It is usually calculated according to the formula of iron core cross-sectional area of double-winding transformer with the same capacity. Kd takes a value between 48 and 60.

The cross-sectional area does not match the capacity, which will cause iron core waste and increase the cost. If it is too small, the magnetic flux density in the core may work in the nonlinear region, and the reactance value will decrease with the increase of current.