This The National Spherical Torus Experiment (NSTX) has been designed and installed in the existing facilities at Princeton Plasma Physic Laboratory (PPPL). Most of the hardware, plant facilities, auxiliary sub-systems, and power systems originally used for the Tokamak Fusion Test Reactor (TFTR) have been used with suitable modifications to reflect NSTX needs. At present, the NSTX power system is feeding thirteen (13) circuits from the Power converters. An upgrade of the NSTX center stack is proposed. This entails a much higher Toroidal Field Current from 71.2kA to 129.8kA, and requires major configuration changes including doubling the number of parallel strings of rectifiers along with associated power loop changes. Also, three additional coils will be installed in the machine. The control and protection of the rectifiers have to be replaced to reflect state of the art features to enhance the performance. This paper describes the details of the proposed Power system for NSTX upgrade.