Last month, I discussed using the grounded conductor to ground and bond a separately derived system. In this issue, I discuss using the grounded conductor to ground and bond the distribution panel in a structure when supplied from switchgear in a substation or another structure.
History of using the grounded conductor for grounding
At one time, the National Electrical Code (NEC) required the grounded conductor to be used to ground and bond the distribution panel in a structure supplied from a panel located in another structure. This conductor could also be used as a neutral conductor. An equipment grounding conductor could only be used to ground and bond the noncurrent-carrying metal parts in a panel located in another structure when an exception to the main rule was applied. Application of this exception required the neutral to be isolated.
Beginning with the 1999 NEC, the Code specified the grounded conductor could only be used as an equipment grounding conductor, bonding conductor or neutral conductor. There could be no parallel current paths from one structure to another. Each edition of the NEC changed this rule until the grounded conductor could only be used as an isolated conductor. However, in existing installations, the grounded conductor can be used to perform three functions as outlined.
Grounding separated structures
Section 250.142(A)(2) permits the grounded conductor to be used to ground and bond on the supply side or within the enclosure of the first disconnecting means located in the separated structure only in accordance with the provisions in 250.32(B) Exception.
A feeder supplying a structure must have a grounding electrode installed at the structure supplied from the feeder. Such electrodes must be connected according to the provisions listed in Section 250.32(B) or (C). Where there is no electrode at the structure, a grounding electrode must be installed as outlined in Sections 250.50 (A)(4) through (A)(8).
Sizing the grounding-electrode conductor
Note that the grounding--electrode conductor must be sized from the size of the feeder conductors. For example, if the feeder conductors are 600 kcmil copper conductors, the grounding-electrode conductor is 1/0 copper as required by Table 250.66. Warning: the grounding-electrode conductor must not be selected from the rating of the overcurrent device protecting the feeder conductors.
Connecting grounding and grounded conductor
To properly isolate the grounded conductor (neutral) and bond the equipment grounding conductor to the panelboard enclosure that is installed in structure two, the installer must adhere to the requirements in Sections 250.24(A)(5) and 408.40.
Sizing the grounded conductor
When the grounded conductor is used as a neutral, it must have enough capacity in amperes (A) to supply the neutral load as calculated in Section 220.61. Exception 2 in Section 215.2(A)(1) permits the neutral conductor to be calculated at 100 percent instead of 125 percent of the load. Section 310.15(B)(5)(c) does not consider the neutral to be a current-carrying conductor if the major portion of the load on the neutral is not of the nonlinear type. Section 400.5(A) defines a major portion such as when the nonlinear load is more than 50 percent. Designers can refer to the Example 3(a) in Annex D to help understand how to perform calculations of sizing the neutral conductor at 100 percent of the neutral load. Note the explanatory material that covers the neutral conductor termination to the bus bar in the panelboard. Therefore, the neutral has no responsibility to trip or open an overcurrent-protection device, so the 80-percent or 125-percent rule does not have to be applied.
Sizing the equipment grounding conductor
The equipment grounding conductor must be sized from the rating of the overcurrent--protection device ahead of the feeder conductors. For example, a 450A overcurrent-protection device will require a 2 AWG copper equipment grounding conductor selected from Table 250.122. This conductor is used as a bonding conductor to bond all noncurrent--carrying metal parts and service as an effective path for fault to travel over, if a ground fault should occur.
Grounding with a single branch circuit
The NEC does not require a grounding-electrode conductor and grounding electrode to be installed at a structure supplied from a panelboard located in another structure. In other words, the equipment grounding conductor run with the branch circuit conductors is used to ground and bond the noncurrent--carrying metal items in the second structure. This permissive rule is found in the Exception of Section 250.32(A)
Except as permitted for existing installations that were designed and installed by previous NEC editions, the grounded conductor must not be used to ground and bond the noncurrent-carrying metal parts. This documentation can be reviewed in section 250.32(B)(1).
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.