The Dowell model is widely utilized for copper loss analysis of forward transformers. However, this model is not directly applicable to transformers with parallel-connected windings. The reason is that the Dowell model requires determining the AC current of all the windings in advance of analysis, although the AC current distribution in parallel-connected windings is severely affected by disposition of the windings. This issue is addressed in this paper by employing the Dowell model in combination with a novel insight that the AC current is distributed in parallel-connected windings to give an extremum of the magnetic co-energy of the transformer. Simulation and experiment were carried out; and, the results supported appropriateness of the proposed copper loss analysis method.