Is It an Emergency? Not All Systems Are Created Equal

By Mark Earley | Mar 15, 2019
An optional standby system is intended to supply on-site generated power to selected loads, either automatically or manually. Photo Credit: Michael Johnston




A few years ago, I had a long discussion with an official from a government agency about emergency generators. He referred to small, portable generators as emergency generators. When I explained the difference, he insisted that everyone called them that, so he would too. There is a big difference between emergency situations that require prompt connection to a standby-power source, situations where it is important to have a connection soon, and situations where it would be nice to be connected at some point.

The National Electrical Code (NEC) covers a variety of standby systems. What we call them may be based on whether other codes or local ordinances. Requirements for healthcare occupancies also differ from other occupancies. Important features distinguish each of these standby systems.

Emergency systems

Section 700.2 of the NEC defines an emergency system as legally required and classed as emergency by municipal, state, federal, or other codes or by any governmental agency having jurisdiction. These systems are intended to automatically supply illumination, power or both, to designated areas and equipment in the event of failure of the normal supply or in the event of accident to elements of a system intended to supply, distribute and control power and illumination essential for safety to human life.

An emergency system isn’t necessarily intended to keep everything running continuously. An informational note to the definition indicates they “are generally installed in places of assembly where artificial illumination is required for safe exiting and for panic control in buildings subject to occupancy by large numbers of persons, such as hotels, theaters, sports arenas, health care facilities, and similar institutions.”

The note states they “may also provide power for such functions as ventilation where essential to maintain life, fire detection and alarm systems, elevators, fire pumps, public safety communications systems, industrial processes where current interruption would produce serious life safety or health hazards, and similar functions.” The key phrase here is “where essential to maintain... .” A generator may provide some functions that are not essential to prevent serious life safety or health hazards, as long as they are provided through a separate transfer switch.

Emergency loads must be automatically transferred to the emergency system within 10 seconds. The simplest emergency system is known as unit equipment. Unit equipment typically consists of self-contained lamps mounted on a box that contains trickle-charged rechargeable batteries. Loss of power causes the lamp to illuminate. Obviously, unit equipment must not be installed on a switched circuit. Otherwise, the lamps will illuminate routinely. Unit equipment is designed to supply illumination for at least 1½ hours.

I recall being at my local gym during an outage. Several individual units illuminated. After about 20 minutes, the staff determined it would likely be a longer-term outage, so we were told the gym would close. I appreciated the fact that they had procedures in place to ensure everyone would be able to exit the building while illumination was still available. Storage batteries also can be used to supply luminaries in more than one location in a building. Storage batteries and unit equipment have limitations. Their duration of use is specified as 11/2 hours without dropping below 87.5 percent of the battery voltage. Their primary purpose is for area evacuation. They will not be a good solution for a longer term outage if the building is to remain occupied.

Generators commonly are used in buildings to supply emergency loads, especially where there is a need to serve additional emergency-related functions. As noted previously, some industrial processes may be supplied by the emergency system if the loss of power to the equipment could result in serious life-safety or health hazards. Some emergency power supplies will include generators, along with uninterruptible power supplies. This may be because the generator may not be fully functional in 10 seconds or to prevent the loss of data for life-safety systems, such as fire alarm systems.

Wiring for emergency systems is required to be kept separate from the wiring of the normal system. Separation is intended to eliminate the likelihood of simultaneous failure of the emergency and the normal system. Emergency system wiring must be identified as a component of the emergency to help prevent intermingling with normal system wiring and inadvertent disconnection.

Generators for emergency systems are permitted to serve non-emergency loads. More on that later.

Legally required standby systems

Section 701.2 defines legally required standby systems as systems required and so classed as legally required standby by municipal, state, federal or other codes or by any governmental agency having jurisdiction. These systems are intended to automatically supply power to selected loads (other than those classed as emergency systems) in the event of failure of the normal source.

The informational note further explains what a legally required standby system is: “Legally required standby systems are typically installed to serve loads, such as heating and refrigeration systems, communications systems, ventilation and smoke removal systems, sewage disposal, lighting systems, and industrial processes, that, when stopped during any interruption of the normal electrical supply, could create hazards or hamper rescue or fire-fighting operations.”

Some of this may sound a lot like an emergency system. However, legally required standby systems are not the same. These loads may be important, but they are much less urgent than those consisting of equipment that must continue to operate long enough to evacuate the building and for critical equipment, such as fire alarm systems and fire pumps. It can include lighting, but this is not the lighting needed for illumination essential for safety to human life. In addition, loads of legally required standby systems must automatically transfer within 60 seconds, as opposed to the 10-second requirement for emergency systems. Legally required standby system wiring can occupy the same raceways, cables, boxes and cabinets with general wiring.

Optional standby systems

Section 702.2 defines optional standby systems as systems intended to supply power to public or private facilities or property where life safety does not depend on the system’s performance. These systems are intended to supply on-site generated power to selected loads, either automatically or manually.

Optional standby systems are what the name indicates: optional. No codes require them. In most instances, a transfer switch is required in accordance with 702.5, but it can be manual or automatic. Several years ago, a newspaper reporter contacted me to ask how to use a portable generator to backfeed a home through a range or dryer receptacle. I told him a transfer switch was required. He told me transfer switches are expensive, he knew that it was possible to connect safely through the range or dryer receptacle, and he wanted me to provide recommendations for home owners who could follow instructions. With that clarification, I told him the homeowner would need a transfer switch.

If the transfer switch is automatic, it must be capable of supplying the full load that can be transferred to the alternate power source, or it must have a load management system. There is no minimum time requirement for transferring the load. An optional standby system can be supplied by a portable generator.

While an emergency system can be very simple, an optional standby system can be simple or very elaborate. Many financial service and internet companies will invest in sophisticated systems to prevent any power interruption because continuous operation is important to business continuity. Their optional system may include generators and uninterruptible power supplies to prevent any momentary interruption. This is common for data processing operations and companies that provide website hosting.

As indicated previously, a generator can supply emergency and other non-emergency loads. Section 700.10(B)(5) provides requirements for maintaining the integrity of the emergency system, legally required standby system, and the optional standby system, in that order.

Critical operations power systems

Article 708 was created during the 2008 NEC cycle. It is intended to provide a more robust standby system than the emergency systems of Article 700.

Section 708.2 defines critical operations power systems (COPS) as power systems for facilities or parts of facilities that require continuous operation for the reasons of public safety, emergency management, national security or business continuity.

A COPS supplies power to a vital infrastructure facility that must continue to operate during an incident or event. The typical incidents or events include severe weather, such as hurricanes. These systems are deemed necessary to supply vital infrastructure facilities, the loss of which could affect national security, the economy, public health or safety.

A typical application of COPS is a police or fire department communications center. The decision of whether or not Article 708 would apply to specific installation is a decision that must be made by the authority having jurisdiction.

Informational Note No. 1 states critical operations power systems are generally installed in vital infrastructure facilities that, if destroyed or incapacitated, would disrupt national security, the economy, public health or safety; and where enhanced electrical infrastructure for continuity of operation has been deemed necessary by governmental authority. While Section 700.10(C) requires emergency system circuits to be designed and located to minimize exposure to hazards such as flooding, fire, icing, vandalism and other adverse conditions for emergency systems, Section 708.10 has more detailed requirements for protection from physical damage, including protection from floodplain protection because of the importance of these facilities.


The type of system installed will depend on the loads being served, how quickly those loads need to be served, and how critical the facility is. It is not unusual for commercial and institutional buildings to have emergency, legally required standby and optional standby loads.

About The Author

EARLEY, P.E., is an electrical engineer. Retired from the National Fire Protection Association, he was secretary of the National Electrical Code Committee for 30 years and is president of Alumni Code Consulting Group.





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