optimizing genset sizing

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.

Tom Divine, PE, LEED AP, Smith Seckman Reid Inc., Houston


Whether 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. 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.

Optional 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. 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.

Health care facilities

Alternate power sources for hospitals are covered in NEC 2014, Article 517, health care facilities. The portion of the electrical system served by the alternate source is called the "essential system." As of the 2014 edition, health care facilities no longer have an emergency system. Instead, they have three branches: life safety, critical, and equipment, each serving different load categories. The requirements for the life safety and critical branches, and the functions of those branches, are substantially similar to emergency loads in non-health care facilities. The equipment branch supports loads that would be categorized as both legally required standby and optional in other systems.

The requirement for genset sizing for health care facilities is described in NEC, Article 517.30(D), as "capacity and rating to meet the maximum actual demand likely to be produced by the connected load." The article allows hospital generators to be sized in accordance with "prudent demand factors and historical data," and it explicitly states that Article 700.4 and 701.4, describing capacity requirements for emergency and legally required standby loads, don't apply to hospitals. This requirement is extended to other health care facilities in Article 517.45.

This requirement is much less prescriptive than the corresponding rules for emergency and legally required standby systems. The purpose of the additional flexibility in sizing hospital systems arises from the fact that hospitals typically have a large portion of their total load on the generation system, and the actual demand loads are generally substantially less than their connected loads. Sizing hospital systems to serve their entire connected load would result in vastly oversized generation systems, representing a substantial stranded investment in generating capacity, and leading to operational issues and difficulties in testing and exercising systems.

There are more methods of establishing generator size for hospitals than there are engineering firms practicing in the industry. Those methods depend on variables such as climate, local practices, and AHJ interpretations. All are modified by special conditions for each project, owner preferences, and individual experience.

With the 2012 edition of NFPA 99: Health Care Facilities Code and the 2014 edition of the NEC, the definition of emergency system in health care facilities has been deleted from the codes. Prior to the 2014 edition of the NEC, the portion of a health care electrical system supported by the alternate source was defined as the "essential system," with the life safety and critical branches comprising the emergency system. The remaining, mostly optional loads, comprised the equipment system. In 2012, the definition of the emergency system was deleted from NFPA 99, and the equipment system was redefined as the equipment branch. The same changes appear in the 2014 edition of NEC, Article 517.

The overall effect of these changes on electrical design for health care is minimal. Requirements for the life safety and critical branches remain unchanged. The name of the equipment system was changed to "equipment branch," but its requirements are generally unchanged as well. The apparent intent of these changes is to clarify that health care systems do not fall under certain specific requirements of Articles 700 and 701, particularly those affecting genset sizing.


An oversized generator is difficult to test properly. NFPA 110: Standard for Emergency and Standby Power Systems sets specific requirements for operational testing of standby generators. The 2013 edition requires each unit to be operated at least once monthly, either at 30% of its nameplate power rating, or at a level that causes the exhaust temperature to reach the manufacturer's recommended level. If the building load is insufficient to reach those targets, each unit must be run annually with supplemental loads at 50% and at 75%. The purpose of these load requirements is to reduce the likelihood of "wet stacking," a condition caused by incomplete fuel combustion and other effects of running at light load (see "Preventing wet stacking"). Operational testing requirements are found in NFPA 110, Section 8.4.2.

With standby systems sized to meet maximum demand load or more, it can be difficult to reach the required load levels using only building load. If those levels can't be reached, the owner must install or rent load banks to supplement the building load. It's clearly advantageous to the owner for the system to be designed to provide adequate test load. One strategy is to connect optional loads to the system beyond those needed for operation during a blackout, with the intent that those additional loads will be connected for testing, rather than operational, purposes.

Moving forward

Different rules for generator sizing apply to health care facilities and other facilities. Requirements for non-health care facilities can result in gensets sized much larger than their demand loads. Requirements for health care are, to some extent, intended to reduce that effect, but can still result in gensets substantially larger than their loads. By adding non-essential loads to the generator system, the negative effects of lightly loading a genset can be ameliorated.

Tom Divine is a senior electrical engineer and project manager at Smith Seckman Reid Inc. He is a frequent contributor to Consulting-Specifying Engineer magazine and Pure Power, and is often a speaker at CSE Webcasts.



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