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□ Now, if this kind of topic interests you, why not have a look at our free online webinars? □ In practice both 2D and 3D models have their use cases, and with COMSOL you can study both. The longitudinal B field too, will not show up in a 2D model. Notice how the numeric AC resistance starts off slightly lower than the analytically determined DC resistance. Some effects will not naturally follow from a 2D simulation, though. Admittedly, much of the problem can be analyzed in 2D as well - when the strands have been properly twisted and the frequencies are within reasonable limits. The simulation and the rendering have been performed in full 3D with COMSOL Multiphysics. When the animation switches to a 3D flyover, the magnetic fields are shown too: First the azimuthal field Bφ that encircles the bundle, and then the longitudinal field Bz (forming the star shapes). The 1D graph shows the skin depth with respect to the strand radius, and the effective AC resistance of the bundle. At low frequencies the current simply oscillates, but at high frequencies a pattern emerges (as dictated by the skin effect, and the proximity effect). The animation shows the currents in the strands, where yellow is "positive" and blue is "negative". This design is typically used in high voltage power cables, often carrying hundreds of amps. An extreme case is the Milliken conductor, where strands are twisted in large groups and then compressed together. So then the question is would it be OK to have a less perfect mixing, to allow room for a few extra strands? What compromise is best, will depend on the application. Also, a super helix has a lot of air pockets in it, meaning that only a small portion of the cross section is copper. If the strands are twisted, they will follow a helical path that is longer than the bundle itself. Twisting does not come for free, however. To make sure that the currents are uniformly distributed the strands will need to be "mixed", and the best mixing is given by a super helix: a twist of twists. A bundle of straight strands will not work, though: the outer strands will simply carry all the current, giving rise to the same problem as before. Since the effect is less pronounced for thin conductors, it makes sense to replace one thick wire with many thin strands and bundle them together. This makes the conductor less effective at high frequencies. The skin effect is the tendency of an alternating current to flow near the surface of a conductor, leaving the center with little or no current. Litz wires are designed to reduce the skin effect. When looking at litz wires in more detail, you soon realize there is more to it than twisting a few copper strands together.