Do You Know How to Size an Emergency Generator for Your Office?

Anuj Srivastava
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    How to Size an Emergency Generator in Office Buildings

    Emergency generators play a very important role in office buildings, given the large number of occupants typically found inside them. In New York City office buildings are classified under occupancy Group B (Business), and the requirements for their emergency power systems are covered in Chapters 4 and 27 of the NYC Building Code. Further information and requirements are provided by the NYC Electrical Code, which is based on the National Electric Code by NFPA, adding some amendments that are specific for NYC. In addition, emergency and standby power systems must meet the following standards:

    • NFPA 110: Standard for Emergency and Standby Power Systems
    • NFPA 111: Standard on Stored Electrical Energy Emergency and Standby Power Systems
    • NYC Fuel Gas Code, whenever the emergency or standby power system is based on combustion
    • The best way to ensure compliance with all applicable standards is to hire the services of a qualified engineering consultant or design firm. Ensuring code compliance from the design and specification stage of a project is much faster and less expensive than having to modify the installation because it failed an inspection by the NYC Department of Buildings.

    When is Emergency Power Required in NYC Office Buildings?

    There are two main cases where the NYC Building Code makes an emergency power system mandatory for office buildings:

    • All high-rise office buildings in New York City must have an emergency power system, with no exception.
    • Office buildings that are not high-rise require an emergency power system if they meet the two following conditions: having occupied floor space located less than 75 feet above the lowest fire truck access, and a gross area that exceeds 15,000 ft2 per floor or at least 100,000 ft2 for the entire building.

    When sizing the generator, all equipment and building systems designated as emergency loads by the NYC Building Code must be considered, and the generator must have an on-site fuel supply of sufficient capacity to operate for six hours at full emergency load. However, emergency lighting systems may be powered by batteries, as long as the following performance requirements are met:

    • The batteries must be sized to power emergency lighting systems at full rated power, for a minimum period of 1.5 hours
    • The battery system must be designed so that voltage output never drops below 87.5%
    • An automatic recharging system must be included.
    • Lead-acid batteries can be used as long as they meet the requirements above, but automotive batteries are not allowed.

    Emergency loads other than lighting systems can only be powered by batteries after receiving special permission from the NYC Department of Buildings, which involves a requesting an approval procedure, and this applies even for Uninterruptible Power Supplies (UPS). Otherwise, all emergency loads must be counted when sizing the emergency generator.

    Emergency power systems should not be confused with standby power systems, which provide backup power for building systems that do not create life-threatening conditions when they stop working, only discomfort and inconvenience. However, the NYC Building Code makes standby power mandatory in all office buildings where an emergency power system is required. Standby generators do not require an on-site fuel supply, and can use gas from the local utility instead, as long as the valves controlling the supply are independent from the rest of the installation.

    All emergency or standby generators used in NYC must be listed according to the UL 2200 Standard for Stationary Engine Generator Assemblies. Neither the generator nor its energy distribution systems are allowed to be located in the same room as the main electric service entrance. The room where they are located must be independent, with 2-hour fire barriers or horizontal assemblies.

    Sizing an Emergency Generator in High-Rise Office Buildings

    All high-rise office buildings require an emergency generator of sufficient capacity to power the following loads:

      • Any exit signs or means of egress lighting systems required by the NYC Building Code in Chapter 10.
      • Elevator car lights.

    Communication equipment and networks used by emergency voice and alarm systems, including Auxiliary Radio Communication systems (ARCs) for use by the NYC Fire Department.

    Fire protection systems: automatic fire detection, fire alarms and electrical fire pumps. This includes manual, automatic and sprinkler booster pumps.

    The standby power system, which is also required by the NYC Building Code, must be sized to include the fire command center power and lighting system, ventilation and automatic fire detection systems for smokeproof enclosures, stair pressurization systems and at least one elevator serving all floors. If multiple elevator banks serve different building areas, the standby generator must be able to power one elevator per bank.

    The NYC Building Code does not make sliding doors mandatory in office buildings, but if present they must be considered a standby load and included when sizing the standby generator.

    Sizing an Emergency Generator in Low-Rise Office Buildings

    If an office building is not high-rise, an emergency power system is only required when the building meets the minimum height and floor area conditions previously described: occupied floor space less than 75 feet above the lowest fire truck access, and an area above 15,000 ft2 per floor or at least 100,000 ft2 in total.

    The specific loads to consider when sizing the emergency generator are exactly the same as in high-rise office buildings, presented in the previous section. The standby power requirements in this case are similar, with only two differences:

    • The standby power must be sized to power at least three elevators at once per building, with a manual transfer switch to power other elevators as required, keeping at least three in operation at all times
    • The requirement for smoke venting and control is general in this case, and not specifically for smokeproof enclosures.


    The requirements for emergency and standby generators in New York City are complex, but this is necessary given the critical role they perform. If you get in touch with a qualified engineering consultant or designer from the start, you project will proceed without issues. In addition, the NYC Department of Buildings requires a testing report submitted by a registered professional, so it is better to get proper technical assistance from the start.

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    Tags : Design Building Code new york

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