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 250-122, 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.