Grounding techniques and National Electrical Code requirements for systems and equipment operating at more than 1,000V, such as 5-kilovolt (kV) and 15-kV systems, differ slightly from those for systems of 1,000V or less. A typical 5-kV system has a phase-to-phase voltage of 4,160V. The phase-to-ground voltage is approximately 2,400V. An example of this system is one with a phase-to-phase voltage of 12,470V and a phase-to-ground voltage of about 7,200V.

These systems are called medium-­voltage. Those of more than 35 kV are high-voltage. The NEC provides several rules related to grounding of these systems and equipment. 

Part X of Article 250 provides the rules for grounding and bonding systems of more than 1,000V. Section 250.180 indicates that if systems over 1,000V are grounded, they shall comply with all applicable requirements in sections 250.1–250.178 and 250.180–250.194, which supplement and modify the preceding sections.

Why ground?

The reasons for grounding systems of more than and less than 1,000V are the same. They are grounded to limit voltages imposed by lightning events, line surges or unintentional contact with higher-voltage lines and to provide voltage stabilization during normal system operation. If systems of more than 1,000V are grounded, the requirements in 250.180 through 250.194 must apply accordingly, depending on the type of grounding.

It is important to realize that grounding and bonding provisions in parts I through IX are only modified or supplemented by Part X of Article 250. Also, remember that 90.3 indicates that chapters 5, 6 and 7 of the NEC can modify or amend any of the requirements within Article 250. This is important as it relates to establishing minimum sizes of equipment grounding conductors (EGCs) for feeders and branch circuits, which is accomplished using the same methods as those for feeders and branch circuits rated 1,000V or less. Apply the minimum sizes using Section 250.122 and Table 250.122 based on the rating of the overcurrent protective device.

Grounding and bonding of equipment associated with medium- and high-voltage systems is required for fences, enclosures, housings, support structures, etc., and for all noncurrent-carrying metal parts of fixed, portable or mobile equipment.

Grounding and bonding of equipment associated with medium- and high-voltage systems is required for fences, enclosures, housings, support structures, etc., and for all noncurrent-carrying metal parts of fixed, portable or mobile equipment. Note that equipment isolated from ground that cannot be contacted by people in contact with the ground is not required to be grounded. 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 are based on 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 or 4 AWG aluminum.

EGCs

Feeders and branch circuits of more than 1,000V often include EGCs, which must be of sufficient capacity. 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 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 is not permitted as an EGC because of its inadequate size. This shielding material is typically under the minimum capacity necessary to perform during ground-fault conditions, as EGCs must be able to provide an effective ground-fault current path to facilitate overcurrent protective device operation.

The EGC is sized using Table 250.122 based on the rating of the overcurrent protective device protecting the feeder circuit. As an example, if a pad-mounted transformer is single-point grounded and includes overcurrent protection at the bushing on the output side of the transformer, the rating of the overcurrent protection establishes the minimum size of the EGC. A 150A bushing results in a minimum 4 AWG copper EGC for this feeder circuit.

The overcurrent rating for a circuit breaker in these systems is typically a combination of the current transformer and current pickup setting of a protective relay system in the circuit breaker assembly. 

Remember that the minimum size of the EGC for systems of 1,000V and higher is 6 AWG copper or 4 AWG aluminum if an EGC is not already an integral part of a cable assembly.