Dry-type transformers fundamentals

What is a transformer?

A transformer is an electrical device that is used in AC power distribution systems to safely “step up” or “step down” voltages to meet the incoming power requirements of connected equipment. Transformers achieve this goal without modifying the frequency or amount of electrical power to help ensure connected devices can operate reliably.

Transformers are static devices, meaning they contain no continuously moving mechanical parts. This contributes to their long lifecycle and minimal maintenance requirements. They can range in size from enormous devices that weighs several tons, commonly used in power substations, to very small devices that weigh only a few ounces and are commonly found in compact electrical equipment.

Thanks to their design flexibility and critical role in the electrical system, transformers are used in nearly every power distribution system application including commercial buildings, schools, healthcare & institutions, water/wastewater, industrial, oil & gas and data centers.

How do transformers work?

The typical transformer has several secondary windings or coils of insulated wire conductor wrapped around a laminated steel core. When voltage is introduced to one coil, called the primary, it magnetizes the iron core. A voltage is then induced in the other coil, called the secondary or output coil. The change of voltage (or voltage ratio) between the primary and secondary depends on the ratio of the turns in the two coils.

There are two primary types of transformers that are defined by the insulation used within the enclosure: dry-type and liquid-filled. On this page, we will explore the basics of dry-type transformers and their most common configurations.  


What is a dry-type transformer?

Unlike their liquid-filled counterparts, dry-type transformers do not incorporate liquid to dissipate excess heat and meet temperature classification requirements. Instead, the coils within a dry-type transformer are constructed of a gaseous or dry insulation medium. There are three common types of dry-type transformers:


These transformers use natural or fan-assisted circulation through ventilation openings to maintain temperature requirements.


Also known as compound-filled or potted transformers, encapsulated designs enclose windings within a mixture of solid-insulating materials such as resin, gravel, sand or epoxy to help dissipate heat.


The completely enclosed design of non-ventilated transformers is made for applications where the atmosphere may contain conductive, corrosive or combustible materials that could damage the transformer, and works by dissipating heat through the surface area of the enclosure. 

Common dry-type transformers 

Although there are three primary types of dry-type transformers, there are multiple configurations that can be applied to meet a wide variety of application needs. The four most common configurations include: General purpose ventilated, General purpose encapsulated , Totally enclosed non-ventilated  and Mini-power centers.
General purpose ventilated transformers are commonly used in commercial and light industrial general-purpose applications, with single- and three-phase configurations. Typical ventilated transformers range in size from 15-1000 kVA, with primary voltages ranging from 380-575 V and secondary voltages ranging from 120-240 V. Coils are wound with aluminum or copper conductors and 220°C insulation materials. These transformers operate within a temperature rise range of 150°C to 80°C and at a frequency of 60 or 50 Hz. Most enclosures are National Electrical Manufacturers Association (NEMA) rated Type 1, 2 or 3R.
Commonly used in industrial and light industrial general purpose applications, single- and three-phase general purpose encapsulated dry-type transformers are of the two-winding type, encapsulated (potted) in a mix of epoxy coated sand and gravel. Typical encapsulated transformers range in size from 0.05-75 kVA, with primary voltages ranging from 380-575 V and secondary voltages ranging from 120-240 V.  These transformers operate within a temperature rise range of 115°C to 80°C and at a frequency of 60 or 50 Hz. Most enclosures are NEMA rated Type 3R or 4X.
Totally enclosed non-ventilated transformers are highly suited for applications where the atmosphere contains conductive, corrosive or combustible materials that might damage a transformer, or lint and dust, which might block the ventilation passages. With no openings in the enclosure, heat is dissipated through the surface area of the enclosure. Typical totally enclosed non-ventilated transformers range in size from 15 – 300 kVA, with primary voltages ranging from 380 – 575 V and secondary voltages ranging from 120 – 240 V. These transformers operate within a temperature rise range of 150°C to 80°C and at a frequency of 60 or 50 Hz. Most enclosures are NEMA rated Type 3R, 4X or 12. 
Mini-power centers incorporate an encapsulated transformer, interior, primary and secondary main circuit breakers into a single assembly to reduce cost, footprint and installation complexity. Mini-power center transformers range in size from 3 – 30 kVA, with primary voltages ranging from 380 – 575 V and secondary voltages of 120/240 V (single-phase) or 120/208 V (three-phase). These solutions are ideally suited for applications where 120 volts are required at a remote location, such us parking lots, workbenches and temporary power on construction sites.

Special dry-type transformers 

There are also specifically designed dry-type transformers that are engineered to meet the needs of unique applications, including: 

1. K-Factor rated

K-factor transformers are specifically designed to withstand the harmful overheating effects caused by harmonics generated by nonlinear (non-sinusoidal) loads. These loads include computers, laser printers, copiers, and other office equipment, as well as video monitors and other electronic equipment. K-factor transformers are not simply oversized transformers. The core and coils of K-factor transformers are designed for reduced induction levels, which results in less stray losses. K-factor transformers do not reduce or mitigate harmonic levels but are specially designed to withstand the harmful heating effects of the harmonic currents produced by nonlinear loads. Oversized (200% rated) neutrals and electrostatic shielding are typical features of K-factor transformers. 


2. Harmonic mitigation

Harmonic currents produced by nonlinear loads can cause additional heating which may overload transformers, generators and conductors. Harmonic mitigation transformers, when properly applied within an electrical system, will cancel out harmonic currents to keep the loads operating the way the manufacturer designed them and keep the facility’s electrical system free from voltage distortion. These devices are suited for installations rich in harmonic loads, such as educational facilities (K–12 and universities), government buildings, commercial office buildings, medical facilities and call-center applications. 

3. Breaker Integrated

Breaker Integrated Transformers (BITs) provide a simple, cost-effective approach to reducing arc flash risk and increasing flexibility for mounting secondary panels. By integrating the secondary molded-case circuit breaker into the low-voltage dry-type distribution transformer package, arc flash incident energy values for secondary panels can be greatly reduced (usually to less than 8 cal/cm2). Additionally, with the transformer secondary protection integrated into the transformer, placement of the secondary panel is not required to be within 10 feet to meet NEC requirements. This allows total flexibility in the placement of the secondary panels to be closer to loads, reducing individual branch conductor lengths/voltage drop or placement in any available location when existing space is limited. BITs can be supplied with integrated primary breakers, secondary breakers or both, providing ultimate flexibility to meet specific application disconnect and overcurrent protection needs.


4. Motor drive isolation

Motor drive isolation transformers are designed for three-phase variable  speed drive load profiles. Sized by horsepower and common motor voltages, motor drive isolation transformers are braced to withstand the mechanical stresses associated with AC or DC  variable speed drives. The two-winding drive isolation transformers provide:

  • Electrical isolation between the incoming line and drive SCR circuitry
  • Voltage conversion on input line to standard drive input voltages
  • Help to minimize line disturbances caused by variable speed  drive rectifiers
  • A reduction in short-circuit currents and voltage line transients 

Motor drive isolation transformers are manufactured as ventilated or encapsulated. Ventilated transformers include a normally open, dry contact temperature sensor installed in the coils that provides advance warning of potential transformer overheating. 

5. Class 1, Division 2 hazardous area

Many petrochemical plants, refineries and other industrial facilities have atmospheres unsuitable for the safe application of either liquid-filled or general-purpose dry-type transformers. Transformers for Class I, Division 2, Groups A, B, C and D locations meet the stringent standards set forth by NEC® Article 501, with NEC-recommended installation procedures for dry-type transformers rated for under 600V nominal operation.

These solutions are used in locations where volatile flammable liquids or gases are handled, processed or used, and might become harmful to the transformer following failure of ventilation equipment.


6. Marine-duty 

Marine-duty transformers are “Type Approved” by the American Bureau of Shipping (ABS) for on-board use in steel vessels (not for propulsion systems or combat vessels). These transformers are typically installed below deck in electrical or mechanical rooms where the ambient temperature is greater than normal. Marine-duty transformers are especially designed for operation in 50 °C ambient locations and copper windings are standard.

7. Buck-boost transformers 

Buck-boost transformers are suited to applications where the available voltage needs to be slightly boosted or bucked (reduced). When buck-boost transformers are wired as autotransformers, they can be used to accomplish this bucking or boosting of voltage automatically. Buck-boost transformers are single-phase encapsulated transformers, available in three-voltage combinations:

  • 120 V x 240–12/24 V
  • 120 V x 240–16/32 V
  • 240 x 480 V–24/48 V

Specific combinations of these buck-boost transformers are applied to output the desired voltage level and load kVA requirement. Since buck-boost transformers are only slightly boosting or bucking voltage levels and are typically wired as autotransformers, the kVA size of the actual buck-boost transformers is typically very small compared to the kVA requirement of the load. Buck-boost transformers only range in size from 0.05 kVA through 7.5 kVA, but can be applied to boost or buck the voltage to loads as large as hundreds of kVA.

These transformers can also be used at their nameplate voltages for applications such as low voltage interior or landscape lighting.


Enclosure options for dry-type transformers

There are many different enclosure options available for dry-type transformers to meet specific environmental challenges of intended applications. All enclosures provide a degree of protection to personnel against access to hazardous parts. Here are a few of the most common enclosure ratings as described by NEMA relative to  environmental protections.
  • Type 1

Constructed for indoor use to protect against dust, light and indirect splashing, but not dust-tight

  • Type 2

Constructed for indoor use to protect against ingress of solid foreign objects (falling dirt), and harmful effects on the equipment due to the ingress of water (dripping and light splashing)

  • Type 3R

Typically used in outdoor applications to protect from rain, sleet, snow and external ice formation

  • Type 3RX

Same as Type 3R except that provides an additional level of protection against corrosion.

  • Type 4

Intended for general purpose indoor or outdoor use to provide protection against windblown dust and rain, splashing water, and hose-directed water, and to be undamaged by the formation of ice on the enclosure

  • Type 4X

Most often constructed of stainless steel and intended for indoor and outdoor use to provide protection against corrosion, windblown dust and rain, splashing water, and hose-directed water, and to be undamaged by the formation of ice on the enclosure

  • Type 12

Constructed for indoor use primarily to provide protection against dust, falling dirt and dripping noncorrosive liquids


To learn more about various enclosure types, please review NEMA’s published definitions

Custom dry-type transformers

When you need more than “off-the-shelf,” Eaton's Transformer Flex Center is available to help meet unique general-purpose and distribution transformer needs that cannot be met with standard offerings.

With the ability to engineer or modify transformers to meet nearly any application requirement, our expert team solves critical application challenges. 

Common transformer customizations 





Custom paint

Compliance with industry and regional standards


NEMAT 4X, 304 or 316 stainless steel enclosures

Configurations with quick connectors (military grade)

Special labeling

Infrared viewing windows to ease diagnostics


NEMA 12 totally enclosed, non-ventilated

Pre-terminated cable for ease of installation

Tailored dimensions

Hinged front cover to simplify maintenance and testing


Hazardous location ratings with Class I, Division 2 encapsulated transformers

Primary or secondary breakers and ventilated transformer in a single component

Efficiency, sound, altitude, operating ambient, impedance and temperature rise


Integrated breakers on secondary side to reduce arc-flash hazard





Low-voltage transformer applications

See the types of transformers at the Eaton experience center and the practical application considerations you should evaluate before applying transformers in your power system.

Application considerations and calculations

See the application considerations and calculations for transformers and voltage regulators, such as impedance, fault current, motor starting, tap changer, inrush current and winding configuration.