A distribution transformer has a wound magnetic core (50) of overall circular shape and rectangular cross-section with between two and four overall rectangular shape electric coils (20, 30) extending through the core window. The coils (20, 30) are pre-formed and assembled so that their parts (20A, 30A) which meet form a circular section solid cylinder. A mandrel (40) is then located around this cylinder (20A, 30A) and continuous non-amorphous steel strip is wound thereon to form an unannealed, uncut wound magnetic core of axial length in the range 250mm to 1m.