The grounded conductor at the service provides two essential functions for the premises wiring system. The first is to serve as a current-carrying conductor for the load supplied. Second, it functions as the intentionally constructed, low-impedance and effective ground-fault current path during a ground-fault event as addressed in Section 250.4(A)(5). This is an essential element of effective overcurrent protective device operation for any ground fault that occurs in grounded systems. This is why 250.24(C) requires that the grounded conductor be brought to a service disconnecting means enclosure and bonded to the enclosure. This requirement applies for grounded systems, whether or not the grounded conductor supplies a load.
The grounded conductor of a service is usually a neutral conductor, but it can also be a phase conductor, depending on the type of system supplied. For example, a corner-grounded delta system has a grounded phase conductor and no grounded neutral conductor present. The grounded neutral conductors typically carry the maximum unbalanced neutral current to the system neutral point.
The term “neutral” can relate to either a conductor or a system-connection point. “Neutral conductor” and “neutral point” are defined in Article 100. These definitions help determine what constitutes system neutrals and the conductors to which they are connected. System neutral conductors are usually grounded, but not all grounded conductors are system neutrals. At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that use the neutral, with respect to the neutral point, is zero (ground) potential.
During normal operation, the grounded (neutral) conductor is carrying any unbalanced neutral current to the source windings. The same load characteristic applies to grounded phase conductors, except a grounded phase conductor typically carries the same current as the ungrounded phase conductors, as would be the case for a three-phase motor supplied by a corner-grounded system. Section 250.24(C)(3) requires the grounded conductor of a three-phase, 3-wire, delta-connected service to have an ampacity not less than that of the ungrounded conductors of this service.
The NEC generally restricts grounding connections only to those required or permitted on the line side of and up to the service disconnecting means enclosure. This restriction applies on the load side of the grounding point at the service equipment, or the point where the main bonding jumper connection is made in the service equipment.
Section 250.24(A)(5) restricts load-side grounding connections to the grounded conductor. Connections on the load side of the service disconnecting means are not permitted. The same restriction is included for separately derived systems, as provided in 250.30(A).
The informational notes following 250.24(A)(5) and 250.30(A) indicate a few other installations where using the grounded conductor for grounding is permitted. Load-side grounding of the grounded conductor is generally prohibited to minimize the paths that current can divide between while returning to the source windings. Once the grounded conductor(s) leaves the service equipment enclosure routed with either feeders or branch circuits, installers must not connect the grounded conductor to ground or to grounded metal enclosures. Doing so creates multiple paths for current to return to the source.
This is often referred to in the field as parallel paths for neutral current. The objective is to isolate grounded conductors, often neutral conductors, from ground and grounded parts everywhere but where the service or system is initially grounded. If this rule is not complied with, current will be introduced on conductive parts, raceways and equipment that are not intended to carry current during normal operation. This condition is a common cause of many power quality problems experienced today.
There are very few allowances for load-side grounding connections to the grounded conductor of services and premises wiring systems. Obviously, these are permitted for separately derived systems because a new system-grounding point is established at the source. There are also allowances for use of the grounded conductor to ground equipment, such as existing ranges and dryer installations, as provided in restrictive conditions in Section 250.140. Using the grounded conductor for grounding at separate buildings or structures was recognized in the NEC prior to the 2008 edition. However, the trend now is to migrate away from using the grounded conductor for equipment grounding purposes on the load side of the service disconnect or the load side of a grounding point for a separately derived system.
To meet the NEC minimums for grounded conductor connections, exercise care in the terminations of grounded (neutral) conductors and respect the performance concepts discussed above by keeping the neutrals and grounding connections separated in wiring installations on the load side of the service main bonding jumper or system bonding jumper for a separately derived system.