Standby Systems, Part I

This is the first of two parts about installing emergency, legally required and/or optional standby systems. This first part will cover the basics of the three systems and the second part will cover requirements for transfer equipment. A basic understanding of emergency, legally-required and optional standby systems will help in the overall understanding of the requirements found in the National Electrical Code.

Article 700 applies to the electrical installation, operation and maintenance of emergency systems. These systems are intended to automatically supply, distribute and control electrical power and illumination to designated areas and electrical equipment essential for life safety when normal power is interrupted.

The authority having jurisdiction or federal agency mandates the emergency classification of these systems. These governmental agencies use nationally recognized codes and standards, such as the NEC, NFPA 110 (the Standard for Emergency and Standby Power Systems), the building codes, NFPA 99 (the Standard for Health Care Facilities) and various UL standards, to provide direction for installing and maintaining emergency systems.

Emergency standby systems can consist of storage batteries, generators, uninterruptible power supplies (UPS), a separate service or unit equipment (such as self-contained, battery-operated exit and emergency egress lighting). The type of system installed will depend upon the type of equipment or the function needing power in an emergency situation.

Since emergency sources of power to a building such as generators and separate services may need to be de-energized for maintenance or fire situations, signs indicating the location and type of emergency power are required to be placed at the main electrical service to the building to ensure all sources of power are disconnected. Signs are not required for individual unit equipment, such as battery-operated exit and egress lighting.

Where a grounded conductor from the emergency source is connected to a grounding electrode conductor at a location that is remote from the emergency source, a sign must be installed at the grounding electrode location that will identify all emergency and normal sources connected at that location. The purpose of this sign is to identify the grounding electrode system and the various grounding conductors to prevent inadvertent disconnection of the ground electrode conductor for the emergency system.

Section 700.9 requires all boxes and enclosures for emergency circuits—including transfer switches, generators and power panels—to be permanently marked as a component of the emergency circuit or system. The marking must be by color code, such as painting the raceways, enclosures and equipment red, by marking with the words “emergency system,” or any other method of identification that will adequately identify emergency systems and their related components.

Wiring for two or more emergency circuits from the same emergency power source are permitted in the same raceway, cable, box or cabinet. Wiring from two different emergency sources or from normal power cannot be mixed in raceways, cables, boxes or cabinets. This ensures that emergency power is isolated from other circuits and not affected by a short or a malfunction from another source. This separation does not apply to transfer switch enclosures, wiring to exit or emergency luminaires with normal and emergency supply circuits, in a common junction box attached to exit lighting, emergency lighting or to unit equipment.

Legally required standby systems are covered by Article 701 and are also mandated by municipal, state and federal building 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. Legally required standby systems provide backup power for loads, such as critical heating and refrigeration systems, communications systems for police and fire stations, ventilation and smoke removal systems to help people during emergency exiting of the building, sewage disposal and industrial processes where interruption of the process could create hazards to people or the environment.

Optional standby systems are considered to be all other systems not specifically classified as emergency or legally required systems. Optional standby systems consist of either permanently installed systems or portable alternate power supplies. Providing requirements for portable optional standby systems is new in the 2002 NEC, prompted by the vast number of portable standby systems installed prior to the Y2K changeover.

Optional standby systems routinely provide power to various loads, such as refrigerators, water pumps for domestic water, computers, lights and heating or air conditioning systems within homes, farms and commercial buildings during power outages from storms or other power interruptions.

Legally required standby system and optional standby wiring can occupy the same raceways, boxes and cabinets with normal wiring systems.

Understanding these three systems will provide a basis for a more extensive review of transfer equipment requirements. EC

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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