Last month, we reviewed the basic requirements for grounding separately derived systems in accordance with Article 250 of the National Electrical Code (NEC). This month, we will explore a proposed 2002 Code change and how it might impact safety.
2002 NEC proposal
A proposal was submitted to modify Section 250-30(a) by adding new subsection (3)(d):
“It shall be permissible to connect the grounded electrode conductor of a separately derived system to a continuous grounding electrode conductor which is connected to an effectively grounded metal water pipe within 5 ft (1.52 m) from the point of entrance into the building and extending through the building. All connections shall be made at an accessible location and either by an irreversible crimp, listed for the purpose, or by exothermic weld. The continuous grounding electrode conductor shall not be smaller than 3/0 copper or 250kcmil aluminum. The grounding electrode conductor of the separately derived system must still comply with Sections 250-28(a) through (d).”
The substantiation stated:
“In a high-rise building or a large manufacturing facility where there is no effectively grounded structural metal members it is not practical to run an individual grounding electrode conductor form every separately derived system transformer back to the effectively grounded metal water pipe within 5 ft (1.52 m) of the entrance of the metal piping into the building. The same result can be accomplished by grounding each separately derived system back to a common grounding electrode conductor that is connected to the effectively grounded metal water pipe.”
Interestingly, Code Making Panel 5 (CMP-5) accepted the proposal in principal and added a new paragraph to Section 250-30(a)(2), to read as follows:
“It shall be permissible to connect the grounding electrode conductor of a separately derived system to a continuous grounding electrode conductor which is connected to the building grounding electrode system and extended through the building. All connections shall be made at an accessible location by an irreversible compression connector listed for the purpose, listed connections to copper busbars not less than 6 mm x 50 mm (1/4 in. x 2 in.), or by the exothermic welding process. This continuous grounding electrode conductor shall not be smaller than 3/0 AWG copper or 250 kcmil aluminum. This continuous grounding electrode conductor and the grounding electrode conductor(s) of each separately derived system shall comply with 250-64(a) through (e).”
First, the proposal stated that the application is necessary in “high-rise” buildings or a “large manufacturing facility.” However, the Code doesn’t define either and even if it did, the wording accepted by the panel has no such limitation for these applications.
Second, the substantiation states this method is necessary “where there is no effectively grounded structural metal members.” The proposal was intended to clarify that the point of connection to the effectively grounded metal water piping had to be done within 5 feet of the point of entrance into the building. The accepted wording doesn’t specifically address this application and contains no provisions to limit the scope of the requirement to buildings where there are no effectively grounded structural metal members.
Third, the original proposal specified that the wording be added as a new subsection (3)(d). The panel chose to accept it as a new second paragraph to (a) (2). This may present a conflict with the requirement in (a) (3), which requires that the “grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system.”
Suppose a typical high-rise office building is designed using a 3/0 AWG copper GEC from the basement building steel to a series of transformers located on the 15th, 20th, and 25th floors. When the transformers are grounded on their respective floors they are grounded by using the 3/0 GEC run with the feeder conductors. In essence, the effect is to reference the newly derived systems on the 15th, 20th, and 25th floors to the building steel in the basement. Have we complied with the provisions of (a)(3), which required the connection to be “as near as practicable to and preferably in the same area?”
More importantly, is there a possible difference of potential between the building steel in the basement and that on the upper floors and, in referencing the new system to the steel in the basement, a potential hazard may exist? Consider that Section 250-104(a)(4) requires bonding to the interior water piping system in the area served by the separately derived system. This ensures that where the building steel is nearest a grounding electrode, the water piping would still be required to be bonded. If there is not a limitation to the use of this proposed revision, it could result in applications where building steel in the area served by the separate derived system is not required to be bonded.
CMP-5 has correctly begun discussing a practice that has been used in some parts of the country for many years. The question remains of whether the practice is warranted or is used out of convenience or to cut costs.
Such a change in these requirements should only be based on sound technical data, which establishes that differences of potential cannot exist and there is no detrimental impact on the safety of the electrical distribution system.
CALLANAN is director of Safety, Codes, & Standards at the National Joint Apprenticeship & Training Committee. He can be reached at firstname.lastname@example.org.