An isolation transformer includes a transformer core. First and second through-bores extend through the transformer core from a first surface to a second surface. Each through-bore has an elongated profile with at least a portion of the elongated profile providing a respective flat winding surface. The flat winding surfaces are spaced apart by a central portion of the transformer core. The transformer is wound with a six-wire cable having a central non-conductive core. First, second, third, fourth, fifth and sixth conductive wires are positioned around and adjacent to the central non-conductive core in a substantially equally spaced angular relationship. The second conductive wire is positioned between the first conductive wire and the third conductive wire; and the fifth conductive wire is positioned between the fourth conductive wire and the sixth conductive wire. The conductive wires are twisted about the central non-conductive core at a selected twist density.