Section 250.30 is the topic of much discussion in every workshop that I instruct on the changes in the 2011 National Electrical Code (NEC). The section has been completely rewritten in an effort to make the requirements easier to read, understand and apply.
Section 250.102(D) outlines an equipment--bonding jumper (EBJ) that is installed on the load side. However, this conductor is sized from Table 250.122 because these conductors have an overcurrent-protection device (OCPD) installed ahead of them. See Section 250.102(D) for more information concerning the sizing and installation of the EBJ. For example, if a fused disconnect with a set of 225-ampere fuses is installed ahead of these conductors, the EBJ is selected from Table 250.122 and sized at 4 AWG copper. However, a supply-side bonding jumper must be sized from 250.102(C)(1) and requires the 12 percent rule to be exercised when the kcmil rating of any one phase exceeds 1,100 kcmil for copper and 1,700 kcmil for aluminum.
Grounding separately derived alternating current system—250.30
Section 250.30 has been completely revised and reorganized in an effort to provide better clarity and usability. This new layout provides a better structure for requirements pertaining to the grounding and bonding of separately derived systems, and it alerts designers and installers that separately derived systems must be grounded as described in 250.30(A), (B) or (C). For example, the information in existing 250.20(D) has been revised and relocated to 250.30, including Informational Notes 1 and 2 that specifically address switching or not switching the grounded conductor within the transfer equipment and how it relates to separately derived systems.
Supply-side bonding jumper—250.30(A)(2)
The term “equipment-bonding jumper” was changed to “supply-side bonding jumper” because it is and always has been located on the supply side. Since there is no OCPD ahead of these transformers’ secondary conductors, Table 250.66, in most cases, is used to size this supply-side bonding jumper, when it is installed in a single raceway per 250.30(A)(2) and 250.102(C)(1). The grounded conductor (may be a neutral per 220.61) is sized per 250.30(A)(3). It was necessary to make this change in order to ensure that the proper installation of bonding conductors installed within or on the supply side per 250.102(C) were properly identified. For example, if 4/0 AWG output conductors per 240.21(C)(2) are tapped (my words) from the transformer’s secondary, a 2 AWG conductor from Table 250.66 is used.
My students often ask about the grounding electrode (GE) and why the metal water pipe was listed first instead of the structural building steel. In my opinion, the metal waterpipe system may be closer to the separately derived system and must comply with 250.30(A)(4). Another explanation may be that the metal water pipe could be sliced with PVC piping, which would isolate the pipe from earth-ground. A separately derived system requires a grounding electrode to be installed in order to provide as close as possible to an equipotential-grounding plane.
The new 250.30(C) has been accepted to cover a separately derived system that has been installed outside on the premises of the building or structure supplied. The GE connection must be made at the source, and one or more of the GEs from 250.50 must be selected. Furthermore, the grounding electrode system must comply with 250.52 and 250.53. For the definition of “premises,” see Article 100 of the 2011 NEC. This change prevents the wrong interpretation from being made for installations where the grounded conductor is routed from the system to and connected to the service-grounding electrode. This type of grounding scheme had to be clarified that it was not for the separately derived system.
Other review sections—Article 250
A review of 250.20, 250.21, 250.22, 250.26 and 225.32 is important because they all tie together and apply to grounding and bonding a separately derived system.
Section 250.30 has been completely reorganized and revised with material added to the requirements for grounding such a system. These revised rules provide designers and installers with a better understanding of this section and its intended application.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.