There are a number of substantial changes in Article 250 of the 2002 National Electrical Code (NEC) covering grounding and bonding. One of the most significant changes involves the method of grounding a separately derived system and the permission to use grounding electrode taps for multiple separately derived systems.
Sections 250.30(A)(2) and (A)(3) have been expanded to cover the primary grounding electrode conductor and grounding electrode conductor taps for multiple separately derived systems. Section 250.30(A)(2) provides two choices for installing the grounding electrode conductor: either grounding a single separately derived system or grounding multiple separately derived systems.
A grounding electrode conductor for a single separately derived system must be sized in accordance with Section 250.66 and the accompanying Table 250.66. Its size is based on the largest ungrounded conductor from the secondary of the transformer or other type of separately derived system. This part is much the same as it has been in previous editions of the NEC.
For example, a 75-kVA transformer with a 480-volt primary and 208/120 volt, three-phase, four-wire secondary would have a maximum primary current of 90 amps and a maximum secondary current of 208 amps. The secondary conductors would be required to be 4/0 copper using 75 degrees Celsius insulated (or better) conductors.
The “75” column in Table 310.16 provides the ampacity of the 4/0 copper conductors as 230 amps without any derating. If 4/0 copper conductors are used on the secondary of the transformer, Table 250.66 would require a minimum of a No. 2 AWG copper, 1/0 aluminum, or 1/0 copper-clad aluminum grounding electrode conductor. There are some exceptions.
The use of a ground rod, a concrete-encased electrode, or a ground ring may permit the size of the grounding electrode conductor to be reduced in some cases. The connection of the No. 2 AWG copper grounding electrode conductor to the separately derived system must be made at the same point on the separately derived system. This is the point where the bonding jumper connects the grounded conductor to the equipment-grounding conductors of the secondary of the transformer.
For multiple separately derived systems, the 2002 NEC permits a common or primary grounding electrode conductor to be established in accordance with Section 250.30(A)(3). The common grounding electrode conductor will still be sized in accordance with Section 250.66, but the size of the grounding electrode conductor will be based on the total area of the largest phase conductor from each separately derived system.
Basing the common grounding electrode conductor on the total area of the largest phase conductor from each separately derived system should provide a common grounding electrode conductor of sufficient size. This should provide a zero-resistance reference point for the derived system.
For example, assume there are six transformers located in an18-story, concrete-and-glass building. Each transformer feeds the floor where the transformer is located, the floor above the transformer and the floor below the transformer. Each transformer is a 75-kVA transformer with 4/0 AWG copper conductors on the secondary side. A 4/0 AWG conductor has a circular mil area of 211,600 as determined from Table 8 in Chapter 9.
Since there are six transformers, each with 4/0 AWG conductors as their largest-phase conductors, determining the total area involves multiplying 211,600 circular mils x six transformers (1269.6 kcmil). Since the combined phase conductors exceed 1,100 kcmil, Table 250.66 requires the common grounding electrode conductor to be a minimum size of 3/0 AWG copper.
Section 250.64(C) requires this common grounding electrode conductor to be continuous without a splice, unless spliced by an irreversible compression-type connector listed for that purpose, or spliced by the exothermic welding process.
Section 250.30(A)(3) would then permit each separately derived system to have a grounding electrode conductor tapped to the common grounding electrode conductor. Each tap would be sized based on the largest ungrounded conductor from the secondary side of the separately derived system.
Using Section 250.66 and the accompanying Table 250.66, the grounding electrode conductor tap from the common grounding electrode conductor for each transformer would be required to be a minimum of a No. 2 AWG.
All tap connections to the common grounding electrode conductor must be made at an accessible location by an irreversible compression connector listed for the purpose, by the exothermic welding process, or by a listed connection to a copper busbar.
This copper busbar must be sized at least as large as a ¼ inch by 2 inches. No matter which tap connection is made to the common grounding electrode conductor, the common grounding electrode conductor must remain without a splice or a joint.
Finally, if the building has exposed structural steel that is interconnected to form the building frame, a bond must be established from the building steel to each transformer secondary in accordance with Section 250.104(C). Also, if there is interior metal piping in the area of the separately derived systems, a bond must be installed from the piping to each transformer secondary.
This code change should provide an alternative method of establishing a grounding electrode connection point for transformers in a multi-story building.
ODE is staff engineering associate at Underwriters Laboratories, Inc., in Research Triangle Park, N.C. He can be reached at (919) 549-1726 or via e-mail at firstname.lastname@example.org.