Grounding methods and requirements for systems operating at more than 1,000 volts (V), such as 5- and 15-kilovolt (kV) systems, differ slightly from those for systems of 1,000V or less. Systems in these voltage ranges are commonly referred to as medium-voltage systems. The NEC provides several rules related to the grounding of these systems and associated equipment. Part X of Article 250 provides the rules for grounding and bonding systems of more than 1,000V. The reasons for grounding systems of more than 1,000V are the same as the reasons for grounding on systems of 1,000V or less. If systems of more than 1,000V are grounded, the requirements in 250.182 through 250.191 must apply accordingly, depending on the type of grounding used for the system. Grounding and bonding provisions in Parts I through IX are only modified or supplemented by Part X of Article 250.
A few grounding methods are permitted to ground systems greater than 1,000V. These systems can be solidly grounded, grounded through an impedance device, grounded through surge arrestors, or grounded through a set of grounding transformers that create a reference to ground. Part X of Article 250 provides specific rules for systems grounded at a single point and systems that are grounded at multiple locations. The following are general requirements for solidly grounded, single-point grounded and multipoint grounded neutral systems.
A solidly grounded electrical system has a direct electrical connection to ground with no intentional impedance installed between the earth connection and the system. A commonly grounded system operating at greater than 1,000V is a 4,160V, three-phase, 4-wire, wye-connected system. In this system, there is a derived neutral that is the grounded conductor. The requirements for grounding such systems are found in Section 250.184(A). Generally, the neutral of such systems must be an insulated conductor with 600V-rated insulation. Bare neutral conductors of such systems are only permitted if it is installed with service entrance conductors, or if installed with a service lateral, or if installed with the direct-buried portion of a feeder. The neutral conductor of solidly grounded neutral systems can also be bare when installed as overhead conductors. Only the portion installed overhead is permitted to be bare in this case. Exception No. 3 to Section 250.184 also permits a bare neutral conductor for solidly grounded neutral systems if the neutral is isolated from the phase conductors and protected from physical damage. See exceptions 1 through 3 to Section 250.184.
The neutral conductor of a solidly grounded system must have sufficient current-carrying capacity for the load served and, generally, must not be smaller than one-third of the ampacity of the ungrounded phase conductors supplied by the system. By exception, the NEC permits the neutral for these systems to be sized no smaller than 20 percent of the ungrounded phase conductor ampacity only in commercial and industrial establishments where conditions of engineering supervision are in place.
Single-point grounding means the system is grounded at only one point and no neutral-to-ground connections can be made downstream of that initial connection location. In a single-point grounded neutral system, the neutral typically is grounded at the source, such as at a transformer. The connection to the earth for a single-point grounded neutral system is made through a grounding electrode meeting the requirements in Part III of Article 250. A grounding electrode conductor is required from the neutral conductor of such systems to the grounding electrode.
An equipment grounding conductor (EGC) is typically run with the feeders and branch circuits from the single point of system grounding and then connected to equipment required to be grounded. The EGC must be routed with the ungrounded phase conductors of the system and cannot carry any continuous load current. This EGC can be insulated or bare and must have current-carrying capacity for the maximum fault likely to be imposed on it. A word of caution: ribbon shielding or metal tape shielding on medium- and high-voltage cables is usually not of sufficient size to serve as an EGC. The shielding serves a different purpose.
Multigrounded neutral systems
The NEC also addresses multigrounded neutral systems. As the term implies, there are multiple grounding points to the neutral of such systems. In these systems, the neutral typically is derived and grounded at the source and then distributed for long distances, usually outdoors. Grounding is required from the neutral at multiple points along its route at distances not exceeding 1,300 feet.
Three common applications for multigrounded neutral systems are installations where the system supplies buildings or structures, such as in a campus distribution system. Multigrounded neutral systems also are permitted for use in underground systems where the neutral conductor is exposed and run as an overhead circuit conductor between poles, for example. The complete rules for multigrounded neutral systems are provided in Section 250.184(C).