This paper presents a numerical analysis of losses generated in the tank wall surrounding the high-current bushings of pad-mounted transformers using a three-dimensional finite-element approach. Two cases are analyzed to study the impact of inserting small plates of different geometry (located near the high-current phases) on the reduction of tank losses. Significant reductions in stray losses occurring in the tank wall are obtained with plate inserts of low-cost. With the "T" configuration proposed in this paper we reduce the tank wall losses by a factor of 1/21 at rated current when compared with the case of tank wall without the stainless steel plate. A three-dimensional time-harmonic finite-element model is used to determine the losses in the tank wall. Two load loss tests were carried out on experimental transformers to validate the simulations and effectiveness of the low-cost inserts.