Now we will discuss the savings of copper in auto transformer compared to conventional two windings electrical power transformer. We know that weight of copper of any winding depends upon its length and cross - sectional area. Again length of conductor in winding is proportional to its number of turns and cross - sectional area varies with rated current. So weight of copper in winding is directly proportional to product of number of turns and rated current of the winding.
Therefore, weight of copper in the section AC proportional to (N1 − N2)I1