During the summer of 1988, the coolers operating on the number-one transformer at the Niagara Mohawk New Scotland Substation were unable to maintain the temperature of the transformer oil at an acceptable level during a period of peak power and hot weather conditions. As a result of that incident, the Niagara Mohawk Power Corporation requested that the operation performance characteristics of the failed General Electric FOA oil transformer cooler be investigated by the Heat Transfer Laboratory at Rensselaer Polytechnic Institute. A theoretical and experimental analysis has been performed on the performance of a transformer cooler. The theoretical study involved the formulation of a model of the cooler, which predicted that the performance is extremely sensitive and dependent upon the air-side flow rate/heat transfer coefficient, as well as the available heat transfer area. The experimental work consisted of the design and implementation of a cooling loop, from which experimental data were obtained to confirm the accuracy of the predictions. The experimental results are in good agreement with the numerical predictions; therefore, they confirm the reliability of the analysis.