(i) Step up transformer — The wire in the primary coil is thicker than that in the secondary coil. It is done because the current in the primary coil is more than in the secondary coil and a thick wire results in less resistance and therefore less loss of energy as heat.
(ii) Step down transformer — The wire in the secondary coil is thicker than that in the primary coil. It is done so because the current in the secondary coil is more than in the primary coil and a thick wire results in less resistance and therefore less loss of energy as heat.