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Installation and operation of optional standby systems

Apr 23, 2004 2:07 PM, By Mike Holt

With last year’s blackout still lingering on the minds of many, the use of optional standby power systems is worth a closer look. Art. 702 of the 2002 NEC covers the installation and operation of optional standby systems that are permanently installed in their entirety or those that are arranged for connection to a premises wiring system from a portable alternate power supply.

The scope of Art. 702 was changed for the 2002 issue of the Code to clarify that it applies to portable generators, which are commonly used for telecommunications facilities, water and wastewater pump stations, as well as to provide backup power to homes and offices. In addition, the sections of this article that deal with transfer equipment and portable generator grounding were also revised.

Transfer equipment – A new sentence was added to 702.6, which calls for a transfer switch to be installed whenever a fixed or portable standby generator is connected to a premises wiring system.

Portable generator grounding – A new section (702.10) was also added for grounding portable generators, which reads as follows:

“(A) Separately Derived Systems. If the transfer switch for a portable generator switches the grounded (neutral) conductor (separately derived system) then the portable generator must be grounded in accordance with 250.30.

“(B) Nonseparately Derived System. If the transfer switch for a portable generator does not switch the grounded (neutral) conductor, then the generator’s equipment grounding conductor must be bonded to the system grounding electrode.”

Section (A) means that the neutral of the portable generator must be bonded to the generator case and grounding electrode in accordance with the requirements of 250.30 (Fig. 1).

Section (B) means that the grounded (neutral) conductors of the generator must not be bonded to the case [250.142], but does require the case to be bonded to a system grounding electrode. The grounding electrode system for the building can be used for this purpose. Effectively, this rule is merely repeating the grounding requirements noted in Article 250 (Fig. 2).

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