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A three phase transformer test facility has been used for measuring the localized power loss and flux distribution within the core. The flux and loss distributions in the T-joint of a core with simple double overlap joints have been investigated. The flux density distribution was measured by an array of search coils wound on individual laminations, and the localized power loss was found by measuring the initial rate of temperature rise using miniature thermocouples. For measurements made in the range of flux densities from 1.1 to 1.6 T, the inner edges of the yoke and limb laminations were the highest loss areas, up to 30% higher than the mean core loss. These regions of high power loss were found to be due to a high third harmonic flux density component being present. The fundamental component of flux density tended to follow the rolling direction of both yoke and limb laminations; however, some deviation from the rolling direction was present in the case of other harmonics.