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Guide to Dry Type Transformers

Guide to Dry Type Transformers

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A transformer is a device that transfers energy from one alternating-current circuit to another. Dry type transformers are used for many applications to reduce the voltage of conventional power circuits. Learn more about what a dry type transformer does and where we use dry type transformers.

What is a Dry Type Transformer?

A dry type transformer is cooled by normal air ventilation instead of a liquid such as mineral oil or Envirotemp FR3. The early transformers manufactured by Westinghouse and General Electric were air cooled because they operated at low power and voltages. Most instrument transformers and low-voltage power chargers today are also dry types. Liquid cooling inside sealed tanks and metal enclosures became the standard for AC power distribution transformers due to the higher induction heat losses and exposure to all-weather conditions. Because of improvements in design, materials and manufacturing methods over the past three decades, dry type transformers not only compete with liquid-cooled transformers in the medium voltage range (2.5KV – 34.5 KV) but have significant advantages in many commercial and industrial applications.

Dry-Type Vs. Liquid Cooled (Padmount) Transformers

When designed and built to meet current industry standards, medium-voltage dry type and padmount transformers offer essentially the same levels of power efficiency and BIL for a given KVA rating. The differences in operation depend on the application and the installation environment.

Liquid-cooled padmount transformers are used by utility companies for both medium and higher voltage applications because they are designed for all-weather outdoor installations like substations. They have a long history of providing safe, reliable power, and can be installed in outdoor public spaces (with appropriate enclosures). One of their disadvantages is that, since the mineral oil used as a coolant is flammable, a transformer failure (due to insulation failure, overheating or a lightning strike) can result in an explosion and a fire. Thus, they are not recommended for indoor installations or anywhere a fire could have catastrophic results.

Dry Type transformers are cooled by clean ambient or forced air and contain no liquids. They present a minimal fire hazard and are suitable for indoor installations or anywhere fire hazards must be avoided. Because they are designed and built to run cool and quiet, they are ideal for indoor installations in office buildings, universities, hospitals, and manufacturing clean rooms. They can replace liquid-cooled transformers in many of these settings.

Dry types may be used in certain outdoor installations with moderate heat and humidity conditions throughout the year if protected in a NEMA type 3 enclosure.

Benefits of ELSCO Dry Type Transformers

The benefits of our dry type transformers originate from both the design and the materials used in their construction.

Benefits of ELSCO Dry Type Transformers

1. Experience, Quality Design, Materials and Craftmanship

ELSCO Dry Type transformers are hand-wound in 100% copper, with a disc-wound round coil, slotted comb, sheet wound secondary and a step-lap mitered core with low flux density. Nomex or film covered wire insulation is rated at 200°C and solid insulation is SG 200 fiberglass. or Nomex. Each unit is inspected and tested five times in the manufacturing process and again before shipping. This uncompromising commitment to quality produces tangible user benefits.

2. Highly Efficient Performance

ELSCO Dry Type Transformers exceed Energy Star and DOE energy efficiency standards and meet the latest NEMA, ANSI and IEEE standards. Their Basic Insulation Level (BIL) ratings are comparable to liquid-cooled transformers.

3. Run Cool and Quiet

The combined effect of precision hand-wound copper coils, a step-lap mitered core with lower flux density, and varnishing of the entire unit ensures minimal noise and vibration after installation. This is an advantage in many applications.

4. Long Useful Life

Superior quality materials, superior design and expert craftsmanship deliver long-standing performance. ELSCO Dry Type transformers come with a five-year warranty on materials and workmanship for indoor installations, and have an expected useful life of 35 years or more.

5. More Cost Effective

A superior quality design and construction results in:

  • Higher energy efficiency and quieter unit.
  • Lower utility and maintenance costs.
  • A longer useful life.
  • 5 – 7 year payback due to higher efficiency and lower operation costs.

Industries That Use Dry Type Transformers

Industries That Use Dry Type Transformers

There are a variety of industries that utilize dry type transformers.

Discrete Manufacturing

  • Machine shops
  • Sheet Metal Fabrication
  • Welding
  • Plastic and Rubber Molding
  • Machinery
  • Electroplating

Automation and Continuous Process

  • Electronics Fabrication
  • Steel Mills
  • Small Appliances
  • Paper Mills
  • Chemical Plants
  • Food Processing
  • Bottling Plants
  • Plastics and Rubber

Colleges & Universities

  • Classrooms
  • Laboratories
  • Data Centers
  • Auditoriums
  • Power Plants
  • Residence Halls

Hospitals & Medical Centers

  • Surgical Centers
  • Critical Patient Care
  • Diagnostic Centers
  • Critical Equipment
  • Laboratories
  • Power Redundancy

Electrical Utilities

  • Power Plants
  • Distribution
  • Enclosed Substations
  • Customer Feeds

Examples of ELSCO Dry Type Transformer Applications

In Manufacturing

A large manufacturer had a dry-type transformer fail. Production was down and they had to send workers home. We had a new dry type in stock and they ordered it. We got good photos of the HV cable, the LV bus with braided shunts, and the base. We obtained all the critical dimensions, put everything on CAD and were able to modify our core and coil unit to create a plug-and-play retrofit. In less than a full day their unit was on the road to a seamless installation which got them back up and running. Our quick response time eliminated days of costly downtime.

At a University

A major university had one of their dry-type transformers fail, shutting down operations in a campus medical building. We had a new dry type in stock which they ordered. We obtained photos and all the necessary measurements and modified their unit for a quick shipment. The installation took only hours. They were more than happy with our response and the quick turnaround.

In a Hospital

A hospital in Tennessee lost one of their dry-type transformers due to frozen water pipes which leaked and got the transformer wet. The hospital was running temporarily on a rented generator at reduced power and higher costs. They ordered one of our new in-stock dry types which we modified with bus work for a custom retrofit. They were happy to get back to full power in 48 hours and get rid of the generator and additional costs.

These are typical examples of an ELSCO dry type transformer application:

ELSCO supplies new dry type transformers from stock to quickly restore power to business customers who experience costly downtime due to transformer failures.

When To Choose a Dry Type Transformer

ELSCO dry type transformers are ideal for indoor installations with people nearby because they run cool and quiet and pose much less of a fire hazard.

The most important factor in choosing between a dry type and a liquid-cooled transformer is the installation environment.

  • Is it an indoor or outdoor installation?
  • Are fire hazards a concern?
  • Do you need to avoid possible fluid leaks from an oil filled?
  • Will people be walking around near the site?
  • How much space is available?
  • Will the floor safely support the weight?
  • Is the installation space dry, well ventilated and free of dust and chemicals?

ELSCO dry type transformers are ideal for indoor installations with people nearby because they run cool and quiet and pose much less of a fire hazard. They can be installed indoors close to the load, which is an advantage in many industrial applications. They can also be installed outdoors inside a NEMA type 3 in moderate weather conditions, but will be only as reliable as the enclosure.

Dry type transformers require adequate surrounding space for circulating cooling air and maintenance access (NEC 450.21.) A transformer’s size dimensions, weight and location of infrastructure connections should be obtained from ELSCO and compared with your installation space.

Suitable indoor applications for air-cooled transformers include hospitals, universities, retail space, office buildings, and residential areas. Other applications include:

  • Environmentally sensitive areas,
  • Areas of high fire risk,
  • Performance halls and conference centers
  • Oil, chemical and gas industries
  • Renewable generation areas.

If the space where your transformer is to be installed meets the following requirements, then a dry type transformer will be an ideal choice for the application because all the advantages of a dry type will be realized in that environment:

  • Is indoors,
  • Overhead or side electrical connections,
  • Moderate temperature & humidity all seasons,
  • Clean ambient air due to HVAC,
  • Restricted access or a separate room,
  • Operates in proximity to people,
  • Needs quiet operation,
  • Needs to minimize fire hazard.

Dry Type Transformer Installation Tips

Dry Type Transformer Installation Tips

The following information applies only to indoor installations of ELSCO dry type transformers, which are also covered by NEC Article 450.21-28. Here are some additional tips on preparing for an indoor dry type installation. Outdoor installations of dry type transformers are covered by NEC 450.41-48.

Handling Upon Delivery

Lifting with hand trucks or forklift trucks is permissible if the forks are long enough to pass completely under the case. Since most transformers have a high center of gravity, extreme caution should be exercised when lifting or moving the units. Larger transformers will require a crane and a rigging crew to offload and move the unit. If a dry-type transformer is to be moved outdoors during inclement weather it should be thoroughly protected against moisture.


Storage areas should be as clean and dry as possible without extreme temperature variations. Protective wrappings should be left intact until the transformer is moved to its permanent location. Dry type transformers stored under damp or dusty conditions must be dried out and cleaned before being installed and energized.

Installation Location

Important factors in choosing the indoor location for a dry type transformers are:

  • Accessibility
  • Ventilation
  • Environmental conditions
  • Noise level

Other tips include:

  • The Installation area should be dry, free from dust, excessive moisture, fertilizers, chemicals and other corrosive fumes or vapors, and separated from flammable materials in accordance with NEC 450.21.
  • Access pathways to the location must have enough clearance to move the transformer in place. Avoid stairways and elevators if possible.
  • The floor area should be smooth and level, and able to support the weight (in pounds/sq. ft) of the transformer. Reinforced concrete is preferable.
  • Dry type transformers must be installed in a level upright position in an area with free air circulation. The minimum required clearances from adjacent equipment and structures are specified in NFPA 60 (NEC Article 450.21.) Installing the transformer in a NEMA enclosure locking it in a room for safety and security is strongly recommended.
  • Avoid locations that could be subject to flooding or exposed to moisture or ambient temperatures in excess of 30°C.
  • Do not install dry type transformers beneath wet fire suppression sprinklers. Use a dry or foam fire suppression system if needed.

Room Requirements

Indoor installations should comply with the applicable requirements of NEC Article 450.21. A room containing multiple dry type transformers should be large enough to permit sufficient space between them (as required by NEC 450.21) to permit air circulation. Sufficient space should be provided around each transformer to allow personnel access for routine inspection and maintenance.

Adequate ventilation is essential for the proper cooling of transformers. Room ventilation is part of the cooling system and should support normal circulation of air through the transformer, so that the transformer temperature will not exceed its nameplate rating. Clean, dry air is preferable. Filtered air at or above atmospheric pressure may reduce maintenance if dust or other contaminants are normally present. The required area of ventilator openings depend upon the height of the room, the location of openings and the maximum loads to be carried by the transformer. The room airflow should be evaluated by an HVAC technician to ensure compliance with NFPA 90A, sections 4.1 – 4.3 and IEEE C57.12.01 – 2020.

When transformers are located in rooms or other restricted spaces, sufficient ventilation should be provided to dispose of the transformer’s full-load heat loss without allowing a temperature rise higher than the nameplate rating. If necessary, forced air ventilation should be used to accomplish this.

Key Points in Maintenance for Dry Type Transformers

Key Points in Maintenance for Dry Type Transformers

The information presented here is only a summary of key points and is not comprehensive. The standards and requirements for maintenance and testing of medium voltage dry type transformers contained in NEC Article 450, Chapter 21.3 and IEEE Std. 57.12.91 should be followed.

1. Maintain a Temperature Log

Periodically read and record the transformer temperature controller at normal load and compare the reading with the log history.. All coils should read approximately the same temperature with a 3 phase load. A large difference or a persistent increase in operating temperature at constant load may indicate a problem with either the air cooling system or the transformer.

2. Perform Maintenance Only On De-Energized Units

Caution: The transformer must be de-energized (both primary and secondary circuits disconnected) and all terminals grounded before performing any inspection, cleaning, servicing, or testing on the unit. An Arc Flash suit should be worn when de-energizing the transformer, but once it’s de-energized, locked out, tagged out and grounded, an Arc Flash suit is not necessary.

2.1 Visual Inspection

Caution: This is a visual inspection only. It is not necessary to touch any part of the transformer when performing the inspection.

An air-cooled transformer requires periodic visual inspections. The frequency of inspection depends on how intensively the transformer is used. A transformer operating on load continuously 24/7 needs to be inspected more frequently than one operating on load only 8 hours 5 days a week. Since the transformer needs to de-energized for inspection and cleaning, maintenance should be scheduled during a normal shut-down period. Maintain a log of all observations and maintenance operations performed.

  • Look for adequate ventilation space around the transformer. Remove anything in the area that impedes airflow. Nothing should be stored in the transformer room. (NEC 450.
  • Open the enclosure and look for accumulation of dust or dirt on the surfaces of the transformer. These must be cleaned off while the unit is de-energized .
  • The presence of dust indicates that the transformer will require inspections at least every three to six months.
  • Look for any obvious discoloration of the windings which is a sign of overheating. Take a photo of the windings and report any discoloration to the plant engineer.

2.2 Cleaning the Unit

Caution: For safety reasons, physical contact with the transformer should be minimized during cleaning even if the unit has been de-energized , locked out and and grounded.

  • Clean any dust or dirt from the windings and cooling fans using either a vacuum or compressed air to avoid directly touching the components. Start by vacuuming off the parts followed by spraying them with dry compressed air with a pressure of no more than 20 to 25 psi. Do not use chemical cleaners to clean the surfaces of the unit.
  • Inspect all de-energized electrical connections, tighten all accessible hardware and replace any loose insulation or parts. This will prevent screws or nuts from working loose during regular operation.

2.3 Checking for Potential Problems or Defects

  • While cleaning the transformer, look for rust on the clamps and core steel, any carbonization or tracking on the windings and insulation.
  • Examine the windings and surfaces more closely for discoloration which could be a sign of overheating or a loose connection.
  • Check for cracked, discolored or loose insulators.
  • Check for loose, pitted, corroded or discolored electrical connections.
  • If possible, tighten or replace any hardware components; otherwise you should contact a professional to evaluate and repair the transformer.
  • Fans, motors and other auxiliary devices should be inspected and checked for proper operation.
  • Report all abnormal or suspicious conditions in writing to the plant engineer or facilities manager, who will determine if any additional testing or service is needed before re-energizing the unit.

3. Tests During Routine Maintenance

During routine maintenance, several tests of the transformer may be made addition to the inspection and cleaning. Performance of these tests are regulated by NEC 450.11.11. These tests are necessary only if discoloration, carbonization or other evidence of overheating or shorting is found. They will verify the correct operation of the unit and can detect issues that are not apparent from a visual inspection. Failure of any test requires further investigation and service before the unit is put back in service.

These tests are to be performed only while the transformer is de-energized (disconnected from both the primary and secondary circuits) and for safety and accuracy they should be performed only by a qualified engineer wearing protective clothing and using a Megger (or equivalent ) test kit.

3.1 Transformer Turns Ratio (TTR)

A TTR assesses the condition of the transformer windings and core. It tests whether the actual voltage ratio between the primary and secondary windings (for each phase and tap point) agrees with the nameplate or certified values. They should agree within  0.5% (NEC 450.11.2 & IEEC57.12.91.)

3.2 Insulation Resistance Test (IR)

The IR tests the integrity of the insulation between coil winding conductors. Each transformer rating has a minimum required resistance level measured in Mega- Ohms. Readings below this level indicate a degradation of dielectric strength. This is a warning sign of a potential transformer failure. (NEC 450.11.11.3)

3.3 Winding Resistance Test (WR)

The measured resistance of the coil windings for each the voltage taps being used should be within 1% of the factory tested value or the most recent test. (NEC 450/11.11.7)

The normal insulation and winding resistance levels of your ELSCO transformer can be obtained from the documentation supplied with the transformer, or by calling ELSCO at 800-232-9002 and supplying the transformer serial number.

3.3 Other Tests

Other diagnostic tests require special equipment and facilities, and should be performed experienced professional engineers.

ELSCO performs a series of certification tests on every new dry type transformer and documents the results in the certificate that is shipped with the unit. If you have questions about the test performance of your ELSCO dry type transformer or need advice regarding additional diagnostic testing, call ELSCO at 800-232-9002 and talk with one of our transformer engineers.

Useful Economic Life of ELSCO Dry Types

Ownership of an ELSCO dry type transformer is an investment in quality that pays off in four ways:

  1. Lower installed cost due to ELSCO’s simple installation protocol that can save thousands of dollars at the outset,
  2. Higher power efficiency that yields significant savings in annual utility bills,
  3. Lower maintenance costs of a high-quality dry type,
  4. Longer useful life – 35 years or more – due to a cooler design and high-performance insulation.

These benefit payoffs from an ELSCO dry type transformer result in a lower Total Cost of Ownership compared to alternatives.

ELSCO’s Warranties on Dry Type Transformers

At ELSCO, we offer two dry transformer warranties, depending on if the transformer was remanufactured or new. We offer a competitive five-year warranty for new transformers that protects against any complications from regular use or artistry defects.

For the dry type unit, you will need to install the system, ensuring proper airflow and circulation. If you are adding fan controls and thermocouples, you will need to use leads at least 6 inches away from the high voltage coils and bus bar.

We offer a warranty of three years for our remanufactured units. The warranty’s requirements for remanufactured systems are identical to new transformers. You will need to install the remanufactured unit correctly and situate the thermocouple leads away from the high voltage coils and bus bar.

Comparatively, a traditional warranty for a dry type transformer is just one year. At ELSCO, we attain a high level of reliability and provide extended warranties. We pride ourselves on using superior quality and high-quality components in our transformers.

Why Choose ELSCO for Dry Type Transformers

At ELSCO, we work with clients across numerous industries. We are a leading transformer provider offering high-quality systems and decades of experience. Key members of our expert team have over 22 years of experience in the transformer industry and can answer any questions you may have.

Our team uses their expertise to offer market-leading solutions. We also offer rush shipping with quick-ship units that can leave our facility in as little as 24 to 48 hours. Rush shipping is available for dry and oil filled padmount transformers.

We pride ourselves on a history of reliability in designing, manufacturing and shipping dry type transformers without warranty failures or issues related to design, quality, workmanship or materials. Our team is dedicated to upholding this with all future orders.

For our consumers, we offer an extended five-year warranty for dry type transformers. All of our new dry transformers come with a five-year warranty for indoor operation, going far above and beyond a standard one-year warranty.

Request a Quote on New Dry Type Transformers

ELSCO has been committed to repairing and rebuilding failed transformers from other companies for over 100 years. Our team has decades of hands-on experience working with failed transformers and restoring their function. In addition to dry type transformers, we offer padmount transformers and pre-owned transformers.

Contact us online to request a quote for new dry type transformers.
Request a Quote on New Dry Type Transformers