A detailed mathematical model based on circuit approach has been developed for simulating inrush current for core type transformers. The model is capable of computing inrush current for various phase connection schemes under different initial conditions. A three phase thyristor controlled switch has been designed and constructed to be used in consistent recording of inrush current at zero-crossin and positive going voltage. Also a method for determining the zero-sequence permeance of the transformer was introduced. The model can be applied to power transformers for inrush current investigations which are necessary for designing system protection relays.