A method and system for supply voltage regulation in a motor circuit protector (MCP) that includes a current transformer coupled to a rectifier and a stored energy circuit. A solenoid is actuated by that circuit when a sufficient voltage is present. A controller having a configurable input is coupled to the stored energy circuit. Upon startup of the motor circuit protector, the controller causes the stored energy circuit to be charged to a startup voltage level via secondary current from the current transformer. The controller periodically interrupts the charging to measure the secondary current to detect fault levels. During startup, the configurable input is set to a comparator input for rapid current measurements. During run mode, the configurable input is set to an A/D input for accurate measurements. The controller measures the voltage of the stored energy circuit while charging it to a power level sufficient to actuate the solenoid.