Q
We have a data-processing client that requested stand-by power. For our design we provided a 400 kW/500 kVA generator right outside the building. The distance between the building and generator measures less than 10 feet. The feeder from the generator travels underground to an automatic transfer switch located in the middle of the building, in which the distance would measure about 50 feet from the generator to the ATS.
The local inspector told us that it will be necessary to have a disconnecting means for the feeder conductors as soon as it enters the building. He has referenced Article 225-31 “Disconnecting Means: Means shall be provided for disconnecting all ungrounded conductors that supply or pass through the building or structure.”
The inspector understood that the generator was not for emergency life-safety issues. He also admitted that the conductors had overcurrent protection ahead of them, from an enclosed circuit breaker located inside the generator enclosure. He has classified these as feeder conductors. Therefore they need to comply with Article 225-31.
Article 225-31 falls under Section B of Article 225 and is titled “More than One Building or Structure.” The generator, and its manufactured enclosure, is a piece of equipment, not a building or structure. The conductors are protected; therefore there is no requirement for a separate disconnecting means.
To further plead my case, Article 225 is titled “Outside Branch Circuits and Feeders.” The 1999 NEC Handbook actually states, “Part B covers outside branch circuits and feeders on single managed properties with more than one building or structure.” It goes on to outline the areas covered by this new Code section to include “college campuses, multi-building industrial facilities, multi-building commercial facilities and so on.”
From a safety standpoint I believe there should be a method of cutting off the source of power in the event of an emergency. This could be easily accomplished by installing an Emergency Power Off (EPO) button within the electrical room that is properly labelled for proper identification. This EPO could be wired directly to the generator Emergency Off circuit for total shutdown.
Your views on this subject would be appreciated.
A
Although your generator does fall under the Article 100 definition of “equipment,” it may also be considered a “structure” under the ordinary English language definition of structure — essentially, something that is constructed. Although not everyone would treat a generator as a structure, the inspector is not necessarily wrong in this regard. The inspector is correct that the conductors coming from the generator are feeder conductors as defined in Article 100. However, for the purposes of Section 225-32, the point of entrance of the conductors to the building is the point where the conductors emerge from underground inside the building. This is why Section 225-32 refers to Section 230-6. According to this rule, it appears that your transfer switch is already located where the conductors enter the building. Most transfer switches can be operated manually, and thus may be considered to meet the requirements for a disconnecting means.
Section 225-32 was derived from the rules of Article 230. It is written somewhat different from the rules of Article 230. Section 230-70(a) requires that a disconnect be located nearest the point of entry of the conductors only when it is located inside. When it is outside, it need not be nearest the point of entrance. Section 225-32 requires the disconnect to be nearest the point of entrance whether inside or outside. This was not necessarily intended. To illustrate this point, see the proposal for the 2002 code that was made (and accepted) by Code Making Panel 4 (ROP4-22a). This clarification only requires the outside disconnect to be located within sight of the building. An inside disconnect must still be located nearest the point of entrance. I would consider either the generator breaker or the transfer switch to be a disconnect that complies with the intent of Section 225-32 with regard to location.