Foreword?(This foreword is not a part of IEEE Std C67.120-1991@ IEEE Loss Evaluation Guide for Power Transformers and Reactors.) The users of power transformers and reactors have become more concerned about the value of losses as the cost of energy and of installing generating capacity has increased. The evaluation of losses has become a very significant part of the purchase decision for some users. This guide has been written to provide a method of establishing loss evaluation factors for transformers or reactors. With loss evaluation factors@ the economic benefit of a highfirst- cost@ low-loss unit can be compared with a unit with a lower first cost and higher losses. This enables a user to compare the offerings of two or more manufacturers to aid in making the best purchase choice among competing transformers or reactors. Loss evaluation also provides information to establish the optimum time to retire or replace existing units with modern low-loss transformers or reactors. Purpose and ScopeThe 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 or reactor. 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 or reactors. Loss evaluation also provides information to a user for establishing the optimum time to retire or replace existing units with modern low-loss transformers or reactors. 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. This figure represents the maximum amount that can be spent to save a kilowatt of loss. 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@ and 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 manufacturerapos;s guaranteed values@ economic adjustments may be made. Nothing in this guide is mandatory. It should not be inferred from this paper that the methodology described in the following pages 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. The list of terms in Section 2 uses symbols selected for mnemonic effectiveness and might be different from symbols used in other references.