Generators and transfer switches for mission critical facilities

Transfer switches and generators are specified into commercial buildings—specifically health care facilities—to provide optional standby, emergency, and legally required power.


Learning objectives

  1. Understand the basic requirements of generators when used for standby or back-up power.
  2. Learn which code pertains to the design of generators and transfer switches.
  3. Obtain an overview of the types of systems supplied from transfer switches.

Figure 1: This line-up of 1500 kW generators is at a resort on the Las Vegas Strip. A total of seven generators were configured by the team at JBA Consulting Engineers with paralleling switchgear for an aggregate capacity of 10.5 MW of power generation. CMost commercial building applications require some form of an alternate power source for life safety purposes and to comply with NEC 700 and various other building codes. For small facilities, this typically is achieved by using emergency battery packs in select light fixtures for egress, exit signage, and integral battery backup for life safety equipment such as the fire alarm system (see NFPA 70: National Electrical Code (NEC) 700.12(A)&(F)). 

If the facility is larger or maintenance of a large quantity of individual battery packs is a concern, a central battery inverter system could be installed (NEC 700.12(C)). As the name implies, a central battery inverter replaces the individual batteries scattered throughout the building with one or more central locations for the batteries. This type of system must be listed for the purpose and requires dedicated distribution from the inverter system to serve the emergency egress lighting fixtures and exit signs. Alternatively, a standby generator system may be used in lieu of a battery-based inverter system (NEC 700.12(B)). Batteries are expensive and require replacement every 3 to 5 years. Although a generator system also requires scheduled maintenance and testing, it provides the building with additional uses beyond life safety. Before delving into source of power and transfer equipment, we will explore the three different types of code-defined systems.

Segregated systems

NEC Chapter 7 has requirements for three distinctive systems that may be served from a standby generator. 

  1. Emergency
  2. Legally required standby
  3. Optional standby.  

The emergency system loads are addressed in NEC Article 700. Only those loads as defined in Article 700, Part IV are allowed to be connected to this system. Generally, loads specified for “emergency use” that may be connected to this system include emergency light fixtures and exit signs for the designated paths of egress and fire alarm systems. Depending on the codes adopted by the local authority having jurisdiction (AHJ), the loads classified as “emergency use” may differ slightly, but for the most part the International Building Code (IBC), International Fire Code (IFC), and NFPA are fairly consistent.

NEC Article 701 provides the criteria for any legally required standby loads. The NEC defines these loads differently than Article 700 does. They are not designated as “emergency use” and are generally mandated by the AHJ. Legally required standby loads could include other loads considered critical to building evacuation and firefighting, such as the HVAC equipment that provides smoke management during and after a fire, and communication systems.

The systems addressed in the first two articles of NEC Chapter 7 are code required where applicable, but NEC Article 702 allows a third system to be provided for any other loads a building owner may deem critical to its business or when disruption may cause significant financial loss. This system is called the optional standby system.

Health care facilities’ design differs as NEC Article 517 contains more prescriptive requirements for an essential electrical system. This system comprises two components: the emergency system and equipment system (NEC 517.30(B)(1)). These systems may be combined if the total maximum demand does not exceed 150 kVA (NEC 517.30(B)(4)). 

The emergency system is composed of two separate subsystems, one for the life safety system and the second for critical branches (NEC 517.30(B)(2)). NEC 517.32 explicitly details the only functions that may be connected to the life safety branch. NEC 517.33 lists the functions related to patient care that may be connected to the critical branch. Although an exhaustive list is not provided, NEC 517.33(A)(9) gives performance criteria to allow the designer to make judgments for minimal additional loads it deems critical to patient care to also be connected to this branch of distribution. NEC Article 517 is comparable to Article 700, but with additional functions and more stringent wiring methods (NEC 517.26). NFPA 99 Appendix A, A. also is a good reference point. 

The equipment system functions are addressed in NEC 517.34. For the most part, this system serves mechanical equipment deemed necessary for proper function of a hospital with inpatient care. NEC 517.34(B)(8) also allows the engineer to use his or her judgment to connect additional loads not specifically listed. 

Continuous, prime, standby 

Figure 2: Single generator standby system configurations are shown. An ATS is provided for each of the segregated systems. Courtesy: JBA Consulting EngineersThere is often confusion regarding the differences between continuous, prime, and standby power systems—and when each type is appropriate. Most manufacturers publish data for the same generator set with both a prime and standby rating; it’s physically the same piece of equipment. The main differences are rated capacity and warranty implications.

A standby generator is rated with the higher capacity but is warrantied for use only for a limited number of annual hours. The average varying load is typically required to fall below 70% to 80% of the generator nameplate rating. This type of generator is the most common application in typical commercial applications where there is a fairly reliable utility. If the project is located in an area where utility historical data indicates a large number of average yearly outages and/or substantially long outages, the installation may exceed most manufacturer warranty terms. So, for this type of environment, a standby generator may not be the best long-term option. In addition to potential warranty issues, the generator life may be greatly reduced and incurred maintenance costs may be substantially higher. 

A prime rated generator is provisioned for continual run time with a correlating reduction in the nameplate capacity as compared to a standby unit. Again, the average varying load is typically required to fall below 70% to 80% of the generator nameplate rating. The lower average loading placed on the generator puts less stress on the components while operating. The manufacturer de-rates the engine for prime applications due to more hours on the engine to ensure the operational life is not compromised. A short duration overload of up to 10% is generally acceptable. 

A continuous rated generator is just that: These generators have upgraded components and a larger cooling system that are designed to withstand the stress of operating at 100% of nameplate capacity for an unlimited number of hours. The loads, however, need to be fairly steady or nonvarying, per manufacturer guidelines.

Transfer switches 

All of these systems typically are connected to the generator through the use of transfer switches. In most cases, automatic transfer switches (ATS) are code required, except in the case of optional standby (NEC 700.6) and some health care equipment system loads (NEC 517.34(B)) where manual transfer switches are permitted. ATS equipment is available in closed- and open-transition configurations. Because a standby generator system that serves emergency and legally required standby loads (or in the case of health care) requires monthly testing (NFPA 110-8.4.2), a closed-transition ATS provides a system with the least disruption to building occupants during testing. The operation of a closed-transition ATS momentarily parallels the utility with the generator before breaking connection from utility. This is often referred to as a make-before-break action. It maintains a reasonably uninterrupted service to downstream loads during routine testing and transfer back to a restored utility following a loss of power sequence. 

An open-transition ATS typically is used when the serving utility company does not allow the service to connect close-transition with a generator. In this case, as the transfer occurs, the load momentarily breaks from the utility before making it to the generator source. Re-transfer back to the utility also has the momentary break. It is important to coordinate this design aspect with the local utility company early in the design and to keep the client aware of any limiting factors imposed by utility requirements.

Another option when considering the ATS is whether bypass isolation should be provided. This type of transfer switch includes a second “backup” transfer switching mechanism for use when the main transfer mechanism requires maintenance or replacement. The main transfer switch is manually bypassed to the second transfer switch and then isolated from the system. This feature enables the loads to continue to operate while the ATS is essentially taken out of service, while also maintaining manual transfer capability to generator source if utility power is lost. This type of ATS has a premium cost over a standard ATS and is often specified when the loads are considered extremely mission critical such as in hospitals, financial data centers, and casinos. Some jurisdictions mandate the use of bypass isolation switches; such is the case when applying them to health care facilities in California, which are governed by Office of Statewide Health Planning and Development (OSHPD). 

Smaller systems where the generator is located in an outdoor enclosure are usually capable of including a few unit-mounted circuit breakers to serve an emergency, legally required standby, and optional standby ATS without additional distribution equipment. Space is limited, however, and in most cases where the generator is larger than a few hundred kW, a generator distribution switchboard will be required. A requirement that is often overlooked is that the circuit breaker and subsequent feeder that serves the emergency ATS must be located in a separate vertical switchboard section from the legally required and optional standby devices and feeders (NEC 700.9(B)(5)). 

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