Topic relevance is due to necessity of the correct mathematical expressions during the calculation of constants of equivalent circuit of power supply system, including the overhead transmission lines. Research purpose is the allowances thanks to which formula for capacity admittance calculation and of ground fault current in electrical grids with insulated neutral determination were deduced and made common use. As exemplified by numerical computation accepted allowances can lead to significant error (more than 15%), for example, during the determination of single phase-to-ground fault current in electrical grids with insulated neutral.