This paper analyzes the difference between zero-sequence magnetizing impedances (Z(0M)) of a 3-phase core-type transformer. Z(0M) in per-unit is smaller for an external winding than foran internal winding. The difference between both Z(0M) values is similar to the positive-sequence short-circuit impedance between both windings for units without magnetic shunts on the tank wall as well as for units without tank. This fact can be observed in measured values, and in the results of 3D models. However, the results of simple 2D models do not show this behavior; therefore, this paper demonstrates that this fact only can be explained from 3D models. The results are shown for different cases: transformers with and without magnetic shunts on the tank wall, and transformers without tank. In transformers with magnetic shunts on the tank wall, or in magnetostatic simulations of transformers with tank, the difference between both Z(0M) values is not very similar to the positive-sequence short-circuit impedance between both windings.