Skip to main content

What does ONAN mean?

“Oil Natural Air Natural” – Natural convectional circulates the flow of oil for cooling.

  • What are the Advantages and Disadvantages to Using a Fan-Cooled Transformer?


      • Smaller size; fans may add some height but may reduce width and depth
      • Lower costs for larger units (generally above 1000kVA) to add fans instead of conductor and core
      • Potentially better low-load efficiencies



      • Increased complexity and maintenance
      • Increased cost as fan packages may cost more than just adding material in smaller units
      • Additional energy losses and noise when fan motors are operated in higher loads

  • Can transformers be operated above a 1000m/3300′ altitude?

      There are two main considerations for operating transformers at altitudes above 1000m/3300′. Current standards state designs must be valid to these heights. Above this height, the density of air no longer works as effectively to remove heat. As a result the functional kVA of the transformer must be reduced at higher altitudes, typically about .3% for every 100m/330′. The second issue is the dielectric constant of air is reduced at higher altitudes. Dry type transformers use air gaps as an important component of the electrical insulation properties. At higher altitudes, this lower insulation values, typically in medium voltage BIL levels. Ideally, if transformers will be installed above 1000m, inform the manufacturer and the design can be adjusted to meet all requirements at the higher altitudes.

  • What is ANSI C57.12.91?
  • What is ANSI C57.12.51?

      IEEE Standard for Ventilated Dry- Type Power Transformers, 501 kVA and Larger, Three-Phase, with High- Voltage 34.5 kV to 601 V and Low- Voltage 208Y/120 V to 4160 V covering General Requirements. The current standard was updated in 2008.

      This standard is intended to set forth characteristics relating to performance, limited electrical and mechanical interchangeability, and safety of the equipment described, and to assist in the proper selection of such equipment. Specific rating combinations are described in the range from 750/1000 to 7500/10 000 kVA inclusive, with high-voltage 601 to 34 500 volts inclusive and low-voltage 208Y/120 to 4160 volts inclusive. Part I of this standard describes certain electrical and mechanical requirements and takes into consideration certain safety features of 60-Hz, two-winding, three-phase, ventilated dry-type transformers with self-cooled ratings 501 kVA and larger, generally used for step-down purposes. Part Il describes other requirements or alternatives which may be specified for some applications and lists forced-air-cooled ratings for certain sizes.

  • What does Dry-Type Self-Cooled Transformer Class AA mean?
  • What does Dry-Type Self-Cooled Force Air-Cooled Transformer Class AA FA mean?

      A dry-type transformer that has a self-cooled rating with cooling obtained by the natural circulation of air and a forced-air cooled rating with cooling obtained by the forced circulation of air. This is sometimes referred to as fan-cooled. Fan cooling can increase a transformer’s kVA rating by 25% to 50% depending on the type and size of the transformer.

  • What does Combustible Materials mean?
  • What is NEMA ST 20?
  • What does 50/60 Hertz mean?

      Transformers that are designed to specifically run at 60 Hz can’t be run at 50 Hz or in some cases only with significant derating. Magnetic flux is proportional to frequency so a 50 Hz transformer has a core 20% larger to handle 20% more magnetic flux than a 60 Hz unit. A 50 Hz transformer will simply run cooler at 60 Hz given the proper voltage is applied. Transformers cannot change frequency, the primary frequency equals the secondary frequency.

  • What is U.L. 1562?

      U.L. 1562 covers medium voltage dry-type transformers:

      1.1 These requirements cover single-phase or three-phase, dry-type, distribution transformers, including solid cast and resin encapsulated transformers. The transformers are provided with either ventilated or non-ventilated enclosures and are rated for a primary or secondary voltage from 601 to 35000 V.

      1.2 These transformers are intended for installation in accordance with the National Electrical Code, ANSI/NFPA 70.

      1.3 These requirements do not cover the following transformers:

      1. Instrument transformers
      2. Step-voltage and induction voltage regulators
      3. Current regulators
      4. Arc furnace transformers
      5. Rectifier transformers
      6. Specialty transformers (such as rectifier, ignition, gas tube sign transformers, and the like)
      7. Mining transformers
      8. Motor-starting reactors and transformers

      1.4 These requirements do not cover transformers under the exclusive control of electrical utilities utilized for communication, metering, generation, control, transformation, transmission, and distribution of electric energy regardless of whether such transformers are located indoors, in buildings and rooms used exclusively by utilities for such purposes; or outdoors on property owned, leased, established rights on private property or on public rights of way (highways, streets, roads, and the like).

  • What is U.L. 1561?

      UL1561 covers 600 Volt Class Transformers:

      1.1 These requirements cover:

      1. General purpose and power transformers of the air-cooled, dry, ventilated, and non-ventilated types to be used in accordance with the National Electrical Code, ANSI/NFPA 70. Construction types include step up, step down, insulating, and autotransformer type transformers as well as air-cooled and dry-type reactors


      1. General purpose and power transformers of the exposed core, air-cooled, dry, and compound-filled types rated more than 10 kVA to be used in accordance with the National Electrical Code, ANSI/NFPA 70. Constructions include step up, step down, insulating, and autotransformer type transformers as well as air-cooled, dry, and compound-filled type reactors.

      1.2 These requirements do not cover ballasts for high intensity discharge (HID) lamps (metal halide, mercury vapor, and sodium types) or fluorescent lamps, exposed core transformers, compound-filled transformers, liquid-filled transformers, voltage regulators, general use or special types of transformers covered in requirements for other electrical equipment, autotransformers forming part of industrial control equipment, motor-starting autotransformers, variable voltage autotransformers, transformers having a nominal primary or secondary rating of more than 600 volts, or overvoltage taps rated greater than 660 volts.

      1.3 These requirements do not cover transformers provided with waveshaping or rectifying circuitry. Waveshaping or rectifying circuits may include components such as diodes and transistors. Components such as capacitors, transient voltage surge suppressors, and surge arresters are not considered to be waveshaping or rectifying devices.

  • What is ANSI C57.12.01?
  • How do HPS Line Reactors handle Heat Dissipation?
  • What does ONAN mean?
  • What does ONAF mean?

      “Oil Natural Air Forced” – Natural convectional circulates the flow of oil for cooling.  Fan forced air is applied to the cooling surface of the enclosure to improve cooling.

  • What does OFAF mean?

      “Oil Forced Air Forced” – Fan forced air is applied to the cooling surface of the enclosure to improve cooling.  In addition, an oil pump further forces additional oil flow to aid in cooling.

  • What is the hot spot allowance of a transformer?

      The coils of a transformer core do not evenly heated during energization. Parts of one coil will be hotter than the surrounding areas because they are farther from any ventilated openings or closer to the core which also produces heat. The hot spot allowance is a set number as defined by industry standards and is associated with the insulation class. Typically the smaller the transformer, the lower the insulation class and the more uniform the heating.

      • 105C Insulation System: 10C Hot Spot Allowance
      • 150C Insulation System: 30C Hot Spot Allowance
      • 180C Insulation System: 25C Hot Spot Allowance
      • 220C Insulation System: 30C Hot Spot Allowance

      The Hot Spot Allowance is added the expected ambient temperature and full load temperature rise to get the total expected temperature rise of a transformer.

  • Why Are There Physical Clearance (Distance) Requirements on the Nameplate?

      A ventilated transformer’s physical clearance requirements are designed to provide adequate clearance for airflow cooling. Generally, the larger the transformer the more airflow and clearance is needed. The more important areas are the front and back of ventilated transformers where the air may enter in the bottom and exit at the top. Since the sides of distribution transformers generally don’t have ventilation openings, side clearance is less important. There must also be no obstructions that limit airflow into the bottom vents and top clearance from ceilings must also be maintained.

      All smaller clearances should be reviewed by the manufacturer to verify they are adequate. Also note that electrical codes require minimum front panel clearances to allow safe and easy access to the wiring area. Units supplied with factory installed wall-mounting brackets may also have the back closer to the wall than the nameplate requirements; this is acceptable.  These statements may not apply to non-ventilated and/or potted transformers which don’t have ventilation slots and/or may have zero clearance when mounted to wall suing supplied brackets.

  • How does a UPS transformer’s efficiency affect the overall system efficiency?

      ASHRAE 90.4 Section notes that UPS transformer’s efficiencies at given loads must be included in the total losses for evaluation. Active single feed systems should be evaluated at 100% and 50% ITE load while active dual feed systems should be evaluated at 50% and 25% loads.

  • What does ANN mean?
  • What does ANN/AFN mean?

      “Air Natural Convection Cooling plus Forced Air Cooling” – Fans force more air to circulate through a dry-type transformer to cool the unit typically adding 33% additional capacity verses a similar non-fan cooled unit.

  • What does ANC mean?

      “Air Natural Convection Total Enclosed” – The dry-type transformer is enclosed in a non-ventilated enclosure.  Heat is transferred through the sides of the enclosure to the air to cool the unit.

  • What is the duty cycle of a transformer?

      Duty cycle is the amount of load over set periods of time. Transformers are designed to run continuously at full load without exceeding the insulation temperature limits provided that parameters such as ambient temperature, harmonic distortion, power factor, etc., are met. Transformers can also be designed to run for short term duty cycles which may result in a smaller unit. Short term duty cycles will need lower or no-load periods to aid in cooling. High load duty cycles can affect parameters such as impedance and voltage regulation.

  • What is the maximum surface temperature of a transformers terminals wire leads or connections points?

      Per NEMA ST-20 (2014):

      • Transformers <= 10 kVA can be up to 50C above ambient with a typical maximum ambient of 25C.
      • Transformers > 10 kVA can be up to 35C above ambient with a typical maximum ambient of 40C.
      • Transformers with lower than standard temperature rises will have lower maximum connection point temperatures.

  • What is the lowest temperature transformers can be stored in?

      Dry-type transformers with the exception of Cast Coil (HPS Endura Coil) can be stored to temperatures of -50 degrees C. Cast Coil Transformers can only be stored to temperatures of -20C. If a standard cast coil transformer is stored below -20C, the epoxy coils can crack. Transformers which are energized before 0C can be run in ambient temperatures to -40C.

      There are two main concerns with low temperatures occuring during storage when the units are not energized. Energized transformers have load and no-load losses which will keep the core and coils operational to -40C:

      • Contraction and Expansion of the core and coil during low temperatures can crack or damage the insulation.
      • Cold temperatures can cause condensation to form on the transformer which can result in short circuits and insulation damage.

      The minimum ambient temperature designed for is -40C based on the Environment Canada data Extreme Minimum Temperatures for Wiarton, ON (-36.4C on 18-Jan-1977) and Lucknow, ON (-36.7C on 05-Feb-1918). If the temperature of a transformer’s core and coil is below -25C (typically during unenergized storage), please consult HPS for the cold start procedure before any energization. If the temperature of a transformer is below 0C, please follow the HPS Cold Start Procedure. It is recommended that the transformer be meggered to make sure it has a minimum value of 100 Megaohm before energization after storage.