At a recent workshop that I was teaching, a question was asked about how the grounded conductor was to be installed and used when routed from the outside utility transformer to service equipment located inside the facility. Would the grounded conductor have to be isolated at the service equipment and an additional equipment-grounding conductor have to be pulled and bonded at the utility transformer and service enclosure? The person asking the question said that the utility representative told him that the National Electrical Safety Code (NESC) required this type of installation. So, I’ve decided to discuss the use of the grounded conductor on the supply side of the service.
Section 250.142(A)(1) in the 2011 National Electrical Code (NEC) permits the grounded conductor to be used to bond and ground the utility transformer and the service equipment as well as other components. I told the questioner that he might have misunderstood what the utility person really asked him to do. In more than 40 years of designing, installing and inspecting electrical systems, I have never had a utility person tell me to isolate the grounded conductor in the service-disconnecting means. In fact, the utilities that I have worked with would not allow an equipment-grounding conductor to be pulled with the ungrounded conductors and a grounded conductor be used as the bonding means and return path for fault current to travel over, should one develop. Additionally, the grounded conductor can be used as a neutral and provide a bonding scheme and an effective ground-fault path. Naturally, the neutral must be calculated as required in Sections 220.61, 230.42(A)(1) Ex., and 310.15(B)(5). An industrial calculation example is outlined in Annex D3(a) on page 806 of the NEC.
Calculating the size of the neutral
Section 220.61 calculates the neutral load in amperes, and Section 310.15(B)(5)(c) is used to determine if the neutral is considered a current-carrying conductor. Section 220.61 requires the neutral to be sized based on the greater load from any one ungrounded-phase conductor. Section 230.41(A)(1) Ex. is applied if the designer seeks to size the neutral at 100 percent of the load instead of 125 percent of the maximum neutral load based on any one phase. Annex D explains that the neutral is not connected to an overcurrent-protective device, so in normal operation, it is not trying to trip a circuit breaker or open a fuse due to an overload, short circuit or ground-fault condition. The 125 percent rule per 230.42(A)(1) does not have to be applied because the overcurrent-protective device is not required to be reduced to 80 percent of its rating. On a 4-wire, three-phase wye system, the nonlinear load on the neutral must not contain more than the major portion of such load. Most designers consider this value to be more than 50 percent of the neutral loading. See Section 400.5(A) for an example.
Grounding transformer and service
The transformer is grounded outside per Section 250.24(A)(2) and Section 90 of the NESC. The service equipment must be grounded in accordance with Section 250.24(A). Section 250.24(C) requires that the grounded conductor be run to each disconnecting means that is classified as service equipment. This grounded conductor must be terminated in the service equipment at the same point as the grounding-electrode conductor used to ground all the neutrals and -equipment-grounding conductors related to the load-side circuits.
Grounded conductor routing
The grounded conductor per Section 250.24(C) requires the grounded conductor to be routed with the phase conductors from the transformer to the service equipment. The general rule of routing circuits is outlined in NEC Sections 300.3(B), 300.5(I), 250.134(B), 250.102(E), and 300.20.
Sizing grounded conductors
Section 250.24(C)(1) outlines the requirements for sizing the grounded conductor when it is used for bonding purposes, ground fault path and for neutral loads. First, size the neutral to carry the neutral load as required in Section 220.61. Second, size the grounded conductor based on the largest ungrounded conductor for any one phase. Then apply Table 250.66. For example, 4/0 AWG THWN service conductors per Table 250.66 requires a 2 AWG conductor to be selected. If the neutral is calculated to be a 4 AWG THWN copper, the larger of the two sizes must be used. So a 2 AWG copper conductor must be selected. This conductor bonds (bonds together), grounds (fault-current return path) and serves as a neutral conductor as permitted in Section 250.142(A)(1).
Sections outlined in this article permit the grounded conductor to be used to perform three functions on the supply side of the service-disconnecting means without the use of an equipment-grounding conductor.
In the next issue, I will discuss the grounding of separately derived systems using the grounded conductor for bonding and grounding on the supply side.
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.