Patent ID: 7831133

Claim:
A heating system for a subsurface formation, comprising: a first heater, a second heater, and a third heater placed in an opening in the subsurface formation, wherein each heater comprises: an electrical conductor, wherein the electrical conductor comprises: a ferromagnetic conductor; and an outer electrical conductor electrically coupled to the ferromagnetic conductor, the outer electrical conductor at least partially surrounding the ferromagnetic conductor, and the outer electrical conductor providing a majority of a resistive heat output of the heater at temperatures up to a temperature approximately 50° C. below a selected temperature; an insulation layer at least partially surrounding the electrical conductor; an electrically conductive sheath at least partially surrounding the insulation layer; wherein the electrical conductor is electrically coupled to the sheath at a lower end portion of the heater, the lower end portion being the portion of the heater distal from a surface of the opening; the first heater, the second heater, and the third heater being electrically coupled at the lower end portions of the heaters; and the first heater, the second heater, and the third heater being configured to be electrically coupled in a three-phase wye configuration.