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Your questions answered: Backup, standby, and emergency power in mission critical facilities

Kenneth Kutsmeda, Jacobs; and Scott Kesler, CannonDesign


Speakers at the Feb. 16 Critical power: Backup, standby, and emergency power in mission critical facilities webcast addressed questions not covered during the live event.

When utility power is interrupted, standby power system failure is not an option for mission critical facilities. Mission critical facilities, such as hospitals, data centers, and other highly critical buildings, must remain operational. Vital mission critical power system characteristics include availability, reliability, survivability, security, and efficiency. Designing reliable and efficient standby power for mission critical facilities poses unique challenges, such as determining the size of standby generators, determining level of redundancy, calculating the amount of onsite fuel, and anticipating every possible scenario that can affect system performance.

Requirements of mission critical standby power systems exceed those of standard commercial projects, which are typically designed to merely comply with applicable building codes. Many times, high expectations of systems for mission critical facilities are influenced by the desire for increased levels of reliability and efficiency.

The Feb. 16 Critical power: Backup, standby, and emergency power in mission critical facilities webcast presenters addressed questions not covered during the live event. The presenters are:


  • Kenneth Kutsmeda, PE, LEED AP, engineering design principal, Jacobs, Philadelphia
  • Scott Kesler, PE, LEED AP, principal, CannonDesign, Chicago.


Question: Is the mission critical rating an ISO 8528 rating?

Kenneth Kutsmeda: No. The mission critical rating is not an ISO 8528 rating. It is an industry/manufacturer rating to allow customers to obtain certification from Uptime Institute or at a minimum, comply with its requirements. The rating is more for warranty purposes. The mission critical rating allows the generator to run without limitation on hours due to the fact that data center loads generally are constant.

Question: Please discuss Type 1 essential electrical systems (EES) and the need for separated branching.

Scott Kesler: Type 1 EESs are required to be separated into three branches: life safety, critical, and equipment. Depending on the size of the system, one or more transfer switches are required. For systems 150 kVA and smaller, a single transfer switch can be used.

Question: Considering the Uptime Institute’s requirement for Tier 3/Tier 4 generator sets to be able to serve facility loads continuously at 100% of the load, how does the mission critical rating meet Uptime’s requirement for Tier 3/4 systems?

Kutsmeda: A prime rating engine that is derated per ISO can operate continuously at 100% load to meet the requirements of Uptime Institute Tier 4. A mission critical engine is a standby-rated engine that is allowed to run for more hours based on the load profile of a typical data center. Some manufacturers have indicated that this rating was created to allow a standby rating to meet the requirements of Uptime. The mission critical rating still has a limitation of 500 hours so I recommend clarifying “continuously” with Uptime Institute if you intend to get certification.

Question: For hospitals, am I to understand that NFPA 99-2015: Health Care Facilities Code changed the arrangement as shown in NFPA 70-2017: National Electrical Code (NEC), Article 517.31(b) and removed equipment branch from the essential system, and that it now refers to life safety/critical as an emergency system?

Kesler: The EES as defined by both the NEC and NFPA 99 include the three branches of life safety, critical, and equipment.

Question: In an application where you have a fire pump on a generator, have you used that capacity to also support optional standby loads (freezers), the theory being that these optional loads no longer need to be supported in the event of a fire?

Kutsmeda: Yes. The NEC allows automatic shedding of one or more optional standby loads to comply with the fire-pump capacity requirements.

Question: To what extent could onsite stationary battery installations take the place of onsite backup generators?

Kutsmeda: NEC Article 708 allows the use of batteries as the sole source of power for critical operations power systems (COPS). However, it also requires that the alternate source be capable of operating for a minimum of 72 hours at full load.

Question: On what branch do health care facility computer systems and networks fall? For nursing staff, patient information is very important. Also, many health devices these days are more connected to networks.

Kesler: The 2012 version of NFPA 99 added a new chapter titled “Information Technology and Communications Systems for Health Care Facilities” to specifically address this issue. Prior to this, there was no specific code that covered these systems that were becoming more critical to patient care. While there are some exceptions, in general, the computer systems and networks are to be served from the critical branch of the EES, and the HVAC serving the IT spaces are to be served from the equipment branch.

Question: If one is powering the entire facility from the emergency generator, say a 9-1-1 center, does there still have to be a separate transfer switch for the life safety circuits or can a single transfer switch be used for all the loads?

Kutsmeda: In my experience, a separate transfer switch and distribution system is still required for emergency life safety in a mission critical facility where the entire building is backed up by a generator. The NEC does not define COPS loads as part of the Article 700 requirements. On systems with multiple generators or generator paralleling switchgear, I recommend that this be reviewed with the authority having jurisdiction (AHJ) because an AHJ may often have different opinions on how that connection must be made.

Question: Shouldn’t staggering loads added to the alternate power source be a concern in designing an EES?

Kesler: Code stipulates that only the life safety and critical branches need to be re-energized within 10 seconds for health care facilities. The equipment branch and any optional branches can be added outside of this time frame. Adding loads in a staggered manner minimizes the in-rush current and starting kVA applied to the generator, and can result in a smaller generator needed to support the required loads.

Question: What is the requirement for ground fault protection on a mission critical generator? Is there an exception to exclude ground fault protection on standby generators?

Kutsmeda: Any 480 V generator breaker rated at 1,000 amps or more is required by the NEC to have ground fault protection if the generator is serving life-safety load and has the option of alarm only and not tripping on ground fault. Mission critical generators are considered optional standby so a ground fault would be required if more than 1,000 amps at 480 V. NEC Article 708 requires multiple levels of ground fault on feeders to prevent taking down the entire system. On medium-voltage generators, one recommendation would be to use some type of resistance grounding system.

Question: Please discuss the 3 types of EESs in health care and where each is used.

Kesler: The three type of EESs in health care facilities are aligned with the level and type of care being administered.

Category 1, or critical care, is defined where the failure of equipment of a system is likely to cause major injury or death. Category 2, general care, is defined where the failure of equipment or system is likely to cause minor injury. Category 3, basic care, is where the failure is not likely to cause injury but can cause discomfort.

Type 1 EESs are required for Categories 1 and 2 areas of hospitals and other health care facilities where patients ae sustained by electrical life-support equipment. Type 2 systems generally are applicable to nursing homes and limited-care facilities, while Type 3 systems are required for other health care facilities falling under the Category 3 classification.

Question: With 72 hours at full load of fuel storage required for COPS, what are your recommendations to cycle fuel and prevent the fuel from spoilage over time?

Kutsmeda: With any type of mission critical system where there are large amounts of fuel storage, I recommend some type of fuel polishing system. Also, having multiple tanks instead of one large tank allows you to transfer fuel between tanks through a polishing system.

Question: For a six-story building with elevators, do we need a generator?

Kesler: For health care facilities, the determination for elevators being on the EES is not determined by the height of the building, but rather the areas being served. Selected elevator service is required for patient, surgical, obstetrical, and ground floors in Type 1 facilities.

Question: Are levels of reliability for mission critical facilities governed by the AHJ or owner/user?

Kutsmeda: For public safety type mission critical facilities (NEC Article 708), the reliability is governed by the AHJ. NEC Article 708 requires the facility to have a redundant alternate source of power or at minimum a means for a roll up. For private mission critical facilities, the reliability is dictated by the owner based on his or her business case—cost of additional redundancy versus tolerance for risk and outages.

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Engineering Hospital Systems to Work Effortlessly

Amara Rozgus, Editor in Chief, Consulting-Specifying Engineer


While quite a bit of engineering goes into each mechanical, electrical, plumbing, or fire and life safety system in a hospital, it is mostly invisible to the patient or visitor. That is, until you need it.

Hospital food notwithstanding, hospitals are pretty amazing places. People enter—sometimes walking, sometimes under more dire circumstances—and often leave relieved of any ills that they went in for. Some hospitals play celebratory music each time a baby is born, if only to remind those who are suffering in a hospital bed that joyous things also happen in hospitals.

The doctors, nurses, and specialists within the hospital are often the people we think of when it comes to hospital care, but it’s really a team effort. After a brief stay in a local Chicago hospital for an appendectomy and some other complications, I realized that the cleaning team, security staff, and yes, even the hospital nutrition department, were all key to recovery. Each person had a role in curing patients and took that role seriously in trying to make people feel more comfortable and secure in an unpleasant environment.

What fascinated me in this ultra-modern hospital, however, was the intricate set of medical gases, electrical systems, HVAC systems, and fire and life safety equipment. Most people don’t think they need 20 electrical outlets near their bed, yet when something goes wrong, those outlets can make all the difference in the world.

For patients struggling just to breathe, specialized environments like a negative-pressure room offer up that extra pulmonary assistance needed. Positive-pressure rooms, like operating rooms, fall at the other end of the spectrum. And throughout the entire hospital, the hallways, atriums, and general staff or visitor spaces have to meet normal HVAC requirements.

During my stay, I heard the public address system announcements about various hospital emergencies, the paging of specialized practitioners, and even a “toaster smoke reported, alarms temporarily disabled” notification.

All of these systems and people had to sync up perfectly to ensure each patient and staff member was safe and cared for in the best possible way.

My hat is off to those of you who engineer systems in hospitals. After just seeing the maze of measurement equipment, electrical systems, and call buttons involved in an everyday hospital room, I’m amazed that each system works together impeccably make a patient’s visit run smoothly, though there are a million things going on behind the scenes.

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The Future of Health Care Delivery



At Glumac, a discussion was opened on how the AEC industry must adapt to deliver projects that work within the new model of health care business plans.

Since its launch in 2010, more than 20 million people have signed up for the Affordable Care Act. More than 6 million were previously uninsured. These numbers bear out a change in approach to the fundamentals of health care delivery and the need for a shift in what's required of a health care facility.

At Glumac, a discussion was opened on how the architecture, engineering, construction (AEC) industry must adapt to deliver projects that work within the new model of health care business plans. In November, Glumac was joined at the City Club of San Francisco by Jeff Fyffe of Jones Lang LaSalle; Stuart Eckblad, University of California San Francisco Medical Center; Karen H. Vegas, El Camino Hospital; and Bill Whipple, Sutter Health. Glumac Principal and health-care leader David Summers moderated the discussion.

Q: Everyone has a unique way of delivering an optimum project. Describe yours.

Stuart Eckblad: We're a public entity. We don't have the flexibility for what people call "pure IPD (integrated project design)." We do IPD-light. The key for health care is that people really need to recognize the delivery model needs to be flexible enough to accommodate change.

For example, a medical office building (MOB) takes 3 to 4 years to complete; a good facility takes 8. In that period of time, health care laws and practices may have changed two to three times. All delivery methods have issues, but the idea of collaborating gives us tremendous flexibility. That allows us in the middle of the project to make a major change to a building and allow the team to treat that change as a separate project. That delivery model is the most receptive to the amount of change we see in the health care industry. We've saved millions.

The public system is different than the private system. And in health care, it's important to accept change and be flexible to meet it while being sensitive to budgets and resources.

Karen Vegas: We're more interested in what we can get out of our teams. Everybody knows there's a lot of work out there, so we're figuring out ways to get successful teams together and build lasting relationships. That, to us, is more important than how we contract.

Jeff Fyffe: When we look at the different types and sizes of projects, we see the $20 million-and-less projects as ones to pursue. On the larger projects, we are not seeing design-build as a delivery method. History tells us that with clients they lose control and lose participation. Q: How do you procure a project that helps improve speed to market?

Bill Whipple: We're looking to grow our partnerships in the AEC industry. We want to bring the partners together that we want to work with. On very complicated subject matters, we want to partner with you and improve your knowledge of the project subject matter.

We need design professionals who can think in multiple design sets. We don't need long design narratives from engineers-just the key systems that were selected and the thought process that went into it. We're also using a lot more data. There are eight different platforms we use in the planning process. We need to consolidate that, and we're looking at databases on the planning side.

I think the University of California (UC) system is starting to learn this-the value is in the owner's reputation. We have found that we are attracting the teams we want because of the reputation of our projects. What's difficult is that this collaborative process is a great discussion, but there's a gap in who can participate in that sort of process.

Jeff Fyffe: We're looking for entities that can work together through difficult situations and do what's best for the project. That sounds easy, but how many projects have you worked on where the moons were aligned and that project came together perfectly? We're looking for a group that's done it before and successfully. Of course, things are a little different for us as we work across state lines. I would say we probably see other states a couple of years behind in terms of being transparent, opening up to their partners, and understanding the benefit of an integrated approach.

Q: How do market conditions effect the way you want to deliver projects?

Karen Vegas: It forces us to be more flexible with how we want to contract. We're looking to build new relationships with new contractors. Right now, we're all pursuing you-which is great, but it jacks up the price. It's not a buyer's market.

In California, it takes 5 to 7, even 10 years, to build a hospital. Our problem is everything gets more expensive as we wait. The service we get from our design professionals is less, but we are paying increasingly more. We can't move things quickly enough to justify the cost. Our credibility slips with our own administration. So, we need to learn how to make choices on who we work with. It's very hard to make these things happen in real time.

Q: Another delivery model that's newer to the health care market is developer involvement in project delivery. Is that something your organizations look at?

Bill Whipple: Sutter Health has not done a public-private partnership (P3). When we do engage developers, we access money at a 3% interest rate. Developers usually do so around 7%. So, it would have to make a lot of sense for us to go that route.

Stuart Eckblad: University of California, San Francisco (UCSF) is a bit of a two-headed organization. The campus side has done a few P3s and have had some success, mostly with patient care facilities. On the health care side, we can borrow for a lot less. And that gives us more flexibility for change. We haven't found an organization that can meet those requirements.

Q: What about premanufactured buildings? There are companies doing it for all kinds of projects. Can you tell us about your experience there?

Karen Vegas: I've tried to shed light on premanufactured facilities. Everyone likes it, but sales is a challenge. The health care industry is a legacy industry, so it's difficult to try to convince people that they need to do some standardization and premanufacture at volume. We lobbied Congress to make premanufactured buildings possible. But, we still haven't figured out how to do that in the health care industry. People still aren't interested in not being able to do incredible customization. My belief is that the industry will get there, but it's a very slow time change. We have to get there. We've all talked about being faster/better/cheaper, but nothing changes. And premanufacturing isn't a drastic change, since we deal with so much of it in our daily lives.

For example, everyone is building behavioral health facilities. In California, we're going to be so far behind because it takes so long to build, yet we can't accept premanufacturing. We're too busy deciding on what type of bathrooms to use, or we worry it's going to be ugly and not unique. That's not necessary, and that's not true.

Bill Whipple: I've been working on this idea of reducing the deviation in our projects. There's a step we may be missing in figuring out how to limit the deviation before we jump to a premanufactured approach. The more we can standardize, the better. We don't need to redesign the exam room over and over again. Oddly enough, it's on the clinical side where we have five different exam rooms and five different clinical models. I can certainly see a day where we start to narrow the variability, and I think it'll bring a lot of sanity to your work.

Stuart Eckblad: In the academic world, it's much different. We have 40 Nobel Prize winners who have clear ideas of what they want, and none are the same. So, it's a challenge. We wrestle with standardization. What we find is that the research changes health care so quickly that the room is not the issue-the equipment, the training program, how many people need to be in that room, and people are changing what rooms they use constantly. So it becomes hard to standardize. So we're moving away from standardized facilities and aiming to standardize production. We're trying to see what systems within the building can be standardized and work from there. For example, in the cancer building we're doing right now, we're not only talking about how much of the room can be standardized, but also how much of the performance can be standardized.

Jeff Fyffe: We're a long way away from modular buildings. We've seen an increase of the contractor's ability to construct these buildings, which is driving prices down. But constant changes in the industry force us to look in other directions.

Q: How are pharmacy-construction mandates impacting your projects?

Bill Whipple: There are multiple sites and smaller scopes of work. We're in the throes of doing a lot of updates. How do we avoid small Americans with Disabilities Act upgrades? We need to make these kinds of upgrades, but we need a systematic approach to do this type of work. How do we manage these mandates? Leadership is getting the opportunity, even in pharmacy work, to reconsider the patient experience and ask what the competency of a specific site is and how it relates to the patient experience. These types of projects help create a systemwide standardization around certain processes and procedures. We need to understand the fundamental value of the work being done. Can we avoid something as simple but terrible as the wrong prescription being provided to the wrong patient?

Karen Vegas: The mandates are making issues in existing buildings and bringing up a lot of unintended consequences. For example, we have maybe one patient a day who gets chemotherapy in this small Los Gatos pharmacy where we can't easily make these mandated changes. So we decide to send them all the way to Mountain View where we're always doing construction and no one can park. Then, another unintended consequence is our reputation for service is damaged. And these mandates happen at a speed that is unmanageable with the construction process. They can be very expensive and very disruptive for patients.

Stuart Eckblad: Three months after a health care facility opens, you start tearing the building apart: codes change, things need to be moved, new ducts need to be installed regardless of fit. In our capital planning, we try to understand that these changes will come and budget for them. For example, you're going to need to set aside $20 million to manage updates. I can't tell you tomorrow what the money will go toward, but you can bet it'll be for something.

Q: Jeff, as you work in multiple states, how do you see lessons learned transferred to providers in other states. We've talked about how newer facilities are being built in California, but Nevada, for example, hasn't seen that same pace.

Jeff Fyffe: We talk to clients outside of California who are willing to just wait and see what California does and see where health care is going to go. Their facilities are aged, but we're not seeing them (up until the last year or two) changing their care model like we have here in California.

Q: Talking about the Affordable Care Act and how it has changed the delivery model, we're seeing more of a focus on wellness. How is that having an impact on facilities, both in the outpatient and inpatient realm?

Bill Whipple: We've built three smaller walk-in clinics. They were conceived with a new population who would want more access to care. It's worth noting that there are going to be a lot of these, and the speed to market is quick. We're seeing the inclusion of pharmacy, nursing, social work, physical therapy, and then the ability to hand off one patient between the variety of services. We're seeing how access to care is changing. Also, our emergency departments are expanding. There could be a day when we stop seeing a need to expand our emergency department, but we're not there yet.

Karen Vegas: We're looking for areas to open these clinics. And to address our emergency department issues, we're looking for urgent care. As we know, most people who go to emergency departments don't need to be there, and we are seeing the same challenge as others. We were behind with digital implementation. It effected emergency departments notably in wait time. Everything has gone up 2 hours and stayed that way. Patient satisfaction has gone down, so in response, the community wants more emergency departments. It's an unintended consequence. How do we keep up with this demand and still put out a good product and serve our patients?

Stuart Eckblad: We're trying to increase the market by moving patients more to outpatient care. That's not a unique issue. We're a primary care center, and what we see-and I think the industry will see-is that these centers will be based on acuity. What we're seeing are two things. First, we're trying to regionalize. Second, home health-they Skype us, or we send a patient home with a connection so they can call in to their doctor, which means more information technology. It's fascinating. Our patients come from all over the world. How do you use this kind of technology to cure patients? Can we connect with China and other countries where, in some cases, they're ahead of us? I'm very interested in what the future of patient care is.

Jeff Fyffe: I've looked at stats from 2012 to 2015; less than 25% was in clinics, there were more than 60% MOBs in 2016, and we're seeing a potential uptick in 2017.

Q: Looking 10 years out, where are the trends in terms of health care construction?

Karen Vegas: A recent study mentioned that in 10 to 15 years we're going to be under-bedded in the South Bay. That's part of the reason why we're buying new land. Also, when we bought our hospital in Los Gatos, we wanted to replace it onsite, but we needed more land. 

Stuart Eckblad: We have patients in our beds for months. I think we'll continue to see an increase in primary care. Outpatient without a doubt will continue to grow. It's not just the baby boomers. Right now, millennials are not showing up for care, but they will.

Jeff Fyffe: It's cloudy. All the clients we're involved with that focus on master planning think there will be less beds, but nobody really knows just when that'll happen. There just isn't a ton of evidence yet to support that.

Bill Whipple: At Sutter Health, we have branded helicopters and ambulances transporting people to more centers of excellence. I think the reason for that is clinical outcomes. A cancer patient might be receiving a dose of a drug that costs $75,000 to make. By consolidating patients in these locations, we can help people receive different therapies in one day. Also, there are so many complex modalities, for example, hybrid operating rooms. Clinical trials say why that's coming out. New approaches to treatments and cures make design difficult, and the complexity of care is going to increase.



-Brian Stern is a senior energy analyst, and Laurie Michelon is an energy analyst at Glumac, a CFE Media content partner. This article originally appeared on

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Paralleling Generator Systems

By Leslie Fernandez, PE, LEED AP, JBA Consulting Engineers, Las Vegas


When designing generator systems, electrical engineers must ensure that generators and the building electrical systems that they support are appropriate for the specific application. Whether providing standby power for health care facilities or prime power for processing plants, engineers must make decisions regarding generator sizing, load types, whether generators should be paralleled, fuel storage, switching scenarios, and many other criteria.

Learning objectives


  • Learn best practices for paralleling generators, touching on dependability, cost savings, efficiency, synchronization, and other aspects.
  • Know the requirements for emergency, standby, and backup power loads.
  • Explain the benefits of parallel power-generation systems. 




Editor's note: Because of the extent of this topic, this article is divided into three parts:


  • Part 1 covers the need for backup power, code requirements, why diesel is preferred, generator ratings, and the benefit of paralleling generator systems.
  • Part 2 covers paralleling switchgear, their components, and common paralleling modes.
  • Part 3 covers installation considerations, interconnection with the utility, and generator sizing. Also, two existing parallel generator systems will be presented and their paralleling elements highlighted.




Expertise in generator power design for emergency, legally required standby, and business critical loads is an essential skill for an electrical engineer to master. When designing generator systems, electrical engineers must ensure that the generators and the building electrical systems can support the critical loads reliably and effectively. Building codes will dictate the prescriptive requirements for these systems (see Table). For business critical loads, the owner or client must be consulted to identify the nonemergency loads that require backup power. When the business needs outlined by the client require increased reliability, a paralleled diesel-generating system and electrical paralleling switchgear (PSG) typically are employed (see Figure 1).

This article examines standby systems in which generators serve as backup to the main utility source, such as those commonly installed in airports, data centers, hospitality complexes, water-treatment facilities, and in most life safety institutional applications.

The need for backup power

Interruptions of electrical power, even for a short duration, can introduce the potential for situations that could imperil public health and safety. Extreme weather-related disasters often disrupt power to hundreds or thousands of people and businesses, potentially for days. When these situations occur, they call attention to the vulnerability of the nation's electrical grid and the importance of alternatives. Hospitals, airports, data centers, water and sewage facilities, fueling stations, communication, and transportation systems require alternate-power sources to limit the impact and ultimately save lives during times of crisis. The loss of electrical power due to storms, natural disasters, or high-power-demand issues are increasingly common. The loss of business and the associated economic impact from power outages are significant. Emergency generators are necessary to provide the reliable power required to maintain operations during primary supply system failures. 

Why diesel-powered generators are used

Diesel-powered generators are considered among the most reliable approaches to providing backup power. When compared with alternative fuels and technologies, diesel-powered generators provide a steady supply of high-quality power and superior performance for transient or fluctuating power demands due to the high-torque characteristics of diesel engines (see Figure 2). Many international building codes and standards effectively require diesel generators for code compliance because of the need for rapid response time, load-carrying capacity, fuel supply and availability, and reliability. One of the most important and unique features of diesel-powered generators, as compared with other technologies, is quick response time and block-loading capability within seconds of normal source-power failure.

NFPA 70: National Electrical Code (NEC), Article 517.30, as well as the California Electrical Code require hospitals and critical care facilities to have standby power systems that start automatically and run at full capacity within 10 seconds of power failure. Natural gas-powered generators generally are not acceptable as a source of power for generators due to fuel-source reliability. During disasters, such as an earthquake, gas lines are immediately turned off to avoid the risk of fire and explosion in case of a rupture. Lastly, diesel generators are available in a range of sizes to meet facility power needs.

Generator ratings

When evaluating generator sets for parallel operation, ratings are important because the rating directly affects the efficiency and effectiveness of the selected generator set based on the application (see Figure 3). It is especially important to understand the specific application, as this will help in selecting the proper rating. Specifically, the following factors should be taken into consideration:


  • Average load factor
  • Maximum required load
  • Typical load variation
  • Annual run time per genset.


ISO 8528-5-2013: Reciprocating internal combustion engine driven alternating current generating sets, Part 5: Generating sets defines generator ratings. These rating definitions were created for gaseous and diesel generator sets and were developed to provide consistency across manufacturers. ISO 8528 should be considered a minimum standard for all generator set ratings. If the manufacturer determines that a product is capable of higher performance than that of the ISO definition, the manufacturer's rating should be used. Definitions relevant to this discussion are power factor, standby power rating, prime power rating, and continuous power rating.


  • Power factor: The standard power factor for a 3-phase generator is usually 0.8.
  • Standby power rating: The generator set is capable of providing emergency power at times when no other source is available. ISO-8528-1 limits the 24-hour average load factor to 70% of the emergency nameplate rating. No overload capacity is available for the standby or continuous-power-rated generators. The ISO standard gives no limit to run time in the event of a utility power outage; however, manufacturers have limits on their generator run time typically in the range of 200 to 500 hours for an entire year. Standby generators typically operate around 50 hours/year with maximum expected usage of 200 hours/year.
  • Prime power rating: Generator sets rated for prime power are designed for supplying electric power in lieu of commercially purchased power from a utility. These include applications like rental generator sets supplying power for temporary use as well as applications that are typically remote from a utility grid, such as wilderness outposts, remote mining, and petroleum exploration operations. ISO limits the 24-hour average load factor to 70% of the prime rating nameplate. Prime-rated power is capable of providing the power for an unlimited time period to a varying load. Overload is also allowed but only at 10% of the rated value, which is permitted to only once in 12 hours.
  • Continuous power rating: With a continuous power rating, the generator can again provide a power supply for an unlimited period-but only to a non-varying load. But the average output power comes out to be between 70% to 100% of the rated power output. The load should be "relatively steady," which means that there should be no significant variations in it; otherwise, the prime power rating could be a better option to consider. A continuous-rated generator usually does not have any overload capability. 


Using low-voltage generators in medium-voltage systems

For generators rated 2,000 kW or less, it is common to install 480 V 3-phase generators and step up voltage transformers. The cost of medium-voltage generators is significantly higher-in the order of an additional $80,000 to $150,000 per unit. Additionally, medium-voltage generators generally do not have the UL listing necessary to support emergency power loads.

For medium-voltage standby generation systems that undertake closed-transition transfer operations, the medium-voltage side of the step up transformers must match the utility's distribution system voltage.

What paralleling is

Paralleling is the operation in which multiple power sources, usually two or more generators, are synchronized and then connected to a common bus. Also, with closed transition back to the utility, PSG will parallel the generators and synchronize the generator output with the utility source for a short duration before transitioning back to utility power. When connecting the generators in parallel or synchronizing with the utility, the following criteria must be met:


  • Matched/proper frequency
  • Matched/correct phase rotation
  • Phase voltages in phase and within specified voltage range.


Typical parameters that determine synchronization include a voltage difference of less than 5%, a frequency difference of less than 0.2 Hz, and a maximum phase angle of 5 electrical degrees between the sources.

Closed transition is used when it is desirable to transfer loads with zero interruption of power when conditions permit. It is used when the generator system transfers back to the utility and when load testing the generators with building loads. Closed transition can be either a soft load transfer or a make-before-break transfer. The PSG soft-load transfer synchronizes and operates the generators in parallel with the utility and transfers loads in increments between the two sources, thereby minimizing voltage or frequency transients on the generator plant and utility distribution system.

The typical soft-load-transfer overlap time is around 2 seconds. The make-before-break transfer will parallel the generators and perform a transfer of load from the generator to the utility. This can be the transfer of one large block load or the transfer of multiple block loads having time delays between the block loads. Time-delay transfer can either be programmed through the PSG or the downstream automatic transfer switches (ATS). Typical ATS make-before-break transition overlap time is usually less than 100 milliseconds. 

Benefits of parallel power-generation systems

Paralleling multiple sources provides increased reliability, flexibility in load management, and maintenance capabilities with little to no disruption. Multiple generators paralleled to a common bus can better serve emergency and business critical loads, particularly for system response time and dynamic load response once in operation. However, more complex, parallel generator standby systems have significant advantages with respect to reliability and redundancy. These advantages include redundancy, efficiency, expandability, and ease of maintenance and serviceability.

Redundancy: The redundancy inherent in the parallel operation of multiple generators provides greater reliability than a single generator unit for critical loads. If one unit fails, the backup loads are redistributed among other generators in the system on a priority basis. In many environments, the emergency loads that need the highest degree of reliable backup power usually account for only a fraction of the overall power generated by the system. In a parallel system, this means that most emergency elements will have the redundancy necessary to maintain power even if one of the units goes out. If an N+1 configuration is adopted, one generator can be offline for maintenance while serving the required loads. Furthermore, providing a running spare, an N+1 generator configuration will increase the reliability of the generator system from 98% to 99.96% reliability.

One of the primary purposes of redundancy is to eliminate single points of failure. The objective is to remove the single points of failure, and caution must be exercised to ensure they are not simply moved to another part of the system. The controls enabling redundancy must also be analyzed to avoid failure modes that compromise reliability. For example, paralleled generator sets that rely on a single master control for signals to start and close to a paralleled bus actually replace one failure point with two, as the master control and the communication link between the master and the generator sets each represent single points of failure. A well-engineered paralleling system will have dual hot-backup control systems, redundant communication pathways, redundant best battery select dc power supplies, multiple breakers, multiple power pathways, and a well-documented procedure for system recovery whenever a component fails (see Figure 4).

Efficiency: A more efficient system provides more stability and reduces cost and losses. Loads do not remain at a constant level in most installations. Variations in power demand can cause a single larger generator to run at loads of less than 30% of capacity, which could cause wet stacking. The optimum operational point for prime movers is between 75% and 80% of its rated value. At this point, the generator will be at its maximum efficiency. Fuel and maintenance costs will also be reduced. The paralleling control system can be equipped with a generator load control that can add and remove generators in response to the actual load/demand of the system. This functionality is enabled by a generator removal time delay, which can initiate generators being removed from the bus as a function of the acceptable generator percentage loading selected by the operator. If the load changes and demand reach 90% of running capacity, for example, an additional generator can be started, synchronized, and paralleled to the bus with no time delay. 

Expandability: When sizing generators to match system load requirements, it is often difficult to accurately project increases in load and adequately plan for unanticipated additional requirements. If load projections are aggressive, the initial investment in a generator may be higher than necessary. On the other hand, if load projections are inadequate, reliable standby power may be compromised or expensive post-installation system upgrades may be required. Parallel systems offer a level of scalability and modularity that allows for variations in load over time and optimum operation of the installed units. If physical space planning is executed appropriately, generators can be added for additional power supply when required (see Figure 5).

Ease of maintenance and serviceability: In an N+1 paralleled generator system, if a generator in the system fails or requires maintenance, individual units can be dismantled and serviced without disrupting the function of the remaining units. Furthermore, the redundancy inherent in a parallel system provides multiple layers of protection and ensures an uninterrupted supply of power for critical circuits.

It is important to match all of the new paralleled system generators with the same manufacturer, ratings, and type. When modifying an existing system, matching the existing generator manufacturer, type, pitch, and ratings is highly preferred. This matching will avoid load sharing issues between the generators. Moreover, standardizing on one model type will also enhance maintenance and simplify operations of the generator system. 



-Leslie Fernandez is senior project engineer, electrical at JBA Consulting Engineers. He has more than 30 years of engineering, design, and field experience including medium-voltage distribution systems for military, mining, tunneling, food manufacturing, power production facilities, high-rise facilities, and casino-resort complexes.

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Critical Power: Selective Coordination in Health Care Buildings

CFE Media


Health care facilities, especially hospitals, have more stringent selective coordination requirements than conventional building electrical systems, according to some electrical engineers.

Health care facilities, especially hospitals, have more stringent selective coordination requirements than conventional building electrical systems, according to some electrical engineers. The unique constraints of health care facility electrical distribution systems require engineers to be diligent when designing these systems. The code requirements are less stringent, but the function in these buildings is more important. Selective coordination localizes an overcurrent condition to restrict electrical outages to the affected equipment, circuit, or feeder.

In a properly coordinated system, a fault induces operation of the nearest device on the line side of the fault and limits the outage to only the faulted portion of the system. If overcurrent devices are not selectively coordinated, the fault has the potential to impact one or several devices upstream resulting in a much larger scale outage than necessary for system protection. In extreme cases, faults can open the main overcurrent protective device and cause an outage for the entire facility. In addition, electrical power provided by gensets has less available fault current than the utility. Therefore, selective coordination in the health care environment has different criteria when connected to standby power.

Learning objectives:


  • The audience will understand the applicable codes and standards: NFPA 70: National Electrical Code (NEC), Article 517; NFPA 99: Health Care Facilities Code; and NFPA 110: Standard for Emergency and Standby Power Systems.
  • Attendees will understand the selective coordination issues unique to hospitals and health care electrical distribution systems including separation of feeders and branch circuits.
  • Viewers will learn the criteria for selectively coordinating electrical power provided by gensets.
  • Viewers develop a basic understanding of how to design selectively coordinated electrical systems for hospitals and health care facilities using circuit breakers, fuses, relays, and/or a combination thereof.



Tom Divine, PE, project manager, Smith Seckman Reid, Houston

James Ferris, PE, electrical project engineer, TLC Engineering for Architecture, Orlando, Fla.

Moderator: Jack Smith, Consulting-Specifying Engineer, Pure Power, and CFE Media, LLC

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