When and how to design parallel generators

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 and whether generators should be paralleled.


Learning objectives

  • Learn about design aspects that affect decisions to use paralleled generator systems.
  • Understand fundamental steps taken during the planning and design of paralleling generator systems.
  • Review factors that affect emergency power system and fuel system reliability.

As the demand for reliable electricity grows, so does the need for reliable backup power sources. Factors that affect the decision to provide paralleling capability for standby generator systems include required or desired redundancy, additional capacity, maintainability, upgrades to existing systems, and future growth. 

Building systems are increasing in efficiency, but the use of technology and the desire for standby emergency power is steadily increasing. Information technology (IT) equipment, mechanical cooling equipment, building lighting (required and convenience), and branch power loads are all adding to the total emergency power load.

Figure 1: Generators are shown in a revised room in a hospital patient tower, with the fuel-oil trench and discharge plenum visible. Courtesy: IMEG Corp.It is typical for a building to grow in physical size or increase in electrical demand over its lifespan. Designing systems for future growth requires cost-vs.-benefit discussions with the client and usually includes considering 5-, 10-, and 20-year campus master planning visions and goals.

Systems capable of paralleling generators inherently provide system-growth capability with minimum system downtime or outages, which can be a high-value investment. The potential expandability of paralleled generator systems also offers the owner the ability to purchase additional generator capacity as it is needed.

Environmental conditions

Generators are electromechanical machines that require maintenance and experience a wide range of environmental conditions. Generators with unit-mounted radiators require a large volume of outside air to be brought into the room and across the generator surfaces. Outside air temperatures vary widely, as does humidity. Outside air also can contain moisture, humidity, pollen, dust, insects, and other particulates.

Generators themselves generate substantial heat, vibration, and noise. These factors combine to create adverse environmental conditions that will inevitably affect system reliability. It is not recommended to provide screens or filters on intake grills and louvers to alleviate dust and debris, as those methods will become clogged and prevent proper airflow, which may adversely affect a generator system's cooling and capacity. 


When generator systems need to be maintained, they are taken out of service. This prevents them from starting during an outage and necessitates careful planning by the facility, which usually results in work performed outside of normal hours that require interim life safety measures.

The 2016 edition of NFPA 110: Standard for Emergency and Standby Power Systems, Article 7.13, requires that additional sources of standby emergency power be available during generator maintenance. Paralleled generator systems may provide redundancy of required systems. This means that regular maintenance can be provided during periods of convenience, which increases the likelihood that maintenance will be performed on a regular basis.

Expansion of existing systems

Projects that replace or add onto existing emergency power supply (EPS) and emergency power supply systems (EPSS, as defined in NFPA 110) are the most challenging because they require the design team to consider all impacts to the existing system. Emergency power system outages are the most difficult aspects to manage for health care, critical operations, industrial, or mission critical buildings.

Taking a standby emergency power system offline for any amount of time will require risk assessments with the owner. These events usually trigger disaster preparedness teams that outline plans for managing electrical outages without available backup power. Impacts to IT, phone, radio, lighting and egress, and hazardous safety will need to be discussed and evaluated by the team. The nature of the risk also is affected by the proposed duration of the emergency power system outage and how long it would take to make connections in the event of a utility outage.

Due to these complications, providing a new EPS and EPSS may be easier than trying to add to an existing system. Additionally, when connecting to existing distribution systems, impacts to selective coordination should be evaluated as they will likely impact how and where connections will be made. 

Design choices

Figure 2: This shows fuel-oil polishing—along with two day tanks—where fuel oil routes through the in-floor trench. Courtesy: IMEG Corp.Once the decision is made to go with paralleled generator systems, the design team and owner must make decisions regarding physical location, fuel system requirements, electrical-demand requirements, system voltage requirements, desired system redundancy, and the amount of future growth potential. Determining the operating voltage of the EPS and EPSS is sometimes, but not always, done by matching the normal service voltage.

Discrepancies occur when EPSS are located long distances from the building(s) they serve, and in these cases, higher voltages—even medium-voltage levels—may be prudent. Generator voltages above 208/120 V in the United States will result in the need for step-down transformers, which influence the generator load and take up physical space. Unless long feeder distances justify medium-voltage levels, the EPS voltage usually matches the mechanical equipment, process equipment, and any traction elevator systems. 

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