For a given conducting material insulation required depends on the voltage. Hence if High Voltage (HV) winding is placed near to the transformer core, more insulation is required to insulate between the transformer core and the High Voltage (HV) winding. This results in increase in the cost of the insulation material and also size of the transformer increases significantly. Thus Low Voltage (LV) winding is placed near the core which requires less insulation between the core and LV winding.