Spiraling of conductors in a helical winding of a transformer may lead to a catastrophic failure under the action of torsional short-circuit electromagnetic forces. Torsional forces are produced by the interaction of the axial component of the short-circuit current and the radial component of the leakage magnetic flux density. In this paper, 3-D magnetostatic analysis has been performed to determine the leakage magnetic-field distribution in a 130-MVA power transformer. Torsional electromagnetic forces are calculated from the analysis to determine circumferential displacements of conductors of its low-voltage helical winding. Furthermore, mechanical stress components are computed and a factor of safety is defined, which gives an indication as to whether the winding conductors are in the elastic or plastic zone. The numerical results thus obtained by the finite-element method are verified using the first principles from mechanics. The effect of looseness in the inner support structure on the von-Mises equivalent stress and, hence, on the factor of safety has been studied. © 1986-2012 IEEE.