PURPOSE: To give the degree of freedom to the requirment IZ of each load of the subject transformer by a method wherein the sum of the %IZ (impedance) between a power source side first winding and a three-phase second winding and the %IZ between the first winding and a single-phase load side third winding is equalized to the %IZ between the second winding and the third winding.

CONSTITUTION: A three-wire winding is used for a common phase transformer 7, which is consisted of a winding 71 to be connected to the power source located between U and V, a winding 72 to be connected to a three-phase load 1 located between u and v and a winding 73 to be connected to a single-phase load 2 located between a and b. Also, the percent impedance between windings 71W72 is set at %IZ 12, the percent impedance between windings 72W73 is set at %IZ 23 and the percent impedance between windings 73W71 is set at %IZ 31, and when each percent impedance based on the same capacity standard is indicated, each winding is arranged in such manner that the formula of %IZ+%IZ31%IZ23 can be set up among themselves. Through these precedures, a three-phase load, a single load and the like, and impedance required by each load can be selected freely.