Abstract: A dry insulated, impulse voltage resistant, multi-layer series wound coil having air ducts between layers for use in transformers, reactors, and the like is made with minimum solid insulation spacing between adjacent conductors and maximum air spacing radially between coil layers. The method of making includes winding on a form of alternate layers of insulation and conductors. After one layer of winding is complete, cloth-wrapped duct-forming molds are supported on the windings and further insulation and conductors are wound around the whole assembly. When the desired number of layers is complete, the assembly is vacuum cast in a resin and the forms and the molds are then removed.