Ground fault current protection of a 480Y/277V power system compliance with NEC design requirements

The NEC requires that a ground-fault protection of equipment be added when the building service disconnecting rating becomes 1000A or higher. To meet such a requirement, generally, the main service breaker is specified with ground-fault protection capability. This type of breaker provides ground-fault protection to equipment on the load-side of the main breaker, but does not provide protection for a ground-fault on the line-side of the main service breaker. Such a ground-fault may be on the service feeder connections at the main breaker or at the building service transformer. This paper provides a ground-fault protection relay scheme that can provide desired ground-fault protection of the building service feeders. Discussion on the building switchboard bus-rating, when a photovoltaic (PV) ac inverter has connection to the switchboard per NEC 705. 12. is also included.