Part X of Article 250 in the National Electrical Code (NEC) provides the grounding requirements for systems and circuits of greater than 1,000 volts (V). There are various methods to accomplish the grounding required for medium- and high-voltage systems, and Part X also includes important rules for grounding medium-voltage cable shields. The NEC provides essential grounding and bonding rules for systems and equipment over 1,000V, and there are new and more specific rules for providing an effective ground-fault current path for services. This article is based on the 2014 edition of the NEC.
Effective ground-fault current path
Section 250.186 includes requirements for providing an effective ground-fault current path from the supply source to the service equipment. NEC Article 100 defines the term effective ground-fault current path as “an intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors.”
Currently, Section 250.24(C) requires a grounded conductor to be brought to the service for grounded systems 1,000V or lower. This conductor’s main purpose is to ensure that a low-impedance path is provided for ground-fault current to return to the utility supply transformer or source and facilitate overcurrent device operation during a ground-fault event.
The same need exists for services and systems larger than 1,000V, but previous Code editions had no specific requirements. Some jurisdictions previously relied on the performance requirements of 250.4(A) and (B) to ensure that a suitable ground-fault return path was provided. As revised, this section establishes a clear requirement for installing suitable, effective ground-fault current paths for services and systems over 1,000V and accounts for instances where the serving utility may or may not provide a neutral (grounded) conductor with its distribution system.
Where a utility does not provide a neutral conductor, there is generally a static line or other ground-fault return path where the supply-side bonding jumper can be connected to complete the return circuit. This new provision enhances consistency with essential performance requirements in 250.4 for both grounded systems and ungrounded systems above 1,000V.
Grounded systems supplying a service
For grounded systems, 250.186(A) requires a grounded conductor to be installed and routed with the ungrounded conductors to each service disconnecting means. This grounded conductor must be connected (bonded) to the service disconnecting means using a main bonding jumper. The minimum size required for the grounded conductor must not be less than the required grounding electrode conductor and not smaller than the sizes specified in Table 250.66. If the size of the ungrounded service conductors exceeds the values in Table 250.66, use the 12.5 percent rule for establishing the minimum size. Sections 250.186(A)(1) and (2) provide minimum sizing requirements for the grounded conductor when routed in a single raceway or enclosure or when routed in a parallel arrangement to the service equipment.
Ungrounded systems supplying a service
For ungrounded systems, there is no intentionally grounded conductor. Section 250.186(B) requires a supply-side bonding jumper to be installed and routed with the ungrounded conductors to each service disconnecting means. This supply-side bonding jumper must be connected (bonded) to the equipment grounding conductor (EGC) terminal bus in each service disconnecting means. The minimum size required for the supply-side bonding jumper must not be less than the required grounding electrode conductor and not smaller than the sizes specified in Table 250.66. If the size of the ungrounded service conductors exceeds the values in Table 250.66, use the 12.5 percent rule for establishing the minimum size. Sections 250.186(B)(1) and (2) provide minimum sizing requirements for the supply-side bonding jumper when routed in a single raceway or enclosure or when routed in a parallel arrangement to the service equipment.
Impedance grounding alternative
Another method of grounding systems of more than 1,000V is through an impedance device. Impedance grounding means there is intentional opposition to current inserted between the grounded (neutral) conductor of the system and the grounding electrode conductor. The NEC refers to these systems as impedance-grounded neutral systems. The impedance intentionally limits the amount of current that will return to the source in ground-fault conditions, thus increasing continuity of service. The impedance device is typically a resistor or impedance coil inserted into the alternating current circuit. Impedance-grounded neutral systems are installed in an effort to limit current to the source in ground-fault conditions, thereby providing a measure of protection for equipment as well as minimal downtime. By limiting the current through an intentional impedance device, any arcing condition is also kept to a lower magnitude. Special equipment is manufactured for use on these types of installations.
Impedance-grounded neutral systems are only permitted under controlled conditions. First, conditions of maintenance and supervision must be in place, ensuring only qualified people service these installations. This is an important condition because, in a ground-fault condition, qualified people must understand the proper response and course of action to remove the faulted condition. Second, ground-fault detection systems are installed to notify qualified people of a first phase-to-ground fault event. The third condition that must be met for use of this type of system is that no line-to-neutral loads are served. See sections 250.186(1) through (3).
The grounding impedance device for an impedance-grounded neutral system must be installed in series with the grounding electrode conductor and the neutral point of the system source, which could be located at a transformer or a generator. The system neutral grounding connection is only permitted through the impedance device. The neutral conductor has to be fully insulated, with insulation equivalent to that of the ungrounded phase conductors supplied by the system. The grounded conductor is also required to be identified, typically white or gray, and in accordance with applicable provisions in Article 200. The neutral conductor of these systems extends from the center point of the wye connection to the impedance device’s line side, and a grounding electrode conductor extends from the load side of the impedance device to the grounding electrode.
The neutral conductor of an impedance-grounded neutral system is not a circuit conductor. It does not supply a load. It is the conductor that connects from the impedance device to the system neutral point. It is the point of grounding for this type of system.
Grounding electrode requirements
Equipment grounding associated with medium- and high-voltage systems is required for fences, enclosures, housings, support structures and so forth, and it is required for all noncurrent-carrying metal parts of fixed, portable or mobile equipment. Note that you need not ground equipment that is isolated from ground and cannot be touched by people in contact with the ground. This exception applies to pole-mounted equipment such as elevated transformer and capacitor cases.
Grounding is accomplished through a grounding electrode conductor. Section 250.190(B) provides installation requirements for grounding electrode conductors for systems of more than 1,000V. The sizing requirements for grounding electrode conductors is based on the use of Table 250.66, using the largest ungrounded service, feeder or branch-circuit conductor supplying the equipment. The minimum size required for the grounding electrode conductor is 6 AWG copper.
Equipment grounding requirements
Just as with other supply circuits from grounded systems, a properly sized EGC must be installed. The EGC connection at the source has to be made on the load side of the impedance device. The EGC can be a bare conductor, or it can be insulated. It has to connect to the grounding electrode conductor and the equipment grounding terminal bus of the source equipment enclosure. The EGC is installed for grounding equipment supplied by the system and serves three important functions. First, it grounds the equipment. Second, it performs bonding, and, third, it serves as an effective ground-fault current path in ground-fault conditions.
Feeders and branch circuits of more than 1,000V typically must include EGCs as provided in 215.6. The EGCs must be of sufficient capacity. As previously stated, EGCs installed with circuits of more than 1,000V cannot be smaller than 6 AWG copper or 4 AWG aluminum unless they are an integral part of a cable assembly. If a cable assembly shield is a concentric neutral type and is suitable for ground-fault current performance, the shield can serve as the required EGC. For solidly grounded systems, a cable ribbon shield or tape shield of the cable assembly generally is not permitted as an EGC because of its inadequate size. The shielding material of these cable assemblies is usually smaller than the minimum capacity necessary to perform during ground-fault conditions. EGCs must provide an effective ground-fault current path to facilitate overcurrent device operation. The EGC must be sized using Table 250.122, based on the rating of the overcurrent device protecting the feeder circuit.
For example, if a pad-mounted transformer (nonutility installation) is single-point grounded and includes overcurrent protection at the bushing of the transformer’s output side, the rating of the overcurrent protection integral with the busing establishes the EGC’s minimum size. A 150-ampere bushing requires a 4 AWG copper EGC for this feeder circuit. The overcurrent rating for a circuit breaker in these types of systems is typically the combination of the current transformer and current pickup setting of a protective relay system in the breaker assembly. The minimum size of the EGC for systems of 1,000V and higher is to be based on the sizes in 250.122 but no smaller than 6 AWG copper or 4 AWG aluminum if an EGC is not an integral part of a cable assembly.
New rules in Part X of Article 250 specifically require an effective ground-fault current path for services supplied by grounded and ungrounded systems. If the system is grounded, the grounded conductor must be installed with the service conductors and serves as the ground-fault current path. If the service is supplied by an ungrounded system, a supply-side bonding jumper must be installed with the service conductors and serves as the ground-fault current path. These revisions in 250.186 provide needed clarity in Part X for grounding medium- and high-voltage systems over 1,000V, specifically the new requirements for ground-fault paths.
About The Author
JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected].