Dry-Type vs. Oil-Filled Transformers: What to Confirm Before Ordering

Updated: August 7, 2026

TL;DR

Dry-type and oil-filled transformers differ in construction, cooling method, environmental requirements and maintenance considerations. Neither type is automatically right for every indoor, outdoor, commercial or industrial application. Before ordering, confirm the approved transformer type, kVA, primary and secondary voltage, phase, frequency, enclosure, efficiency requirements, accessories, dimensions and project approvals.

This is a pre-order planning guide. Transformer sizing, system design, protection, installation, testing and maintenance must be handled by qualified professionals using the approved plans, current requirements and manufacturer instructions.

What Is a Dry-Type Transformer?

A dry-type transformer uses solid insulation and air rather than insulating liquid around its windings. Available designs include ventilated, nonventilated, encapsulated and cast-coil transformers.

The correct construction depends on the approved design, surrounding environment, enclosure, temperature requirements, sound expectations and other project conditions. The words “dry-type” alone do not establish where a transformer may be installed.

What Is an Oil-Filled Transformer?

An oil-filled transformer uses insulating liquid around its internal components. Mineral oil is one option, but some transformers use other dielectric fluids. For this reason, manufacturers and project documents may use the broader term “liquid-filled” or “liquid-immersed transformer.”

The fluid type, tank construction, environmental rating and applicable project requirements must be confirmed before purchase. Never assume that every liquid-filled transformer is interchangeable or approved for the same location.

Is One Type Better Than the Other?

Not universally. The better choice is the transformer specified or approved for the project. The decision may involve:

• Installation environment
• Available space and access
• Required enclosure and construction
• Sound limitations
• Efficiency requirements
• Maintenance planning
• Fire and environmental considerations
• Owner, engineer, utility and AHJ requirements

If the plans do not identify the required transformer type, obtain clarification from the engineer of record or other responsible design professional before ordering.

What to Confirm Before Ordering

1. Approved transformer type

Confirm whether the project calls for dry-type, cast-coil, encapsulated, liquid-filled or another specific construction. Record the complete description rather than relying on a general term such as “distribution transformer.”

2. Required kVA

Use the kVA stated in the approved schedule, specification or replacement documentation. Do not calculate or estimate transformer capacity from incomplete information. If the rating has not been established, return the question to the project’s qualified designer.

3. Primary and secondary voltage

Provide both voltage values exactly as shown on the plans or existing nameplate. Similar-looking voltage descriptions are not automatically interchangeable.

4. Phase and frequency

Confirm whether the transformer is single-phase or three-phase and record the required frequency. These details must match the approved system information.

5. Winding and connection information

If the project specifies winding material, connection configuration, impedance, electrostatic shielding or other construction details, include them in the quote request exactly as written.

6. Taps

Record the required tap arrangement from the specification or existing nameplate. Do not assume that two transformers with the same kVA and voltage provide the same taps.

7. Enclosure and environment

Identify whether the equipment will be installed indoors, outdoors or in a demanding environment. Note known exposure to moisture, salt air, dust, chemicals, flooding or physical damage. The enclosure and complete transformer must meet the applicable listing, labeling, approval and environmental requirements for the intended conditions.

8. Temperature-rise and insulation requirements

These values affect product selection and should come from the approved specification or manufacturer documentation. Include them when they appear in the transformer schedule.

9. Efficiency and sound requirements

Check the specification for efficiency, sound-level or project-specific performance requirements. The U.S. Department of Energy maintains efficiency requirements for covered distribution transformers, but product eligibility and project compliance should be verified before purchase.

10. Dimensions, weight and mounting

Confirm the available space, access route, mounting method and any maximum dimensions or weight restrictions. A replacement transformer with matching electrical ratings may still have a different footprint.

11. Terminations and conductor compatibility

Record the required terminal arrangement, entry locations and conductor material identified by the project. Confirm available accessories and lug provisions with the manufacturer.

12. Accessories and monitoring

List required temperature sensors, fans, shields, enclosures, mounting hardware, monitoring equipment or other accessories. Accessories may be factory-installed, field-installed or unavailable on certain models.

13. Listings and project approvals

Confirm required listings, labels, efficiency certifications, approved manufacturers and submittal requirements. Obtain written approval before ordering a substitute for a specified transformer.

Replacing an Existing Transformer

A clear photograph of the complete nameplate is the best starting point. Also provide:

• Manufacturer and full catalog number
• Serial number, if available
• kVA rating
• Primary and secondary voltage
• Phase and frequency
• Tap information
• Enclosure or tank information
• Overall dimensions and mounting arrangement
• Terminal and conduit-entry locations
• Project plans or an approved replacement specification

Do not select a replacement based only on physical appearance, kVA or voltage. Differences in construction, impedance, dimensions, terminations and accessories can make an otherwise similar unit unsuitable.

Questions to Ask Before Accepting a Substitute

• Does the complete model match every required electrical and mechanical specification?
• Is it approved for the installation environment?
• Does it carry the required listing and efficiency documentation?
• Are all required accessories included?
• Will its dimensions, weight and connection locations work with the approved layout?
• Has the engineer, owner or other responsible party approved the substitution in writing?
• What are the lead time, freight terms, cancellation restrictions and return conditions?

Common Ordering Mistakes

• Providing only one voltage instead of primary and secondary voltage
• Leaving out phase or frequency
• Assuming every dry-type transformer is suitable indoors or outdoors
• Assuming every oil-filled transformer uses the same fluid
• Ordering from an incomplete or unreadable nameplate
• Ignoring dimensions, weight or connection locations
• Accepting a substitute without written approval
• Ordering before required submittals have been approved

Frequently Asked Questions

Is a dry-type transformer always the right choice indoors?

No. Location alone does not determine the correct transformer. Use the approved plans, specifications, product listing and manufacturer documentation.

Are oil-filled transformers only used outdoors?

No universal rule applies to every product or project. The fluid, construction, location, occupancy and applicable requirements must be evaluated by the responsible professionals.

Can two transformers with the same kVA and voltage be substituted?

Not automatically. They may differ in impedance, winding configuration, taps, enclosure, efficiency, sound, dimensions, terminations and accessories.

What if I do not have a complete specification?

Request the missing information from the engineer, equipment manufacturer or other qualified party responsible for the system. Revco can help source a product after the required specifications are established.

What should I bring to a Revco branch?

Bring the approved transformer schedule or specification, a clear nameplate photograph for replacement work, the required date, delivery information and any approved manufacturer or catalog number.

Ordering Transformers on Long Island

Start with Revco’s transformers and accessories. You can also review available distribution dry-type transformers.

For help preparing a quote request, call (631) 283-3600 or contact one of Revco’s Long Island locations. Product availability, lead times and specifications should be confirmed when the order is placed.

Important Safety and Design Notice

This article provides general purchasing information only. It does not provide transformer sizing, electrical-system design, installation, protection, grounding, testing, energization, troubleshooting or maintenance instructions. Follow the approved plans, applicable requirements, equipment listing and current manufacturer instructions. Consult the engineer of record, qualified electrical professional, equipment manufacturer, utility and Authority Having Jurisdiction when required.

About Revco Lighting + Electrical Supply

Since 1978, Revco Lighting + Electrical Supply has supported contractors, builders and facilities teams across Long Island with electrical products, lighting solutions and dependable local service. With branches in East Hampton, Southold, Southampton, Riverhead, Bohemia and Rocky Point, Revco helps customers source the materials specified for residential, commercial and industrial projects.

Helpful Resources

• Revco Transformers
• Revco Distribution Dry-Type Transformers
• Revco Long Island Locations
• U.S. Department of Energy: Distribution Transformers