The current paper presents the validation of a finite element (FEM) design methodology for a CORE type power transformer. Any new methodology can only be implemented when it generates significant added value. The application of FEM to the design of power transformers leads to lighter, more balanced mechanical structures that are more economical to manufacture. To be sure that the methodology is a good predictor of the stress and strain effectively encountered by the power transformer, it is necessary to measure stress and strain during loading and compare those measurement values with the original FEM predictions. The current paper begins by first presenting the FEM predictions for a CORE type power transformer. Secondly, by means of extensometry, the strains induced in the tank during loading were measured and the stresses present in the tank were estimated from basic stress-strain relationships. The feedback given by the strain measurements served as a validation tool for the FEM design predictions.