Lately, I have received questions about the number of disconnects permitted for a building or structure. In one query, an inspection found more than six disconnects were present in an oil and gas refinery building, and it took more than six throws of the hand to disconnect incoming power supplies. The questioner was in charge of determining if the National Electrical Code (NEC) ever permits more than six. I received other questions pertaining to demand factors, diversity factors and the connection of the circuit conductors in the switchgear compartment, but let’s start at the beginning.
Demarcation of service point
Before I could reply about the six disconnects, I needed to know the location of the service point for the incoming power. The questioner informed me that the service point and power center, as defined in Article 100 of the NEC, as far as he could tell, was located in the substation. He stated that the utility power supplying the power center is a 12,470-volt, 4-wire, three-phase system that has several feeders. All are protected with properly sized overcurrent-protective devices. Such feeders are routed from the substation to each individual building.
Service equipment disconnects
The six disconnects rule applies at the service equipment per sections 230.70, 230.71 and 230.72. These sections require the disconnects to be identified and grouped.
As far as I could tell, the service conductors complied with Section 230.42(A)(1) and were protected properly per Section 230.90. The individual asked another question about whether the NEC permits a demand or diversity factor to be applied to feeder conductors and, if so, how these terms are defined. Of course the NEC allows demand factors to be applied. It outlines demand factors and considers them less than 1. However, the NEC does not address diversity factors, but the McGraw-Hill Engineering Handbook defines them as more than 1. Whether they can be applicable to a particular installation is up to the authority having jurisdiction.
Feeder equipment disconnects
After performing the load calculations, the feeders were sized properly by the provisions outlined in sections 215.2(A)(1), 215.3 and 220.61, respectively. Now, does the neutral conductor(s) have to be calculated at continuous duty (125 percent of connected load) or at noncontinuous load (100 percent of connected load)? This documentation can be found in sections 220.61, 215.2(A)(1) and Ex. 2, while the Example D3 (a) in the Annex D of the 2011 NEC outlines the calculation procedure in detail. With these feeder conductors having an overcurrent--protective device installed at the substation, they met the definition of the service point, and Article 100 defines a feeder. These conductors, having the classification of feeders and not service conductors, allow for the application of a different set of requirements.
Remember definitions: if they were classified service conductors, only six disconnecting means could be located at each building, per Section 230.71(A). Since they met the definition of feeders, the disconnect location can be elsewhere on the premises in accordance with Section 225.32, Ex. 1, but not Section 225.33(A), which would have otherwise applied. Section 250.32(D) covers feeders and requires that an equipment grounding conductor be run with the circuit conductors when the six disconnects or fewer are located in the substation. This type of installation permits more than six disconnects to be installed at each facility.
The feeders were routed from the substation through manholes and tunnels until they entered the open bottom switchgear. The installation satisfied high-voltage requirements located in Article 490 and the last part of applicable articles.
Terminating circuit conductors
Circuit conductors consist of ungrounded conductor(s), neutral conductor(s) and equipment grounding conductor(s). The procedures for terminating new feeder conductors are located in sections 408.40 and 250.32(B). And the Ex. to Section 230.32(B) applies to existing installations. Note, with new installations, the neutral conductor(s) must be isolated from the noncurrent-carrying parts of equipment, while the equipment grounding conductor(s) are grounded and bonded to the metal of the switchgear’s enclosure. When the feeder wiring exists as outlined in strict compliance with the Ex. to Section 250.32(B), the grounded neutral conductor(s) can serve as a bonding means, as a path for fault-current, and for the flow of neutral current.
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.