Sizing equipment grounding conductors based on thermal damage curves

In solidly grounded power systems, where separate equipment grounding conductors are used inside conduit or other raceways, sizing the equipment grounding conductor (EGC) per the minimum specified in NEC Article 250-122, Table 25D-122, can result in an inadequate ground-fault circuit design. Depending upon the magnitude of the ground-fault current and the specific rating or settings of the overcurrent protective device, an EGC can be damaged or completely destroyed before an upstream overcurrent device can clear the fault. Maximum thermal damage to an EGC often results from low-level ground faults, where currents fall below the short delay pickup setting for the protective circuit breaker. It should be recognized that Table 250-122 of the NEC serves only as a guide in determining the minimum size of an EGC. This is emphasized by the note added beneath the cable, which states "Equipment Grounding Conductors may need to be sized larger than specified in this cable in order to comply with Section 250-2(d)." Section 250-2(d), states than an effective grounding path shall "have the capacity to conduct safely any fault current likely to be imposed on it." Unfortunately, Table 250-122 is often used as the only basis for selecting an EGC. Here we present an example that illustrates how selecting an EGC based only on Table 250-122 can result in a ground-fault circuit design that may violate Section 250-2(d)