10 Factors To Consider When Specifying Transformers

Guidelines to specifying transformers

The following ten factors MUST be considered when specifying transformers //

  1. Kilovoltampere (kVA) Rating
  2. Voltage Ratings, Ratio, and Method of Connection (Delta or Wye)
  3. Voltage Taps
  4. Typical Impedance Values for Power Transformers
  5. Insulation Temperature Ratings
  6. Insulation Classes
  7. Sound Levels
  8. Effects of Transformer Failures
  9. Harmonic Content of Load
  10. Paralleling transformers

1. Kilovoltampere (kVA) Rating

Table 1 gives the preferred kVA ratings of both single-phase and three-phase transformers according to IEEE C57.12.00-2010, IEEE Standard General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers (ANSI).

10 Factors To Consider When Specifying Transformers
10 Factors To Consider When Specifying Transformers (photo credit:
Transformer nameplate
Transformer nameplate

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2. Voltage Ratings, Ratio, and Method of Connection

All the preferred kVA ratings in Table 1 are obviously not available as standard at all voltage ratings and ratios. In general, the smaller sizes apply to lower voltages and the larger sizes to higher voltages.

Voltage ratings and ratios should be selected in accordance with available standard equipment that is indicated in manufacturers’ catalogs. This is recommended, if at all possible, both from the viewpoint of cost and time for initial procurement and for ready replacement, if necessary.

In most small size commercial projects, the 208Y/120V secondary voltage is used because the majority of load is lighting and small appliances. A secondary voltage of 480Y/277 V, in addition to the 208Y/120 V circuits, may be required when loads are electric motors or have large lighting requirements.

Table 1 – Preferred Kilovoltampere Ratings

Single-phase [kVA]Three-phase [kVA]

Generally, a three-phase transformer secondary voltage should be selected at 480Y/277 V. This has become standard and is compatible with three-phase motors, which are now rated 460 V standard.

Under normal circumstances, a 460 V rating for the transformer secondary should not be selected unless the load is predominantly older motors rated 440 V and located close to the transformer. Phase-to-neutral 277 V circuits can serve fluorescent and high-intensity discharge (HID) lighting.

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3. Voltage Taps

Taps are used to change the ratio between the high- and low-voltage windings. Manual de-energized tap changing is usually used to compensate for differences between the transformer ratio and the system nominal voltage. The tap selected in the transformer should be based upon maximum no-load voltage conditions.

For example, a standard transformer rated 13 200 V to 480 V may have four 2.5% taps in the 13 200 V winding (two above and two below 13 200 V). If this transformer is connected to a system whose maximum voltage is 13 530 V, then the 13 530 V to 480 V tap could be used to provide a maximum of 480 V at no-load.

Tap changers are classified as follows //

On-load tap changers

Taps can be changed when the transformer is energized and loaded. These taps are used to compensate for excessive variations in the supply voltage. They are infrequently associated with commercial building transformers except as part of outdoor substations over 5000 kVA.

Load tap changers can be controlled automatically or manually.

No-load tap changers

Taps can be changed only when the transformer is de-energized. Tap leads are brought to an externally operated tap changer with a handle capable of being locked in any tap position. This is a standard accessory on most liquid filled and sealed-type transformers.

On very small liquid filled transformers and most ventilated-dry-type transformers, the taps are changed by moving internal links that are made accessible by a removable panel on the enclosure.

Manually adjustable (handle- or link-operable) taps are suitable for correcting long-term voltage conditions. They are not suitable for correcting short-term (hourly, daily, or weekly) voltage variations.

Automatic tap changing or voltage regulating transformers are relatively expensive so that one of the following solutions might be more appropriate //

  • Request improvement of the utility power supply regulation.
  • Segregate the circuits so that heavy variable loads are separated from more sensitive loads. When a source transformer constitutes a significant part of the impedance to a sensitive load, use a separate transformer (or secondary-unit substation) for such loads.
  • Use voltage regulating supplies for just the sensitive loads.

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4. Typical Impedance Values for Power Transformers

Typical impedance values for power transformers are given in Table 2. These values are at the self-cooled transformer kVA ratings and are subject to a tolerance of ±7.5%, as set forth in IEEE C57.12.00-1987 (ANSI).

Non-standard impedances may be specified at a nominally higher cost: Higher impedances to reduce available fault currents or lower impedances to reduce voltage drop under heavy-current, low-power factor surge conditions.

When specifying transformers, best would be to consult manufacturers’ bulletins for impedances of small transformers because they can vary considerably.

Table 2 – Transformer Approximate Impedance Values

Design impedance (percent)
High-voltage rating (volts)Low voltage,
rated 480 V
Low voltage,
rated 2400 V or higher
Power Transformers
2400 to 22 9005.755.5
26400, 34 4006.06.0
43 8006.56.5
67 0007.0

Rated kVADesign impedance (percent)
Secondary-Unit Substation Transformers
112½ through 225Not less than 2
300 through 500Not less than 4.5
Above 5005.75
Network Transformers
1000 and smaller5.0
 Above 10007.0

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5. Insulation Temperature Ratings

Transformers are manufactured with various insulation material systems (as shown in Table 2).

Performance data with reference to conductor loss and impedances should be referenced to a temperature of 40°C over the rated average conductor temperature rise as measured by resistance.

While Table 3 represents the limiting standard requirements, transformers with lower conductor losses and corresponding lower temperature rises are available, when longer life expectancy and reduced operating costs are desired.

A Class 105 insulation system allows for a 55°C rise with a total ultimate temperature of 105°C. A Class 120 insulation system allows a 65°C rise with a total permissible ultimate temperature of 120°C. An 80°C rise is allowed for a Class 150, a 115°C rise is allowed for a Class 185, and a 150°C rise is allowed for a Class 220. Materials or combinations of materials that may be included in each insulation material class are specified in IEEE C57.12.00-2010 (ANSI).

Table 3 – Insulation Temperature Ratings in °C

Average conductor temp. rise * 
Maximum ambient temperature
Hot-spot temperature differential *
Total permissible ultimate temperature
Class of insulation system
150 †4030220220

* Maximum at continuous rated load.
† Dry-type transformers using a 220°C insulation system can be designed for lower temperature rises (115°C or 80°C) to conserve energy, increase life expectancy, and provide some continuous overload capability.

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6. Insulation Classes

Voltage insulation classes and BILs are listed in Table 4.

Table 4 – Voltage Insulation Classes and Dielectric Tests

  Dry transformersOil immersed distribution transformersOil immersed power transformers
Nominal system voltage (kV)Insulation classBasic impulse level (kV)Low- frequency test (kV)Basic impulse level (kV)Low- frequency test (kV)Basic impulse level (kV)Low- frequency test (kV)

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7. Sound Levels

Permissible sound levels are listed in Tables 5 and 6. Transformer sound levels can be a problem in commercial building interiors, especially where relative quiet is required, such as in conference rooms and certain office areas.

Technical specifications can require transformer sound levels to be below those specified in these two tables.

The effects of transformer sound levels can be minimized by placing the transformers in separate rooms, avoiding direct attachment of transformers to structural members, use of sound isolating pads or vibration dampers for mounting, and avoiding the mounting of transformers near plenums or stairwells where the sound will be directed into work areas.

For large units, providing flexible connections from the transformer to long busway runs will reduce the transmission of vibrations.

Table 5 – Sound Levels for Dry-Type Transformers in dB

Equivalent two-winding
Self-cooled ventilated
Self-cooled sealed
Forced-air cooled ventilated
8334-10 00076

Columns 1 and 2 — Class AA rating, column 3 — Class FA and AFA rating.

Table 6 – Sound Levels for Single-Phase and Three-Phase Oil Cooled Transformers in dB

Equivalent two-winding
Without fans
With fans
7500-10 0007074

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8. Effects of Transformer Failures

Transformer failures are rare. However, in high-rise buildings and in other buildings where the conditions for evacuation are limited, the effects of the failure of larger transformers can be serious. Air from transformer vaults should be exhausted directly outdoors.

Dry-type transformers will usually be preferred to liquid filled transformers (even the less-flammable, liquid insulated types) where fire and smoke considerations are critical.

Well-designed transformer protection can minimize the extent of damage to any type of transformer. Dry-type transformers, including the cast-coil-type, if subjected to faults for an extended period, can burn and generate smoke. Liquid filled transformers can burst, burn, and generate smoke. Provisions can be made for dealing with these rare but still possible failure modes for large transformers in critical areas.

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9. Harmonic Content of Load

Very recent developments have indicated failures of certain types of transformers due to nonlinear loads, which cause third and higher harmonics to flow through the windings.

When these harmonics are present, due to loads like computers, variable speed drives, electronic ballasts, HID lighting, arc furnaces, rapid mode switching devices, and similar electrical loads, consideration should be given to specifying a special transformer that is designed to withstand these harmonic currents and the fluxes they produce in the cores.

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10. Paralleling transformers

When a transformer is able to be paralleled with another transformer, specifying %IR, %IX, and %IZ is required. Read more about matching transformers for parallel operation.

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Reference // Recommended Practice for Electric Power Systems in Commercial Buildings (IEEE STD 241)

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About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on


  1. Pradeep Kumar
    Apr 25, 2016

    Impedance Temperature Ratings and Voltage Ratings are some of the important things we need to consider in the specification of a transformer as they are the basic entities that determines the type of transformer for our needs and requirements.

  2. Manuel Bolotinha
    Mar 29, 2016

    To specify a power transformer it is also necessary to define type (oil immersed – with or without conservator – or dry type), type of installation (indoors; outdoors; pole mounted; pad mounted; etc.), standards that shall be accomplished, primary and secondary voltages, vector group, cooling method and built-on protections (gas, temperature and pressure of oil or windings temperature).
    In Europe, according to IEC standards preferred rated power for MV/LV transformers are: 25, 50, 100, 125, 160, 200, 250. 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000 and 2500 kVA.

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