Note: This article is based on the 2023 NEC.

Optional Standby Systems supply power where life safety does not depend on the system. These provide an alternate power source for industrial and commercial buildings, farms, and residences to serve loads such as heating and refrigeration, data processing, and industrial processes that if stopped (or suddenly stopped) can cause discomfort or economic loss.

Stringency differences
Many of the requirements for these systems are alike. For example a sign at the service disconnect must identify the location of each of these power sources. And for each system, it components must be marked to show it is part of that system.

But they also differ in key ways, with the idea being emergency and legally required systems have more stringent requirements than optional systems do. For example, Emergency Systems and Legally Required Standby Systems must meet specific commissioning and maintenance requirements [700.3 and 701.3], Optional Standby System do not.

Optional systems are excluded from the requirement that the OCPDs must be selectively coordinated with all supply-side and load-side OCPDs. The design must be made by a qualified person and it must be documented and made available to those authorized to design, install, inspect, maintain, and operate the system [700.32(A) and 701.32(A)]. If OCPDs are replaced, they must be re-evaluated to ensure selective coordination is maintained [700.32(B) and 701.32(B)]. If modifications, additions, or deletions to the system(s) occur, selective coordination must be reevaluated [700.32(C) and 701.32(C)].

Emergency systems
Article 700 covers the installation, operation, and maintenance of emergency systems. Figure 01

An Emergency System must have adequate capacity per Article 220 or by another approved method [700.4(A)].

The emergency system power source(s) can supply emergency, Legally Required System, and Optional Standby System loads where a load management system includes automatic selective load pickup and load shedding to ensure adequate power to each system.
Audible, visual, and facility or network remote annunciation signaling devices must be installed where applicable for the purposes enumerated in 700.6(A) through (D). For example, to indicate the emergency source is carrying load.

Emergency system conductors cannot be installed within any cabinet, enclosure, raceway, cable, or luminaire with nonemergency loads, except as noted in 710(B)(1) through (6). For example, wiring within a traveling cable to an elevator.

Line voltage supply wiring and Class 2 power-limited emergency lighting control devices must comply with 700.10. Class 2 power-limited emergency circuits must comply with 700.11(B) through (D). For example, exposed cable, cable tray, or raceway systems must be marked to be identified as a component of an emergency circuit or system within 3 ft of each connector and at intervals not to exceed 25 ft.

Emergency power must be available within 10 seconds upon failure of normal power. The emergency power supply must be one or more of the types described in 700.12(C) through (H). For example, a generator.

In selecting an emergency power source, consider the occupancy and the type of service to be rendered, whether of minimum duration, as for evacuation of a theater, or longer duration, as for supplying emergency power and lighting due to an indefinite period of current failure [700.12A)].

The emergency power source must be of suitable rating and capacity to supply and maintain the total load for the duration determined by the system design. In no case can the duration be less than 2 hours of system operation unless used for emergency illumination in 700.12(C)(4) or unit equipment in 700.12(I).

Emergency circuits must supply only emergency loads [700.15]. Emergency illumination (egress lighting, illuminated exit signs, etc., per 700.16(A) must be designed and installed so the failure of any illumination source will not leave in total darkness any space requiring emergency illumination [700.16(B)]. For any emergency system, emergency illumination is required for an indoor service disconnect [700.16(D)].

Legally required
Article 701 covers the installation, operation, and maintenance of Legally Required Standby System.

Equipment for an Legally Required Standby System must be suitable for the available fault current at its terminals [701.4(A)]. The alternate power supply must have adequate capacity per Parts I through IV of Article 220 or by another approved method. The system capacity must be sufficient for the rapid load changes, and transient power and energy requirements associated with any expected loads [701.4(B)].

The alternate power supply can supply Legally Required Standby System and Optional Standby System loads if there is adequate capacity, or where a load management system includes automatic selective load pickup and load shedding is provided that will ensure adequate power to the Legally Required Standby System [701.4(C)].

Audible and visual signal devices must be installed where practicable for the four purposes listed in 701.6. For example, to indicate a malfunction of the standby power source.

If the normal supply fails, legally required standby power must be available within 60 seconds. In selecting the power source for an Legally Required Standby System, consider the type of service to be rendered, whether of short-time duration or long duration [701.12(A)]. Also consider the location or design, or both, of all equipment to minimize the hazards that might cause complete failure due to floods, fires, icing, and vandalism [701.12(B)].

A generator approved by the AHJ and sized per 701.4 is permitted as the Legally Required Standby System power source if it has the means to automatically start the prime mover upon failure of the normal power source [701.12(D)(1)].

Power supply systems must consist of one the types listed in 701.12(D) through (F). For example, a storage battery.

Optional systems
Article 702 covers the installation requirements for permanent and portable Optional Standby System [702.1].

If the connection of the loads to the Optional Standby System is manual or nonautomatic, size the Optional Standby System to supply all the loads that are intended to operate at one time [702.4(A)]. If the connection for a load to the Optional Standby System is automatic, size the Optional Standby System as follows:

(a) Full Load. The Optional Standby System must be capable of supplying the full load that is automatically connected as determined by Article 220 or another approved method.

(b) Energy Management System (EMS). Where a system is employed per 750.30 that will automatically manage the connected load, the Optional Standby System must have a capacity sufficient to supply the maximum load that will be connected by the EMS.

Interconnection equipment or transfer equipment is required for the connection of an Optional Standby System to premises wiring [702.5(A)]. Interconnection and transfer equipment must be listed and installed to prevent the inadvertent interconnection of all sources of supply.

In other than dwelling units, the short-circuit current rating of the transfer equipment, based on the specific OCPD type and settings protecting the transfer equipment, must be field marked on the exterior of the transfer equipment [702.5(C)].

Optional Standby System installed in parallel with other power production sources must comply with Parts I or II of Article 705 [702.5(D)].

Where a power inlet is used for the connection of a portable generator, a warning sign must be placed near the power inlet to indicate the type of generator permitted to be connected to the inlet. The warning sign must comply with 702.7(C).

Options
As you can see, the NEC requirements for standby power systems that are classed as legally required or emergency are more stringent than the requirements for those that are optional. But the NEC isn’t concerned with optimizing equipment selection, design, or installation [90.1]. If your system is optional, it might make sense to apply at least some of the Emergency System or Legally Required Standby System requirements to it.

A given site might have all three systems. For example, a plant may have an emergency system to facilitate quick evacuation. It may have a legally required system to facilitate the safe and orderly shutdown of chemical processes. And it may have an optional standby system to prevent financial loss due to power loss. It would be necessary to coordinate all three while keeping them separate, so it is critical to understand the purpose and requirements of each.