Optimizing genset sizing

When designing generator systems, consulting engineers must ensure that the generators and the building electrical systems they support are appropriate for the specific application.


This article has been peer-reviewed.Learning objectives

  • Understand the requirements for genset sizing for health care and non-health care facilities.
  • Learn the operational testing requirements for diesel generators.
  • Know how to design systems for proper testing.

Figure 1: The building in the photo is part of Contract Power Australia’s (CPA) Cloudbreak mine power station the Pilbara region of Western Australia—a remote and harsh environment with ambient temperatures that can reach 122 F. The building houses CPA’sWhether providing standby power for health care facilities or prime power for rural processing plants,engineers must make decisions regarding generator sizing, load types, whether generators should be paralleled, fuel storage, switching scenarios, and many other criteria (see Figure 1). In addition to being up to speed on the applicable codes, consulting engineers must work with the authorities having jurisdiction (AHJ) to ensure approval for the generator system is attained.

Generator system sizing

The code requirements for generator sizing are convoluted, and sometimes ambiguous. Sizing requirements vary for different types of loads. Rules for non-health care facilities are different-and often more demanding-than for health care. Genset sizing has ongoing operational impacts, particularly with regard to testing requirements.

This article addresses requirements for different types of loads described in the codes, their impact on operational testing requirements, their application in health care and other types of facilities, and ways to ameliorate testing difficulties. Wherever ambiguity exists in the codes, and sometimes even where it doesn't, there's a chance that an AHJ will enforce something unexpected. It's important to consult AHJs in the early stages of design to determine the code interpretations that will be enforced.

Non-health care facilities

The National Electrical Code (NEC) 2014 edition, Articles 700, 701, and 702, define three types of loads supported by an alternate power source: emergency, legally required standby, and optional loads.Emergency loads are those that are both required and specifically defined by a code or an AHJ as emergency loads. These loads generally directly protect the lives of occupants under blackout conditions.Examples include egress lighting, fire alarm systems, and specially defined process equipment that must be shut down safely to avoid disaster. Emergency loads must be energized within 10 sec of a power loss,as described in NEC, Article 700.12. Requirements for systems serving emergency loads are defined in NEC, Article 700.

Legally required standby loads are required by an authority, but not specifically defined as emergency loads. They don't generally protect human life directly, but are necessary for rescue operations. Examples include lighting of electrical rooms, mechanical rooms that house smoke removal equipment, and selected elevators. Legally required standby loads must be energized within 60 sec of a power loss.Requirements for these loads appear in NEC, Article 701.

Figure 2: These gensets provide standby power for a data center and cloud service provider in Louisville, Ky. Courtesy: Generac Industrial PowerOptional loads are, generally, any other loads that the owner values highly enough to support with an alternate source. Service to these loads isn't required by any authority, and occupant safety and rescue operations don't depend on them. Examples include mission critical data center loads, refrigeration in a restaurant, and cash registers in a retail store (see Figure 2). Requirements for optional loads appear in NEC, Article 702.

The language of the code for the capacities of each of these systems is:

  • Emergency: Adequate capacity and rating for all loads to be operated simultaneously, Article 700.4(A)
  • Legally required standby: Adequate capacity and rating for the supply of all equipment intended to be operated at one time, Article 701.4
  • Optional: Calculations of load on the standby source shall be made in accordance with Article 220 or by another approved method, Article 702.4(B).

In addition, 700.4(B) declares that a single alternate source may serve all three load types, provided that the system is equipped to selectively reject and shed loads that would overload the source, with emergency loads having highest priority, and optional loads having the lowest. One strategy for selective load rejection and shedding is to serve each type of load from a separate automatic transfer switch, with logic to inhibit connection of low-priority loads to the standby source in the event that adequate capacity is not available to support them.

The generally accepted interpretation of NEC 2014, Article 700.4(A), which covers emergency loads, is that the system must be able to serve all of the emergency loads simultaneously, without application of any demand factors. For legally required standby loads, Article 701.4 references "intent," suggesting that limited demand factors may be taken into account, such as non coincident loads in a smoke removal system. However, some AHJs interpret that text to mean that legally required standby loads must be served simultaneously without exception. Article 702.4(B), optional loads, referencing Article 220, offers a greater measure of flexibility. NEC 2014, Article 220.60, non coincident loads, allows the larger of two loads unlikely to operate at the same time as the basis for capacity calculations, possibly allowing optional loads to be locked out during outages, and used primarily for testing and exercising the system.Finally, allowing a single system to selectively serve high-priority loads suggests even more flexibility. Genset control panels can include a contact output that actuates when the generator frequency drops,and that contact can also be used to disengage transfer switches serving optional loads. It's worth noting,though, that AHJs may consider this permissive language as applicable only when the capacity of the alternate system is restricted due to a component failure.

The net effect of these capacity requirements is to drive the size of the generation system considerably higher than the actual demand load, leading to problems with operation and testing.

The codes exhibit plenty of ambiguity on these points. Early consultation with the applicable AHJs to establish the relevant interpretation of the code will avoid rude surprises when the project is submitted for permit, or at final inspection.

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