Critical Power: NFPA 110: Standard for Emergency and Standby Power Systems

In the Consulting-Specifying Engineer's May 8 webcast, Tom Divine, PE, Smith Seckman Reid Inc., and Kenneth Kutsmeda, PE, LEED, AP, Jacobs Engineering, discussed how consulting engineers who specify emergency power equipment for mission critical facilities will be impacted by new code requirements.


Consulting engineers who specify emergency power equipment understand that installations for mission critical facilities, such as hospitals and data centers, are required to comply with NFPA 110: Standard for Emergency and Standby Power Systems, in conjunction with NFPA 70: National Electrical Code. System designers must interpret the requirements of NFPA 110, ensure their designs follow them, and educate their clients about how the standard affects their operations.

NFPA 110 provides performance requirements for emergency and standby power systems that supply reliable auxiliary power to critical and essential loads when the primary power source fails. The standard covers installation, maintenance, operation, and testing requirements as they pertain to the performance of the emergency power supply system (EPSS) including power sources, transfer equipment, controls, supervisory equipment, and all related electrical and mechanical auxiliary and accessory equipment.

The significant changes from the 2010 edition of NFPA 110 inlcude:

  • The scope: the code covers location, maintenance, testing, system characteristics, and the scope ends at load terminals of transfer switches.
  • Key abbreviations: Emergency power supply (EPS) and Emergency power supply system (EPSS)
  • Inspection and testing: Installation testing (EPSS), weekly inspections (EPSS), monthly load testing (EPSS), and testing transfer switches operated monthly are all required.
  • Code changes:
    • 6.1.6: Permits electrically interlocked circuit breakers as transfer switch for medium-voltage mechanical equipment
    • 8.1.2: Requires consideration of portable generator when type criteria can't be met.
    • 8.4.8: Requires that only “qualified persons” perform maintenance, changes from “a properly instructed individual”
  • NFPA 110: Standard for Emergency and Standby Power Systems: Defines how emergency and standby power systems are to be installed and tested. It contains requirements for energy sources, transfer equipment, and installation and environmental considerations. The NFPA 110 divides emergency power systems into three categories: Type, class, and level.

The type refers to the maximum time that an emergency power system can remain unpowered after a failure or the normal source.


The class of an emergency power system refers to the minimum time, in hours, for which the system is designed to operate at its rated load without being refueled or recharged.


The level of an emergency power system refers to the level of equipment installation, performance, and maintenance requirements.

            Level 1: When failure of the equipment to perform could result in loss of
          human life or serious injuries

            Level 2: When failure of the equipment to perform is less critical to human
          life and safety and where the authority having jurisdiction shall permit a
          higher degree of flexibility than that provided by a level 1 system.

Testing Requirements:

  • Installation testing:
  • Start system and record performance
  • Full load test: 2 hr.
  • Engine crank and battery tests
  • Detailed records
  • Monthly load testing:
    • Requirement covers entire EPSS
    • Run generator under load
    • Exercise transfer switches
    • Diesel generators have a 30 min. minimum
    • Recommended exhaust temperature, or 30% of nameplate kW rating
    • Otherwise, annual test with supplemental load, for 30 minutes at 50%, 1 hr at 75%
    • Level 1: 100% test for lesser of class duration or 4 hr, every 3 years, at 30% and at recommended exhaust temperature

Testing strategies:

  • Transfer less sensitive load, if adequate
  • Manage mechanical systems to increase load for test
  • Complete normal power system shutdown
  • Load bank: Start with facility load, supplement with load bank

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