Designing backup, standby, and emergency power in high-performance buildings

Electrical engineers must consider many factors when designing backup, standby, and emergency power systems. Safety, maintainability, code compliance, and economics play crucial roles in determining the topology of an emergency system for a critical facility. Specific requirements for emergency power vary based on building occupancy type, facility use, and critical function.


This article has been peer-reviewedLearning objectives

  • Review design issues that impact backup, standby, and emergency power.
  • Know the codes and standards that outline the requirements for system design.
  • Assess generator systems and the various types that can be specified.

When all other power systems fail in buildings, generators are the workhorses that are expected to operate rapidly and reliably. It is critical for design professionals to have a thorough understanding of proper generator system design. For the generator system to be the “last line of defense,” designers must follow a systematic approach to determine the design criteria, make system selections, and provide a constructible and code-compliant design to construction. This article will highlight design issues that have impacted the author or have been identified by generator manufacturers as “frequent misses” in designs. At the end of the article, we will review how these key design elements were applied to a generator installation at a medical facility.

Codes and standards

It is critical that system designers determine the codes and standards that apply to their designs. Key codes and standards that apply directly to the design of generator systems include:

Proper application of these codes has been covered extensively in other Consulting-Specifying Engineer articles. Plus, additional standards that provide the criteria for the maximum time to energize loads are included in “Paralleling Generator Systems” by Leslie Fernandez, Winter 2016.

Conditions of use

The first step in designing a generator system is to develop the programmatic requirements of the design. It is important to understand:

  • What type of building will this serve? Will this be used for a mission critical application, hospital, industrial facility, or another type of load? The answers here will determine the codes and standards that apply to the design.
  • How will the generator be used? For example:
    - Is it used as backup power in the event of utility failure or will it provide power continuously throughout the year?
    - How will the generator be operated? Is it a 100% standby application or will the onsite staff start the generator during rate curtailment or during a storm event?
    - Will the loads be emergency, legally required, or optional standby as classified in Articles 700, 701, and 702 of the NEC?
  • Site conditions:
    - Ambient conditions: What is the maximum temperature of the cooling air as it enters the machine? National Electrical Manufacturers Association (NEMA) ratings are based on 104°F, so if the installation exceeds those limits, let the manufacturer know. Derating will be required.
    - Altitude: Generators operating at about 3,281 ft. may need to be derated. Make sure to provide the altitude of the site to the generator manufacturer and take altitude into account when performing load calculations. Most manufacturers can provide derating factors for a given altitude.
    - Are there any other site conditions that will impact generator operation, such as a corrosive environment, excessive humidity, or extreme cold


The next question to ask is what type of fuel should be used for the application. The primary fuel types are diesel, natural gas, bi-fuel, and propane. Key points in deciding the fuel type include environmental considerations, code-required starting time, the availability of the fuel type locally, fuel storage, and the commercial availability of the generator size.

Diesel. Diesel is the most common generator for standby applications. Diesel generators are available from many manufacturers in a wide range of frame sizes. Diesel generators are reliable and economical, and the fuel is widely available and easily stored onsite. Diesel generators start quickly and accept large block loads with reasonable voltage dip. Those criteria are vital for emergency applications where the generator must start and accept load within 10 seconds (refer to NEC Article 700.12 and NEC Article 517) and the fuel system must have a reliable supply.

Diesel generator disadvantages include additional emissions, which may require additional equipment and fuel treatment. Additionally, diesel fuel does not store well over a long period, so a fuel-polishing system may be required.

Diesel fuel storage. Diesel fuel is stored either in base (belly) tanks (installed under the generator), day tanks (located near the generator and dedicated to the generator), or centralized storage. Base tanks are very simple to specify and generally do not require many external systems. Potential challenges with base tanks include coordination of underground conduits through the base-tank conduit window and increased engine height as the fuel-storage requirements increase. Day tanks also provide a simple solution, but do require additional space, additional piping, and auxiliary fuel pumps. They are more expensive to install and commission than base tanks. Centralized fuel tanks provide a central location for diesel fuel and are useful when large storage quantities are required. Centralized fuel tanks require a significant investment in additional piping, pumps, and fuel-polishing systems.

Natural gas. When compared with diesel, natural gas is clean-burning and has reduced emissions. Natural gas gensets are available in large frame sizes, are appropriate for base/continuous loads, and are less expensive than diesel when used continuously.

However, natural gas generators do not have the same transient response or acceptance of large block loads as diesel systems. Upon application of load, a natural gas generator must mix fuel and air, move it through the entire air-intake system, and then into the cylinder, which results in delayed voltage and frequency response. This, combined with a lack of onsite fuel storage, generally precludes the design engineer from applying them in emergency applications or for large block loads with fast start requirements. Many jurisdictions do not consider natural gas a reliable supply, so using natural gas as the only fuel most likely will not be allowed where backup power is required by code.  That said, generator manufacturers are rapidly making improvements to natural gas generators to improve start time and load acceptance, with some manufacturers accepting 100% load within 30 to 40 seconds.  As a result, natural gas generators are starting to be deployed as backup power in datacenter applications. 

Bi-fuel. Bi-fuel generators combine some of the best features of diesel and natural gas generators. Typically, the generator will start on diesel and then introduce natural gas into the combustion airstream. The power of the generator is then provided through the combined combustion of as little as 30% diesel and as much as 70% natural gas. Note that these percentages depend on the manufacturer and the load profile that the generator supports. If natural gas is unavailable, the generator can run on 100% diesel. This system can result in lower emissions, fuel flexibility, and fuel storage that meets the requirements of NFPA 99 and NFPA 110.

Bi-fuel generators are not a standard offering of all major manufacturers, so specifying a bi-fuel generator may eliminate some manufacturers from bidding or require a third-party solution that modifies the generator. Purchasing a diesel generator and supplying natural gas to it also introduces an additional first cost.

Propane. Propane generators are good choices for small installations at remote sites or when other fuels may not be available. Propane generators are only available in relatively small sizes. The maximum size depends on the manufacturer, but they are generally limited to 150 kW and lower. Larger sizes are available, but are costly.

Propane fuel tanks can be located onsite and do not suffer from the “unreliable” fuel supply issue that sometimes limits natural gas generators. Propane is commonly used in urban areas as a backup to a natural gas where required by code, or on federal lands due to concerns with diesel spills. When selecting a propane generator, take special care in selecting the fuel vaporizer. Liquid vaporizers allow for smaller tanks and function better in cold weather, but may be a parasitic load on the generator. Vapor-based vaporizers pull the vaporized propane off the top of the tank. These systems are simple and require less equipment than liquid vaporizers.

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