A common task for electrical system designers is to design separately derived systems. It is difficult to imagine a building that does not contain a separately derived system. By definition, a separately derived system is a “premises wiring system whose power is derived from a battery, a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.”
While each year brings more advances in solar photovoltaic and battery system technologies, clearly, transformers remain the most common power source for separately derived systems. Typically, for example, in a high-rise building, power will be distributed at the 480-volt level. The 480/277 voltages can be used for power applications. At intermediate floors, 480Y/277-volt transformers are installed to transform a 480-volt supply to a 208Y/120-volt system for lighting, convenience receptacles, and appliance loads.
This is the first installment of a two-part article that will explore key provisions for grounding separately derived systems. This article covers basic grounding applications for these newly derived systems, while the second will address a proposed addition to the 2002 National Electrical Code (NEC).
Separately derived systems are grounded to limit the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines and to stabilize the voltage to ground (earth) during normal operation. In addition, ensuring a low-impedance ground return path eases the operation of the overcurrent protective devices (Section 250-2 (a) NEC). An electrical ground or reference to ground cannot be “transformed” through the action of the transformer, so each separately derived system that is required to be grounded must re-establish the ground reference point for the new system. Section 250-30(a) (1-4) establishes four basic sets of requirements for grounding the newly derived electrical system.
The four provisions included in Section 250-30 include requirements for the installation of 1) a bonding jumper, 2) a grounding electrode conductor, 3) a grounding electrode, and 4) other grounding methods. The bonding jumper is used to connect the equipment grounding conductors of the separately derived system to the grounded conductor. The bonding jumper must be sized for the derived phase conductors, and in general, is permitted to be installed at any point on the separately derived system from the source to the first system disconnecting means or overcurrent device. If the system has no disconnecting means or overcurrent devices, it shall be made at the source of a separately derived system. The bonding jumper must generally be connected at the same point as the grounding electrode conductor. Exception No. 1 permits two bonding jumpers where a parallel path, for the grounding circuit conductor, is not created.
The second set of rules applies to the grounding electrode conductor (GEC). The GEC is the “conductor used to connect the grounding electrode to the equipment grounding conductor, to the grounded conductor, or to both, of the circuit at the service equipment or at the source of a separately derived system.” (Article 100, NEC). The requirement for separately derived systems is essentially the same as that for an AC supplied system, which is required to be grounded. Like the bonding jumper, the GEC is sized for the derived phase conductors but is never required to be larger than 3/0 copper or 250kcmil aluminum. In general, the point of connection for the GEC is required to be at the same point on the separately derived system where the bonding jumper is installed.
The third set of rules applies to the grounding electrode for the separately derived system. The grounding electrode is the component of the grounding electrode system, which provides for the actual connection to the earth or some other conducting body that serves in place of the earth. These rules were modified during the 1996 NEC cycle to require that the grounding electrode for the newly derived system be the “nearest” one of the following:
a. An effectively grounded structural metal member of the structure
b. An effectively grounded metal water pipe within 5 feet (1.52 meters) of the point of entrance into the building
c. Other electrodes as specified in Sections 250-50 and 250-52 where the electrodes specified by (a) or (b) are not available
A close review of Section 250-26(c) of the 1996 NEC reveals that the sentence structure, intentional or not, did permit a choice of any of the three listed options. The 1999 NEC revision clarified that the “nearest” of the three options was to be the basis for selecting the grounding electrode for the separately derived system. In addition to directing which grounding electrode shall be used, 250-30 (a)(3) also requires that “(T)he grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system.” Due to this article’s generality, two exceptions to this section will not be discussed.
Lastly, Section 250-30(a)(4) requires methods used to ground separately derived systems to be the same as those permitted or required elsewhere in Article 250. For example, provisions for installing the GEC located in Section 250-64 apply equally to GEC’s installed for separately derived systems.
CALLANAN is director of Safety, Codes, & Standards at the National Joint Apprenticeship & Training Committee. He can be reached at firstname.lastname@example.org.