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Full Description


This guide covers the economic loss evaluation of liquid-filled distribution and power transformers, dry-type distribution and power transformers, and reactors.


The purpose of this guide is to provide a method of establishing the dollar value of the electric power needed to supply the losses of a transformer (includes distribution and power transformers as well as reactors in this guide). Users can use this loss evaluation to determine the relative economic benefit of a high-first-cost, low-loss unit versus one with a lower first cost and higher losses. Manufacturers can use the evaluation to optimize the design and provide the most economical unit to bid and manufacture. The evaluated cost of losses also enables a user to compare the offerings of two or more manufacturers to aid in making the best purchase choice among competing transformers. Loss evaluation also provides information to a user for establishing the optimum time to retire or replace existing units with modern low-loss transformers. The user should determine, on a dollars-per-kilowatt basis, the sum of the present worth of each kilowatt of losses of a transformer throughout its life, or some other selected period of time. A portion of this evaluated cost can be paid to the manufacturer to reduce losses. However, this evaluated cost includes other costs associated with owning a more expensive piece of equipment, such as financing costs, taxes, etc. This guide provides formulas by which the costs of energy, power, money, and the loading pattern of a transformer can be converted to dollars-per-kilowatt values of the transformer losses. These dollars-per-kilowatt figures should be furnished to the manufacturer when bids are requested. If the final tested values of losses vary from the manufacturer's guaranteed values, economic adjustments may be made. Nothing in this guide is mandatory. It should not be inferred that the methodology described in this guide is the only valid methodology for computing the cost of transformer losses. Many users have developed their own transformer loss evaluation techniques that are suitable for the intended purpose. This guide uses publicly available information on wholesale electricity prices, transformer costs, and other dollar values. These numbers are provided for illustrative purposes only. Actual price (or cost) will vary by time and location. Each user of this guide should determine the price or cost values that it believes appropriate for making loss evaluations. This guide offers a methodology to determine, and thereby specify, the economic value of no-load, load, and auxiliary losses. The use of this guide allows manufacturers to tailor the design to the unique economic situation of each user, and allows the user to evaluate multiple designs.


Revision Standard - Active. The economic loss evaluation of liquid-filled distribution and power transformers, dry-type distribution and power transformers, and reactors is covered in this guide.

Document History

  1. IEEE C57.120-2017


    IEEE Guide for Loss Evaluation of Distribution and Power Transformers and Reactors

    • Most Recent
  2. IEEE C57.120-1991

    IEEE Loss Evaluation Guide for Power Transformers and Reactors

    • Historical Version