This paper presents the calculation method on the versatile spreadsheet software such as Microsoft® Excel® for instantaneous value of inrush current in three-phase transformers. This method was developed using simplified φ-L magnetizing characteristics and the Ralston's Runge - Kutta method. Inrush current simulation using this method was carried out. Consequently,simulated waveforms reproduced the measured waveforms for a three-phase transformer in field.