Sizing of equipment grounding conductors based on thermal damage curves

Abstract

This paper addresses the sizing of equipment grounding conductors (EGCs) in solidly-grounded electrical power systems, where separate ground fault protection is not used and EGCs are run inside conduits or other raceways. It is shown that using only Table 250-95 or the NEC can result in inadequate sizing of the EGC. An example is presented which illustrates how an EGC can be damaged, or completely destroyed, before an upstream overcurrent device can operate to clear a fault. It is demonstrated that the use of thermal damage curves for EGCs can provide a simple, but effective, way to evaluate the adequacy of a ground fault circuit design.