By Tom Faucette, P.E., LEED AP BD+C and Sara Lappano, P.E., LEED AP BD+C


As terms like big data, business resiliency, and the “internet of things” enter our lexicon, the availability of power has evolved as one of the hottest topics in facility management and engineering. When discussing the uptime of an electrical system, the conversation starts with the utility company and then almost immediately goes to on-site generation. Depending on your building type and loads, on-site generation facilities can take on many different topologies and sources. We are going to focus on generators, using both natural gas and diesel generators and how to successfully determine which fuel type and system topology makes sense for different facility and load types.

We will also discuss some facets of different generator systems that include: understanding loading impacts on configuration and fuel choices, the different topologies for generator set-up, and finally discuss a recent case study in which paralleling became a cost-effective solution for meeting standby power requirements.

A growing stock of commercial office buildings, hospitals, museums, and data centers are requiring some sort of emergency and standby power generators. These systems are required for a variety of reasons, but by code are split into several load categories: life safety, legally required, and optional standby loads.

Understanding these load categories and how your local jurisdiction interprets the code definitions are important steps in choosing fuel type and system topology. Each load category comes with specific requirements for source availability that can range from a maximum of 10 seconds for the life safety branch to 60 seconds for the legally required system. Determining which load categories apply to your project and the size of those loads will assist in providing the building user with the optimal generator system to meet their needs.

For example, due to some limitations with engine size and startup time, it could be problematic to design large load steps when those loads are served by natural gas (NG) generators. Using multiple transfer switches and variable frequency drives can reduce these issues, but this should be carefully considered.   Table 1 provides an overview of the advantages and disadvantages of natural gas versus diesel generators.  We use this table as a starting off point at the kick-off of a project to determine what design concepts to explore. 


Natural Gas Generator Example

Recently, we’ve seen an increase in jurisdictions accepting NG generators for life safety loads.  In major city centers where the NG supply is deemed reliable by the authority having jurisdiction, we’ve seen NG systems utilizing engines up to 400kW or 500kW as a cost effective way of supplying commercial buildings with emergency power, including life safety loads.  

These systems have the advantage of reducing pollutants, reducing or eliminating on-site fuel storage, and reducing typical preventative maintenance costs. NG can also be useful for critical (i.e. optional standby) loads where sufficient on-site fuel storage is not possible. We recently completed a project for a local non-governmental organization where a 500 kW system was installed on the roof top of an existing building. Providing fuel storage on-site was not feasible and the client was concerned about the environmental impact of this mission critical generator. We knew that the load could be easily carried by a single engine set and that we were extremely limited for space. In the end, we were able to engineer the system into the existing electrical distribution and provide a reliable source of NG to generator for less cost than a comparable diesel system. Additionally, we found continuing projected fuel operational savings. 

Two notes of caution, operators may need to secure a letter of non-interruptible service from the local natural gas utility and multiple automatic transfer switches may be required to reduce step loads. The other piece in this puzzle is that we already had a life safety system on-site and we used that system to segregate life safety and critical loads. Figure 1 shows a typical emergency system design using a single generator.


Paralleled Diesel Generator Example

As discussed before, load planning becomes a critical aspect of emergency power system design. We’ve found that meeting with user groups and developing a power system master plan early in the design and planning stage can save time and money.

In a recent multi-phase renovation of an office building, we used this master plan strategy to determine load requirements for both short term needs and for long term requirements. We determined that the life safety loads would be best served by a dedicated life safety and legally required generator installed during the earliest phase of the project and multiple paralleled diesel generators loaded to 70% each would meet the remainder of the project requirements. We specified diesel generators to allow for large step loads, which included multiple chillers, pumps, and other motors on a single step load and a central UPS system that had limited battery capacity. On this particular project, we were not constrained by fuel storage and the site was able to accommodate an emissions treatment system to reduce the impact on the environment. The other critical factor determined during our master planning was that the site needed a high level of resiliency. All these factors combined led us to select multiple engines in a paralleled configuration using a more conventional PLC control and paralleling switchgear. This allowed us the most flexibility and, in the event of a malfunction on one of the engines, we were able to provide load shed options on the switchgear that allowed the user to maintain a majority of their critical load. This solution required more time during the design, construction, and commissioning process to ensure proper operation.


Electronically Paralleled Natural Gas Generator Example

On a recent museum project that was executed through a design-assist delivery method, we worked with the electrical contractor on the project (Ennis Electric Company Inc.) to evaluate two different options for serving the emergency loads in the building. In addition to life safety and legally required loads, there was also a sizeable optional standby load made up of a portion of the building’s central plant. Our original design consisted of three 1MW diesel generators, each serving a separate emergency power branch within the building. 

While there would have been some benefits to paralleling the generators, because of cost and because the owner did not require any redundancy, we did not originally pursue a paralleling switchgear as part of our design. Because of site constraints, the trio of generators would be located on top of the building in a penthouse, with the diesel fuel storage and pumping system located in a below-grade parking garage. The fuel storage was sized to provide 48 hrs of run time for all three of the generators. This approach created several challenges for our team. Having three totally separate emergency branches was creating challenges as the life safety, legally required, and optional standby loads fluctuated up and down in size during the design process.

Numerous automatic transfer switches (ATS) were being used to step the loads on each generator in order to minimize the maximum inrush current experienced by the generators during equipment startup. Also, locating the fuel storage in the parking garage and pumping the fuel up to the roof was expensive and required sacrificing parking spots in the garage to store the fuel.

Once the electrical contractor was engaged, we worked with them to evaluate other design solutions that could maintain the required performance of the emergency system while reducing costs. An alternate design solution using paralleled natural gas generators was evaluated and eventually adopted for use on our project in lieu of the original diesel solution. The maximum size NG generator we could use while still ensuring a maximum 10-second start up for life safety loads was a 400kW engine.

After analyzing our loads, we determined that eight 400kW NG generators would be required. We worked with multiple manufacturers during this process and found several that were capable of electronically paralleling multiple generators without adding the cost of a paralleling switchgear to the project. Figure 2 shows a typical arrangement of multiple generators paralleled through a paralleling switchgear.

The electrically-paralleled option is similar to this, but the paralleling switchgear and electrically operated circuit breakers are no longer required. In this approach, all eight of the generators would feed into a single conventional switchboard located in the penthouse with the generators. The generator manufacturer could only guarantee that one out of the eight generators would start within the 10-sec limit mandated for life safety loads, meaning that we had to ensure that our life safety load and associated ATS were able to be served by a single 400kW engine. This design approach also allowed us to reduce the number of ATS’s on the project from eight to five since a bank of multiple generators can better withstand the impact of higher inrush current than a single generator. 

Once the design of the alternate NG generator system was finalized, the electrical contractor performed a cost analysis comparing the original diesel design to the alternate NG design. They determined that the installed cost of NG system was less than the original diesel system. While the NG generators themselves were more expensive, the deletion of the fuel storage and pumping system and the reduction in ATS’s resulted in a lower overall cost to the project.

In addition, the cost analysis predicted slightly less expensive maintenance costs for the NG generators and a fuel cost savings of $680,000 over the assumed 30-yr lifespan of the generators under normal testing conditions. Table 2 shows the fuel cost comparison between the two fuel types. The consolidation of the generators’ output into a single switchboard also had advantages for load management since we were no longer sizing a single generator for each emergency power branch in the building. Under this new design approach, we still did not provide any redundant generators, but the use of a bank of generators that all supply a single switchboard still adds some flexibility to the emergency power system.

While this solution had numerous cost-related and operational benefits, it created some additional design challenges. Due to the limited footprint of the penthouse space on top of the building, we eventually determined that the only way to locate all eight generators was to stack them two-high inside the penthouse space. We had to work closely with the generator manufacturer and the structural engineer to ensure that the platform and walkway system was designed to provide code-required access and working clearance at key locations on each generator. We also had to work closely with the mechanical engineering and contracting team to ensure proper air intake and exhaust was provided for the engines. Figures 3 and 4 show some of the BIM modeling performed by the electrical contractor to coordinate the locations of the engines, platform structure, mufflers and exhaust piping for the stacked generators.



With natural gas becoming a more widely-accepted fuel type for emergency power systems and with the availability of electronic paralleling for certain engine sizes, there are more options than ever in designing standby power systems. Understanding the advantages and disadvantages of different system types will allow you to design an emergency power system that is tailored to meet the needs of your clients.


Tom Faucette is the Director of Electrical Engineering for the Washington office of SmithGroupJJR. He has electrical and computer engineering experience with an emphasis on electrical distribution design and mission critical systems. His responsibilities have included project management, engineering load analysis and short circuit calculations, distribution system design, and construction services.

Sara Lappano is the Learning Studio Lead Electrical Engineer for the Washington, office of SmithGroupJJR. Sara draws on a wide base of experience in lighting design, electrical engineering, and construction for every project she works on.  Her project experience includes major renovations and new construction projects for numerous universities as well as private and government sector clients.