Grounding a service-supplied AC system after the service equipment is a code violation. The danger is that normal current returning on the grounded conductor will travel on the equipment grounding conductors. This is a parallel path for grounded conductor current. The equipment grounding conductors are not intended to carry normal current. They are often un-insulated, not protected from contact, and sized to carry current only for brief periods of time. Making a load-side grounding connection can energize enclosures, pipes and building steel, and pose a serious electrical hazard.
