating.
Check the rating. At larger kVA the cost and efficiency advantage moves to liquid-filled.
Check the environment. Dusty, humid or corrosive locations affect dry-type ventilation and enclosure choices.
Consider the loss evaluation and the energy-efficiency rules in your jurisdiction.

Common mistakes

Placing a liquid-filled unit indoors without the required fire separation or containment.
Assuming a "less flammable" fluid removes all fire requirements.
Ignoring ventilation and ambient temperature for a ventilated dry-type unit.
Comparing purchase price only and leaving out losses and maintenance.

Two scenarios

A 500 kVA transformer in a basement electrical room of an office building. Dry-type units have no liquid to burn, so the fire separation rules are simpler. A dry-type unit is the usual choice here, and the room needs the ventilation the unit requires.

A 2,500 kVA transformer outdoors beside a factory, with room for a containment pit. A liquid-filled unit is smaller, has lower losses and handles overloads better, and the outdoor siting avoids the indoor fire rules.

Cost and life cycle

Purchase price: dry-type units cost more at larger ratings. Liquid-filled units cost less per kVA.
Losses: over a service life of 30 years or more, the cost of losses can exceed the purchase price, so evaluate losses as well as price.
Maintenance: dry-type units need cleaning and a dry environment. Liquid-filled units need fluid sampling and leak management.
End of life: liquid-filled units need fluid handling and disposal at replacement.

Related tools and labs

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Questions

What is a cast-resin transformer?

A dry-type unit with the windings encapsulated in epoxy resin, which resists moisture and dust and is common indoors in Europe and in many commercial buildings.

Are PCB-filled transformers still in use?

Old units may exist. PCB fluid is regulated and the owner must follow the rules for testing, labeling and disposal.

What is a less-flammable fluid?

A liquid with a higher fire point than mineral oil, such as a natural ester or a silicone fluid. Such fluids lower the fire risk and may reduce the separation or containment requirements, depending on the code.

Can a dry-type transformer be used outdoors?

Yes, in a weatherproof enclosure designed for it. Check the enclosure rating and the ventilation.

How do I compare losses?

Use the guaranteed no-load and load losses and a cost per kW over the evaluation life to find the lifetime cost of each option.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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