This paper introduces a semi-analytical method of predicting AC loss in commercial Litz wires. Simple finite-element simulations are used to compute the resultant proximity fields in a coil system of arbitrary geometry. The approach addresses non-ideal Litz wire construction by applying a surrogate skin effect model to inform and distribute the current over the cross-section into segmented layers. This simplifies the finite-element problem into a two-dimensional, DC simulation with a low number

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... of mesh elements. Analytical solutions are then used to compute frequency-dependent loss due to skin and proximity effect. The method is demonstrated using two 14 AWG equivalent Litz wires with very different constructions and is validated with experimental results from several coil configurations. Finally, an appeal is made to commercial Litz wire manufacturers to provide an empirical "fabrication factor" specification that would allow consumers to predict the performance of a conductor in their application.