Understand the distinctions of generators for health care

Introducing a standby generator system on a project brings new design complexities and application of additional articles of NFPA 70: National Electrical Code, among other applicable codes. Generator systems designed for health care occupancies have a unique set of requirements that differ in many ways from those of other occupancy types.  

It is important to both recognize these nuances and understand how to implement them in design. This article highlights the more impactful distinctions between applicable articles of the NEC but does not cover the full spectrum of code requirements found therein. The referenced version of NEC is the 2020 edition. 

Allowable uses of system 

NEC Article 700 is written specifically to address requirements for emergency power and illumination in the event of an electrical utility outage. Only very specific building loads, classified as emergency, are permitted to be supplied by the emergency system. NEC 700.2 Definitions direct that “municipal, state, federal or other codes, or by any governmental agency having jurisdiction” establishes which loads are classified as emergency. 

The central principal around having the emergency system is to “supply, distribute and control power and illumination for safety to human life.” In practice for most occupancies, such loads on the emergency system within a building typically include egress lighting, exit signage and fire alarm systems. High-rise occupancies will have an expanded set of requirements that are dictated by other applicable codes including the International Building Code. The key takeaway regarding NEC 700 Emergency Systems is that the requirements are centered around the safety and protection of human life. 

In similar fashion, but with a somewhat different focus, NEC Article 701 lays out the use limitations of the legally required standby system. Again, only specific building loads are permitted to be connected to the legally required standby system and those loads are classified as such by applicable locally adopted “…codes or by any governmental agency having jurisdiction.”  

The focus of NEC 701 is to prevent interruption of service to electrical loads that could introduce “hazards or hamper rescue or fire-fighting operations” (701.2 Informational Note). A wide range of loads can fall into this category including elevators, smoke purge systems, communications systems and fire pumps, among possibly many others depending on the occupancy and the jurisdiction. Varying jurisdictions have differing rulings on what is and what is not part of the legally required standby system. Remember that NEC 701 legally required systems are focused on safeguarding firefighting and rescue activities. 

By contrast, NEC Article 702, optional standby systems, is devoted to those loads whose disruption “could cause discomfort, serious interruption of the process, damage to the product or process, or the like” (702.2 Informational Note). In essence the use, as the article’s name implies, is for the standby backup of virtually any optional load; any load that does not specifically have a legal obligation to be connected to the emergency or legally required standby systems (Articles 700 or 701). Most often, these optional systems are deployed to protect valuable production or inventory assets (e.g., refrigerated goods) in commercial and industrial settings.  

The use requirements of NEC 517 Part III, governing the essential electrical system of health care facilities, are of a different variety than those of NEC 700, 701 and 702. The three branches of the essential electrical system — life safety, critical and equipment — are intended to accommodate specific health care operational functions in the event of an electrical utility outage to the facility.  

Figure 2: Typical co-located generator sets in a hospital setting. Courtesy: CannonDesign

NFPA 99: Health Care Facilities Code provides further explanation, beyond NEC 517 Part III, of permitted uses of essential system branches in health care facilities. The life safety branch is similar to the NEC 700 emergency branch of other occupancy types in that its use is to support egress illumination, exit signs, fire alarm and other similar loads focused on protection of human life.  

However, 517.32 prohibits uses other than those specifically identified in this section. Unlike NEC 700 that leaves the specific use load types up to the authority having jurisdiction’s discretion and other applicable code requirements, 517.32 is restrictive in that permitted use loads are explicitly identified, to the exclusion of all others. It is also important to note here that 517.26 requires the “life safety branch of the essential electrical system shall meet the requirements of Article 700, except as amended by Article 517.” Use limitation of the life safety branch is one such amendment included in Article 517. 

The critical and equipment branches of health care essential electrical systems are a different story. These branches of the essential electrical system distribution are exclusive to health care occupancies and have specifically defined permitted uses stated in 517.33 and 517.34, respectively. Their purpose is to provide a limited amount of service to support “… orderly cessation of procedures during the time normal electrical service is interrupted for any reason” (717.25). The critical and equipment branches do not align with Articles 701 or 702 (legally required or optional standby systems).  

Generator system capacities 

Generator capacity is also a point of variation between Articles 700, 701 and the requirements of Article 517, Part III. The alternative power source described in NEC 700, most often a generator (although NEC allows other options), is permitted to not only carry NEC 700 emergency loads, but also NEC 701 legally required and NEC 702 optional standby loads.  

This is contingent on sufficient capacity, or via automatic load management capabilities, to ensure that capacity is prioritized for emergency loads first and foremost (safety to human life). The alternate source capacity must carry, simultaneously, the entire load connected to the emergency system. This is the baseline minimum; obviously, additional capacity is needed if additional branches of distribution (legally required and/or optional standby) are also to be served by this alternative source. 

However, the preceding differs from the requirements found in Article 517, Part III, specifically 517.30 (D) (Capacity of Systems). This portion of code prescribes that the “essential electrical system shall have the capacity and rating to meet the maximum actual demand likely to be produced by the connected load.” This section goes on to permit demand calculations for sizing of the generator(s) to be based on any combination of the following:  

• Prudent demand factors and historical data.  
• Connected load.  
• Feeder calculations.  

Further, the code text explicitly tells us that “the sizing requirements in 700.4 and 701.4 shall not apply to hospital generator set(s).” This may appear to be a subtle distinction between capacity requirements for health care facility essential system generators and those of other occupancy emergency generators, but the implications can be significant.  

The commentary language found in the NEC handbook affirms the author’s intention to “practically” size hospital generators and lessen the effects of under-loaded generator sets (e.g., wet-stacking, etc.). Articles 700 and 701 have no such provision. An engineer must approach the sizing of generators for these two system types (health care essential system versus other occupancy emergency systems) deliberately attentive to the specific nuances of the applicable articles (see Figure 1).  

Figure 1: Capacity of essential system (Article 517) is sized using multiple sources of data. Emergency (Article 700) system capacity is determined by connected load only. Courtesy: CannonDesign

System distribution separation 

While the alternative power source (e.g., generator) is permitted to service emergency, legally required and optional standby loads, the separation of emergency loads (and their distribution) is required from all other loads. This separation begins ahead of the transfer switches and continues through the entire system all the way down to the branch circuit loads. Emergency system transfer switches are also required to be dedicated specifically to emergency distribution and are not permitted to serve loads on either legally required or optional standby distribution.  

The emergency distribution system required physical separation from all other loads (e.g., legally required and optional standby), starts at a distinct point. The specific requirements are laid out in 700.10 (B)(5). This point is physically located between the alternative power source and the transfer switches and can be accomplished in two permissible ways: either by feeding the emergency system’s automatic transfer switches from a separate (dedicated) vertical section within a switchboard or switchgear; or by using individual disconnects (separate, dedicated enclosures) to serve the emergency system’s ATS(s). It is interesting to note that the NEC does not include this same requirement in Articles 701 or 702 for legally required standby or optional standby systems.  

Further, neither Articles 701 or 702 prohibit the sharing of transfer equipment and distribution to serve legally required or optional standby loads. This means, at least theoretically, a single vertical section in a switchboard could be used to feed both legally required and optional standby ATSs; or both branches of distribution (legally required and optional standby) could share an ATS. It is not necessarily a recommended practice, but not a prohibited arrangement be that as it may. 

With regard to system wiring, NEC 700.10 (B) requires that “wiring from an emergency source … to emergency loads shall be kept entirely independent of all other wiring and equipment.” As with most sections, there are a couple of specific exceptions, but by and large, the emergency system wiring (within raceway and enclosures) needs to remain clear of all other wiring in the building.  

It may come as a surprise to learn that this same requirement for wire separation is not found in Article 701 for legally required systems. In fact, quite the opposite, it is explicitly “permitted to occupy the same raceways, cables, boxes and cabinets with other general wiring” as stipulated in 701.10. The same is true for optional standby system wiring (702.10). 

Different rules apply for health care essential electrical systems related to distribution separation. Article 517.30 (B)(1) mandates that hospitals provide separation between their three branches of essential electrical system distribution (life safety, critical and equipment). Further, 517.30 (B)(2) requires that “each branch of the essential electrical system shall have one or more transfer switches.” Smaller essential systems whose maximum demand is less than or equal to 150 kVA are permitted to share a single transfer switch and downstream distribution to serve one or more branches. By and large though, it is typical that maximum demand will exceed this threshold thereby triggering the requirement for separate transfer switches and downstream distribution for each of the three branches.  

While Article 700 requires that transfer switches be fed from separate vertical sections or from separately enclosed disconnect switches, Article 517 has no such requirement for the essential electrical system. However, as discussed earlier, the life safety branch of the essential system is subject to the requirements of article 700, unless specifically amended in article 517. As such, the life safety branch transfer switch is required to be fed from a dedicated vertical section of gear or by separate disconnect switch. The same rule does not apply to the critical or equipment branches of essential distribution.  

Further, health care essential electrical systems are also subject to a different set of requirements for system wiring separation for each of the branches. Article 517.30 (C)(1) requires that life safety and critical branch “… shall be kept entirely independent of all other wiring … and shall not enter the same raceways, boxes, or cabinets with each other or other wiring.” A few exceptions apply, but generally, separation is required.  

The same does not apply to the equipment branch, however. “The wiring of the equipment shall be permitted to occupy the same raceways, boxes or cabinets that are not part of the essential electrical system” (517.30(C)(1)). Put simply, equipment branch wiring can run in the same pathways as normal distribution or optional standby distribution wiring (see Figure 3).  

Figure 3: Article 700 requires separate vertical sections or separately enclosed disconnects for emergency standby system. This separation requirement applies also to health care life safety branch. Courtesy: CannonDesign

Ground fault and surge protection in generators 

Automatic ground-fault protection is not required for the NEC Article 700 alternative power source. In practical terms, a ground fault-protective device on a generator may not be beneficial if it trips and causes an outage of the emergency system when it is needed most. Remember that the purpose of the emergency system is preservation of life. However, 700.6 (D) does require ground-fault indication on larger sized (>150 volts, 1,500 amperes or more) main overcurrent protective device for the alternative source. This simply provides an indication, typically an audible alarm and a flashing LED, but does not interrupt the alternative source (e.g., does not create an outage).  

For health care facilities essential electrical system, ground-fault indication is required, by extension of Article 700’s applicability to the life safety branch. Article 517 does not amend this requirement. However, 517.17(B) does specifically prohibit the use of automatic ground-fault protection on the load side of essential system transfer switches.  

Surge suppression provisions are also a part of emergency system requirements. Beginning with the 2014 edition of NEC, a requirement was added in 700.8 that all emergency system switchboards and panelboards include a listed surge protection device.  

Again, because the life safety branch is subject to the requirements of Article 700 (expect as amended by 517), the life safety branch panelboards of health care facilities are also required to include a listed surge protection device. The same does not apply, however, to the critical and equipment branches of the essential system (see Figure 4). 

Figure 4: Article 700 requires emergency panelboard installations include surge suppression. The same requirement extends to the life safety branch of healthcare essential distribution. Courtesy: CannonDesign

Temporary source of power provisions 

Starting with the 2017 edition of NEC, paragraph 700.3 (F) was added to include a requirement for permanently installed provisions to connect a temporary source of power in the event that the on-site generator, for example, is disabled for maintenance or repair. Some exceptions are included in this paragraph that may apply, so careful review is warranted. Most often, the provisions will include a point of connection (e.g., cam-lock box) for a roll-up trailered generator set and a switching means with interlock.  

No such specific requirement is found in Article 517 for health care essential systems. However, given the interrelationship between the essential system life safety branch and Article 700 discussed earlier, the requirement for temporary source of power provisions is applicable. 

Figure 5: Typical paralleling switchgear for the interconnection of multiple generator sets. Courtesy: CannonDesign

Overcurrent protective device coordination 

One of the most important differences between the requirements of NEC Article 517 for health care essential systems and NEC 700 emergency systems is that of selective coordination. Article 700 requires that the emergency system’s overcurrent protective devices be selectively coordinated will all supply-side overcurrent protective devices.  

Of course, the exception is included that two OCPDs in series need not be selectively coordinated. The stipulation being that no loads are connected in parallel to the downstream device in this arrangement. This makes sense because either of the OCPDs opening will impact the same load, which makes coordination of these two devices unnecessary.  

The extent of the level of coordination is the difference between these two articles (517 and 700) that is most interesting. The term “coordination (selective)” is defined in Article 100 Definitions as:  

“Localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent protective devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current and for the full range of OCPD opening times associated with those overcurrents.”  

The full range of device opening times is a subtle, but important distinction. Note that Article 701 includes this same requirement for legally required standby systems but it is absent in Article 702 for optional standby systems.  

Article 517 Part III (Essential Electrical System) has a somewhat different set of requirements as it relates to OCPD coordination. In 517.30(G), NEC requires that “OCPDs serving the essential electrical system shall be coordinated for the period of time that a fault’s duration extends beyond 0.1 second.”  

As with NEC 700, 517.30(G) provides a couple of exceptions associated with series-connected OCPDs. Unlike NEC 700, however, the coordination requirements for health care essential system OCPDs do not cover the full range of overcurrent conditions. Again, an important distinction. Coordination of a system of overcurrent devices is notably more easily achieved for the period of time that a fault’s duration extends beyond 0.1, as compared to a fully selective coordination as required in NEC 700 (see Figure 6).  

Figure 6: Coordination to 0.1 sec (left) is more easily achieved. Coordination to 0.01 seconds (right) is more difficult to achieve since instantaneous region of trip curves often fall in this region and often overlap. Courtesy: CannonDesign

It is important for the engineer of a standby generator system, especially on health care projects, to become well acquainted with the requirements that are imposed by applicable codes and local amendments to those codes. This article focuses on the requirements of NEC and the more apparent nuances between health care and other occupancy generator systems. Other codes and local amendments will come into play and impact aspects of design. Each system is unique and should be treated as such, with the proper attention given to the role occupancy plays in determining applicable requirements for the installation.  

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