Re: parallel raceway EGC size
I have found that following only the article 250-122 for sizing EGC is not enough. This is just the minimum requirement for an EGC (see title to table 250-122) that you must choose. But if the ground fault available is high enough, the EGC chosen by table 250-122 generally will not be protected by the breaker. Remember that the EGC is permitted to be sized smaller than the phase conductors. If for high fault currents the breaker will not protect the phase conductors, the EGC will be even in worst shape. Sometimes, the breaker will protect the phase conductors for the available ground fault, but will not protect the EGC. The NEC establishes that all conductors should be protected against ANY current likely to be imposed on them, including high ground fault currents. So to make sure that you have specified the right EGC, you should know the available ground fault current, the ampere rating and time of operation of the breaker (for the ground fault current) and the damage characteristics (damage curve) of the EGC.
NEC 110.10 Circuit Impedance and Other Characteristics.
The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the grounding conductor or enclosing metal raceway.
NEC 240.1 Scope.
FPN: Overcurrent protection for conductors and equipment is provided to open the circuit if the current reaches a value that will cause an excessive or dangerous temperature in conductors or conductor insulation. See also 110.9 for requirements for interrupting ratings and 110.10 for requirements for protection against fault currents.
NEC 110.10 Circuit Impedance and Other Characteristics.
The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the grounding conductor or enclosing metal raceway
NEC Definitions:
Bonding (Bonded). The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed.
