12.4 Emergency, Legally Required Standby, Optional Standby and COPS

The Four Classes

The distinction is who requires the system, not what it powers. A hospital generator is an Article 517 essential electrical system built on Article 700 requirements. A data center generator that no authority requires is an Article 702 optional standby system, no matter how important the owner considers it.

Article 700 — Emergency Systems

700.2 Definitions. Emergency Systems are those systems 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.

700.3(F) Temporary Source of Power for Maintenance or Repair of the Alternate Source of Power. Where the emergency system relies on a single alternate source of power that will be disabled for maintenance or repair, the emergency system shall include permanent switching means to connect a portable or temporary alternate source of power, which shall be available for the duration of the maintenance or repair.

700.5 Transfer Equipment. Transfer equipment shall be automatic, listed, and marked for emergency system use, and shall be designed and installed to prevent the inadvertent interconnection of normal and emergency sources of supply in any operation of the transfer equipment. Transfer equipment and electric power production systems installed to permit operation in parallel with the normal source shall meet the requirements of Article 705.

700.10(B) Wiring. Wiring of two or more emergency circuits supplied from the same source shall be permitted in the same raceway, cable, box, or cabinet. 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, with the following exceptions:

1. Wiring in transfer equipment enclosures;
2. Wiring in luminaires supplied from two sources;
3. Wiring within a common junction box attached to luminaires supplied from two sources;
4. Wiring within a listed load control relay enclosure supplying and controlling the same load;
5. Wiring from two sources in exit or emergency luminaires;
6. Wiring from two sources in a listed load control relay supplying exit or emergency luminaires, or in a common junction box attached to exit or emergency luminaires.

700.10(D) Fire Protection. Emergency systems in assembly occupancies for more than 1000 persons or in buildings above 23 m (75 ft) in height with any of the specified occupancy classes shall meet additional fire protection requirements: feeder-circuit wiring shall be installed in spaces or areas that are fully protected by an approved automatic fire suppression system, or shall be a listed electrical circuit protective system with a minimum 2-hour fire rating, or protected by a listed fire-rated assembly with a minimum 2-hour fire rating, or encased in a minimum of 50 mm (2 in.) of concrete.

700.12 General Requirements. Current supply shall be such that, in the event of failure of the normal supply to, or within, the building or group of buildings concerned, emergency lighting, emergency power, or both shall be available within the time required for the application but not to exceed 10 seconds.

700.12(B) Generator Set. Where internal combustion engines are used as the prime mover, an on-site fuel supply shall be provided with an on-premises fuel supply sufficient for not less than 2 hours' full-demand operation of the system, with the prime mover not solely dependent on a public utility gas system for its fuel supply or on a municipal water supply for its cooling systems.

700.12(F) Unit Equipment. Individual unit equipment for emergency illumination shall consist of a rechargeable battery, a battery charging means, provisions for one or more lamps, and a relaying device arranged to energize the lamps automatically upon failure of the supply to the unit equipment. The branch circuit feeding the unit equipment shall be the same branch circuit as that serving the normal lighting in the area and connected ahead of any local switches.

700.16 Emergency Illumination. Emergency illumination shall include means of egress lighting, illuminated exit signs, and all other luminaires specified as necessary to provide required illumination. Emergency lighting systems shall be designed and installed so that the failure of any individual lighting element, such as the burning out of a lamp, cannot leave in total darkness any space that requires emergency illumination.

700.19 Multiwire Branch Circuits. The branch circuit serving emergency lighting and power circuits shall not be part of a multiwire branch circuit.

700.32 Selective Coordination. Emergency system overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices. Selective coordination shall be selected by a licensed professional engineer or other qualified persons engaged primarily in the design, installation, or maintenance of electrical systems. The selection shall be documented and made available to those authorized to design, install, inspect, maintain, and operate the system.

700.7 Signs. (A) Mandated Emergency System. A sign shall be placed at the service entrance equipment for all buildings served by a mandated emergency system, indicating the type and location of on-site emergency power sources. (B) Grounding. Where removal of a grounding or bonding connection in emergency system equipment interrupts the grounding electrode conductor connection to the alternate power source, a warning sign shall be installed at the equipment. (C) Emergency Power Source. A sign shall be placed at the emergency power source, if applicable, denoting the location of any on-site emergency power sources.

Article 701 — Legally Required Standby Systems

701.2 Definition. Legally required standby systems are those 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.

Typical loads include 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 supply, could create hazards or hamper rescue and fire-fighting operations.

701.5 Transfer Equipment. Transfer equipment shall be automatic, listed, and marked for legally required standby system use, and shall be designed and installed to prevent inadvertent interconnection.

701.12 General Requirements. Current supply shall be such that, in the event of failure of the normal supply to, or within, the building or group of buildings concerned, legally required standby power will be available within the time required for the application but not to exceed 60 seconds.

701.12(B)(2) requires an on-site fuel supply sufficient for not less than 2 hours' full-demand operation of the system where internal combustion engines are the prime mover.

701.32 Selective Coordination. Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices, with the same documentation requirements as 700.32.

Article 702 — Optional Standby Systems

702.2 Definition. Optional standby systems are those systems intended to supply power to public or private facilities or property where life safety does not depend on the performance of the system. Optional standby systems are intended to supply on-site generated or stored power to selected loads either automatically or manually.

702.4(A) Available Short-Circuit Current. Optional standby system equipment shall be suitable for the maximum available short-circuit current at its terminals.

702.4(B) System Capacity. The calculated load shall be in accordance with Article 220 or by another approved method.

• (B)(1) Manual Transfer Equipment. Where manual transfer equipment is used, an optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at one time. The user of the optional standby system shall be permitted to select the load connected to the system.
• (B)(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (a) or (b): (a) Full Load — the standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment; or (b) Load Management — where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system.

702.6 Transfer Equipment. Transfer equipment shall be suitable for the intended use and designed and installed so as to prevent the inadvertent interconnection of all sources of supply in any operation of the transfer equipment. Transfer equipment shall be listed, and shall be marked for emergency, legally required, or optional standby use as applicable.

The prohibition on inadvertent interconnection is what makes the ubiquitous suicide cord — a cord with male plugs on both ends used to back-feed a dryer receptacle from a portable generator — a serious Code violation as well as a lineman-killer.

702.7 Signs. (A) Standby. A sign shall be placed at the service-entrance equipment for commercial and industrial installations that indicates the type and location of each on-site optional standby power source. (B) Grounding. Where removal of a grounding or bonding connection in optional standby system equipment interrupts the grounding electrode conductor connection to the alternate power source, a warning sign shall be installed. (C) Power Inlet. Where a power inlet is used for a temporary connection to a portable generator, a warning sign shall be placed near the inlet to indicate the type of derived system that the system is capable of based on the wiring of the transfer equipment. The sign shall display the following: "WARNING — FOR CONNECTION OF A SEPARATELY DERIVED (BONDED NEUTRAL) SYSTEM ONLY" or "WARNING — FOR CONNECTION OF A NONSEPARATELY DERIVED (FLOATING NEUTRAL) SYSTEM ONLY."

That sign requirement answers the question raised in section 9.6 in the field: whether the portable generator's neutral must be bonded depends entirely on how the transfer switch is wired, and 702.7(C) requires the answer to be posted at the inlet.

702.12 Outdoor Generator Sets. (A) Where an outdoor housed generator set is equipped with a readily accessible disconnecting means in accordance with 445.18, and the disconnecting means is located within sight of the building or structure supplied, an additional disconnecting means shall not be required where ungrounded conductors serve or pass through the building or structure.

Article 708 — Critical Operations Power Systems

708.1 Scope. Article 708 applies to the installation, operation, monitoring, control, and maintenance of the portions of the premises wiring system intended to supply, distribute, and control electricity to designated critical operations areas (DCOA) in the event of disruption to elements of the normal system.

Critical operations power systems are those systems so classed by municipal, state, federal, or other codes by any governmental agency having jurisdiction, or by facility engineering documentation establishing the necessity for such a system. These systems include but are not limited to power systems, HVAC, fire alarm, security, communications, and signaling for designated critical operations areas.

708.5 Physical Security. The area containing the equipment shall be restricted to authorized personnel and shall be protected by an approved means.

708.11 Wiring of HVAC, Fire Alarm, Security, Emergency Communications, and Signaling Systems. Wiring for these systems within a DCOA shall be installed in accordance with 708.10.

708.14 Wiring of Equipment. Wiring from an emergency source or emergency source distribution overcurrent protection to critical loads shall be kept entirely independent of all other wiring and equipment.

708.20(F) Surge Protection. A listed SPD shall be installed in or on all distribution panelboards and all critical branch panels supplying critical operations power systems.

708.54 Selective Coordination. Critical operations power system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices, with the same qualified-person selection and documentation requirements found in 700.32 and 701.32.

What Selective Coordination Means

Selective coordination is often confused with a series rating (section 7.4). They are opposites.

For a fire alarm circuit or an egress lighting circuit, losing the whole panel because of a single branch-circuit fault is unacceptable, which is why selective coordination is mandatory in those systems and why a series-rated combination is not an acceptable substitute.