Abstract: Disclosed is a helically wound core of ferrous material used as a differential current sensor core in the ground fault sensor of a ground fault interrupter circuit. The ferrous material comprises a single strand of wire which is wound in helical fashion to create a tubular shaped core comprising a series of wire loops (all part of the single strand) parallel to each other and running the length of the tubular shape. The core is placed around a pair of current carrying lines to be monitored for ground faults (one line leading to and one line leading away from the power source) to interact with the magnetic fields of the lines. Toroidally wound leads wrapped around the wire core act as a secondary and are connected to a ground fault interruption circuit to shut off the power to the conducting lines in the event that the sensor detects a difference in current in the lines.