Russlk said:It is common practice to put the primary first and the secondary on top, I don't know why, it should not make a difference, performance wise.
I believe that's because most transformers are of the stepdown kind, where the primary has higher voltage (and therefore more turns) than the secondary. Winding on the outside always uses up more wire length than winding on the inside for the same number of turns. If the primary were on the outside, you would need a much longer wire than if you wound it inside. If the secondary were outside, the need for additional wire length would be less since the secondary has fewer turns. Of course, the opposite would be true in a step-up transformer.