In Case of Emergency

Certain changes to the National electrical Code (NEC) take more than a single cycle to accomplish due to the complexity of the issues, changes that may involve more than one NFPA committee, changes that involve more than one NEC panel, changes where another NFPA committee and an NEC panel are involved, or the enormous affect the changes may have on the electrical industry.

The issue of separation of emergency circuits from all other premises wiring was called into question, based on industry practice and the actual text in the second sentence in Section 700.9(B) that stated, “wiring from an emergency source or emergency source distribution overcurrent protection to emergency loads shall be kept entirely independent of all other wiring and equipment.” Panel 13 maintained that “the emergency wiring must go clear back to the generator terminals and non-emergency wiring, where permitted, must also go clear back to the generator terminals” and made that position clear in a Panel Statement on Comment 13-71 Log #3103 for the 2005 NEC process.

The NFPA 99 chairman, representing the Committee for the Standard for Health Care Facilities, and the NFPA 110 chairman, representing the Committee for the Standard for Emergency and Standby Power Systems, contacted the NEC Technical Correlating Committee and made it clear that those committees did not agree with the submitted Panel 13 statement. The NFPA 110 Committee felt that emergency power, covered by NFPA 110, originated either at the generator or source of power or at the first point of distribution, such as at a distribution panelboard or switchboard. The NFPA 99 Committee felt that emergency power, as covered in NFPA 99 and Article 517, started at the transfer switches for critical branch, life safety branch, and equipment branch and not at the generator or distribution panelboard or switchboard.

The NEC Technical Correlating Committee (NEC TCC) contacted the NFPA 99 and 110 committees, requesting their participation in a task group to address this issue for the 2008 NEC process. The NEC TCC also stated that NEC Panel 15 provided the following statement in the 1996 NEC Report On Proposals on the same issue: “The feeders from the emergency generators to the first overcurrent device of any load, even nonessential ones, must be considered as part of the emergency system to protect system integrity.” Since separation requirements for wiring of emergency systems (Article 700), legally required standby systems (Article 701), and optional standby systems (Article 702) was at issue, the Joint Task Group involved members of Panel 13, representing the NEC position on these three articles; members of NFPA 110, representing the Standard for Emergency and Standby Power Systems; and NFPA 99, representing the Standard for Health Care Facilities.

The addition of text in Section 700.9(B)(5) in the 2008 NEC was the result of the Joint Task Group as follows: “(5) Wiring from an emergency source to supply any combination of emergency, legally required, or optional loads in accordance with (a), (b), and (c):

“(a) From separate vertical switchboard sections, with or without a common bus, or from individual disconnects mounted in separate enclosures.

“(b) The common bus or separate sections of the switchboard or the individual enclosures shall be permitted to be supplied by single or multiple feeders without overcurrent protection at the source. Exception to (5)(b): Overcurrent protection shall be permitted at the source or for the equipment, provided the overcurrent protection is selectively coordinated with the downstream overcurrent protection.

“(c.) Legally required and optional standby circuits shall not originate from the same vertical switchboard section, panelboard enclosure, or individual disconnect enclosure as emergency circuits.”

The accepted text in 700.9(B)(5)(a), (b) and (c) must all be complied in concert with each other if this application is used. Section 700.9(B)(5)(a) requires separation between the emergency vertical section of a switchboard and the vertical section containing overcurrent devices for legally required and optional standby power. A common bus or a separate bus can be used. Section 700.9(B)(5)(b) recognizes either a single feeder supply to the switchboard from the generator or multiple feeders (one for the emergency loads and one for the legally required and optional standby systems) where overcurrent protection is not provided at the generator.

Remember, compliance with 445.13 requires sizing the conductors from the generator to the first overcurrent protective device at 115 percent of the generator total nameplate rating. The exception to 700.9(B)(5)(b) permits overcurrent protection at the generator if the overcurrent protection at the feeder is selectively coordinated with any downstream devices. Section 700.9(B)(5)(c) specifies legally required and optional standby circuits must originate in a different vertical switchboard from the emergency circuits. Rather than using a single switchboard with separate vertical sections, individual disconnect enclosures can be used. Study these requirements closely before you design or install emergency systems.

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at

About the Author

Mark C. Ode

Fire/Life Safety Columnist and Code Contributor
Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and .

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