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Generators and Transfer Switches for Mission Critical Facilities

by salesassistant@gopower.com 9. September 2014 05:41

By: Robert R. Jones Jr., PE, LEED AP, JBA Consulting Engineers, Las Vegas

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.

Most 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.

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. 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. 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.

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. These systems may be combined if the total maximum demand does not exceed 150 kVA. 

Continuous, prime, standby 

There 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. 

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. 

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 and some health care equipment system loads 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, 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. 

Smarter distribution 

In some larger installations where a single generator is not sufficient or multiple gensets are required to provide system resilience, paralleling switchgear (PSG) provides a scalable and intelligent automation option. Typical PSG systems provide the programming and logic to synchronize the generators and optionally operate a transfer pair of circuit breakers. Some PSG designs include a utility feeder input on the load bus. The utility input circuit breaker must be opened and closed simultaneously with the main generator input circuit breaker such that only one input is closed onto the load bus. In this sense, this equipment could operate like an ATS. The PSG can be programmed to operate in manual open transition, manual closed transition, automatic open transition, and automatic closed transition.

On-site fuel storage 

The generator run time is largely dependent on the amount of fuel storage. The most common fuel source for a standby generator system is diesel. Natural gas and hybrid generators also are available; this article will focus on diesel. 

The first priority for determining the amount of on-site fuel needed to be stored is to study the load applications. If loads are limited to backup for life safety to meet code minimum criteria, the fuel supply must provide a minimum of 2 hours of run time. However, if additional standby loads are served or the client indicates a need to provide additional backup time, the fuel storage must be sized appropriately to address the requirement. A good rule of thumb is 7 gal/hr/100 kW of generator nameplate at full load. Again, fuel consumption should be reviewed with the specific manufacturer for the project’s generator. 

There are multiple methods for on-site fuel storage. For exterior generators, a sub-base (or belly) tank is usually the least complicated. It should be sized to provide the amount of fuel required to operate the individual generator through the specified number of hours. In this sense, it is a self-contained fuel system and is a part of the packaged generator system. 

The fuel storage and supply must be designed by a qualified professional. Each generator will require a day tank in the form of a small fuel storage tank located on or in close proximity to the generator to provide the initial fuel for startup. Day tanks are typically sized to store 15 min to 1 hr of fuel, based on the generator fuel consumption rate. The day tanks will have fuel piping from the bulk storage tank(s) with additional return piping from the generators back to the bulk storage tanks for fuel recirculation.

Determine which system—emergency, legally required standby, or optional standby—is required by code, and carefully select and specify the product to best suit the facility’s needs. 


Robert R. Jones Jr. is an electrical project engineer at JBA Consulting Engineers with more than 10 years of design experience. Jones has experience in market sectors including hospitality, commercial, medical, and government projects. He specializes in medium- and low-voltage distribution systems, emergency/standby power systems, renewable energy design and implementation, circuit analysis calculations, and equipment space planning.

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