Emergency power systems (Art. 700) are at the top of the hierarchy of backup power systems. Legally required standby systems, which fall under Art. 701, hold the No. 2 spot. Optional standby systems (Art. 702) are third in the pecking order. Basically, their level of importance corresponds to the order in which they appear in the NEC.
A system is legally required when any government agency having jurisdiction says it is. The rules that dictate what kinds of emergency loads are legally required are found in the locally adopted building code, such as the International Building Code or NFPA 101 Life Safety Code. Unlike emergency systems, legally required systems do not directly protect the lives of the public at large. They prevent shutting down specific loads — the loss of which would create hazards or impede rescue operations. Hospital communications systems, for example, fall under Art. 701 because evacuation instructions announced over the public address system are part of a rescue operation. Article 701 governs the installation, operation, and maintenance of such systems.
Legally required circuits and equipment supply illumination or power upon interruption of the normal electrical supply. They provide electric power to aid in firefighting, rescue operations, control of health hazards, and similar operations. They typically supply such loads as communications systems, ventilation and smoke removal systems, sewage disposal, lighting, and potentially dangerous industrial processes. Surprisingly enough, they might also include fire pumps and elevators, which many people automatically assume are emergency loads. The definition of “Legally Required Standby Systems” in 701.2 contains a Fine Print Note (FPN) that lists some of the items that might be a legally standby system…but be careful — FPNs are informational only [90.5(C)]. Don't depend on this FPN as a comprehensive list. Always consult the authority having jurisdiction (AHJ) and/or fire marshal to find out what's legally required — never guess.
When it's optional. A standby power system is optional when it's not required by Art. 700 or Art. 701. These systems protect public or private facilities or property where life safety doesn't depend on the performance of the system. These systems are not required for rescue operations.
They may supply on-site generated power to selected loads automatically or manually. These are typically installed to provide an alternate source of power. This can be for a variety of facilities, including industrial and commercial buildings, farms, and even residences. They serve such loads as heating and refrigeration systems, data-processing systems, communications systems, and industrial processes. They may also serve any load that the customer considers important enough to warrant a backup system, but not important enough for a building code to require it, such as a break room refrigerator or that important coffee maker.
These systems may be permanently installed, or they may be arranged for a connection to a premises wiring system from a portable alternate power supply. A portable generator doesn't fall within the scope of Art. 702, unless the generator is connected to the premises wiring (Fig. 1).
It should come as no surprise that the AHJ must approve all equipment used in legally required systems [701.4]. What may surprise you is this same rule applies to optional systems [702.4].
Tests and maintenance. While there are compelling operational and economic reasons to conduct testing and maintenance of optional systems, doing so is not an NEC requirement. But for legally required systems, it is [701.5].
Legally required standby system testing consists of acceptance testing and operational testing. The AHJ must conduct or witness an acceptance test of the emergency system upon completion of the installation, and periodically thereafter [701.5(A)]. The systems must also be periodically tested to ensure they are in proper operating condition. Running the system to power the loads of the facility is a generally accepted method of operational testing. In fact, you must provide a means to test these systems under the maximum anticipated load condition [701.5(E)]. In addition, you must maintain records of testing and maintenance [701.5(D)].
Capacity and rating. Legally required and optional standby systems must have adequate capacity to safely carry all loads that are expected to operate simultaneously [701.6 and 702.5]. Because optional standby systems are not critical, the user of the system can select which load(s) to connect to the system.
Additionally, both legally required standby system and optional standby system equipment must be suitable for the maximum available fault current at line terminals. The legally required standby alternate power source can supply legally required standby and optional standby system loads under either of these conditions [701.6]:
The alternate power source has adequate capacity to handle all connected loads.
Automatic selective load pickup and load shedding is provided to ensure that the legally required system takes priority over the optional system.
Power sources. To protect utility workers, the backup system must have approved transfer equipment. Legally required standby systems and optional standby systems can be on the same transfer switch — emergency systems must have their own [700.6(D)]. On legally required systems, transfer equipment must be identified for standby use and be approved by the AHJ [701.7].
Where an outdoor generator has a readily accessible disconnecting means within sight (within 50 feet) of the structure, you don't need an additional disconnecting means for the generator feeder conductors that serve or pass through the structure [701.11(5) and 702.1].
Signage. To warn emergency response personnel of multiple electrical supply systems, both legally required and optional standby systems are required to have a sign indicating the presence of the system. This signage must be placed at the service disconnecting means for the structure, and must indicate the physical location of the standby systems power source [701.9(A) and 702.8(A)] (Fig. 2).
The following power source requirements apply to legally required systems [701.11], but not to optional power sources.
If the normal supply fails, legally required power must be available within 60 seconds (as opposed to 10 seconds for emergency systems). The supply system for the legally required standby power source must be one of these seven types:
Storage batteries [701.11(A)]. These must be of suitable rating and capacity to maintain the total load for 1.5 hours, without the voltage applied to the load falling below 87.5% percent of normal.
Generator set [701.11(B)]. If prime mover-driven, a generator acceptable to the AHJ (and sized per 701.6) must have the means to automatically start the prime mover upon failure of the normal service. Where internal combustion engines are the prime movers, you need a 2-hour on-site fuel supply.
Uninterruptible power supplies [701.11(C)]. A UPS must comply with the requirements for batteries and generators.
Separate service [701.11(D)]. An additional service installed per Art. 230 can serve as a legally required source of power, but only if it is acceptable to the AHJ (Fig. 3). To minimize the possibility of simultaneous interruption of the legally required standby supply, a separate service drop or lateral must be electrically and physically separated from all other service conductors.
Connection ahead of service disconnecting means [701.11(E)]. Where acceptable to the AHJ, connection ahead of (not within) the same cabinet, enclosure, or vertical switchboard section as the service disconnecting means is permitted (Fig. 4). To prevent simultaneous interruption of supply, the legally required standby service disconnect must be sufficiently separated from the normal service disconnection means. See 230.82 for equipment permitted on the supply side of a service disconnecting means. You cannot have more than six service disconnects, including the disconnecting means for the standby system [230.71(A)].
Fuel cell systems [701.11(F)]. A fuel cell system meeting the requirements of Art. 692 can be used, provided the system is capable of carrying the load for at least 2 hours of full demand operation.
Unit equipment [701.11(G)]. Unit equipment consisting of a battery, a battery charging means, and automatic actuation can serve to supply some legally required loads, such as lighting. Where unit equipment is used, the battery must maintain at least 87.5% of battery voltage for at least 90 minutes.
Wiring and circuit protection. The wiring for these systems can be in raceways, cables, boxes, and cabinets with other general wiring [701.10 and 702.9].
You don't need to provide ground-fault protection of equipment on the alternate source of legally required standby power systems [701.17]. But you must ensure selective coordination [Art. 100] of the overcurrent protection devices (OCPDs) of legally required power systems with all supply-side OCPDs [701.18]. Selective coordination for optional standby systems is not a Code requirement.
Grounding and bonding. If the system is separately derived, ground it per 250.30 [250.20(D)]. If it's not separately derived, bond it to the system grounding electrode. Nothing in Art. 701 explicitly states this, yet it's explicitly stated in 702.10(B).
Sorting it out. When you step back and take a look at what each kind of standby system is trying to accomplish, the requirements make sense. Article 700 applies to systems or equipment required to protect people who are in an emergency and are trying to get out, while Art. 701 applies to systems or equipment needed to aid the people responding to the emergency. Article 700 lighting provides an exit path, but Art. 701 lighting illuminates fire hydrants and switchgear areas. Article 702 systems don't protect people — they protect against financial loss.
To ensure you don't confuse what's optional with what's legally required, walk down your systems and identify which non-emergency circuits and equipment prevent danger or aid in rescuing people. Review the list with the AHJ and/or fire marshal, making necessary additions.