This multi-part article is designed to help engineers, contractors, and other specifiers understand the stringent requirements for hospitals—as people’s lives are on the line when the power goes out. Previously, we addressed Circuit Requirements and Selective Coordination, and Separation of Circuits and Disconnect at Point of Entry. In Part 3, we will consider Generator Sizing and Fuel Options.
The sizing of the generator system for a hospital is influenced by many factors. At the most basic level, the generator system needs to have enough kWs to power the essential electrical systems (life safety, critical, and essential equipment) and any desired non-essential equipment. One of the potentially significant non-essential equipment circuits is air conditioning. Current, relevant healthcare codes don’t mandate that hospital air conditioning be backed-up on the emergency generator system, but many system designers understand the importance of air conditioning to a hospital’s primary function. In addition to general comfort, temperature is extremely critical to many patients in a hospital: open heart surgery, cardiac recovery, intensive care units, intensive care nurseries, etc. For hospitals in warm weather climates, the functional need to have about 60% of the chilling capacity backed-up is fairly compelling.
From a healthcare code standpoint, generator capacity is defined in National Electric Code (NEC) 517.31(D). The essential electrical system shall have the capacity and rating to meet the maximum actual likely load at any given time. This sizing requirement is significantly different than NEC 700.4 for emergency systems which requires sizing for all connected load. Due to the large theoretical size of a hospital’s critical emergency system circuit, NEC 517 specifically identified that NEC 700.4 does not apply. Sizing should be based upon a combination of factors: prudent demand factors and historical data, connected load, and Article 220 NEC calculations. The method of sizing should facilitate a practically sized generator that minimizes potential prime mover operational problems associated with lightly loaded generators.
Most hospital applications are designed around diesel fueled generators. Though diesel generators offer a cost advantage over large natural gas powered generators, the main reason for diesel is a strong code bias for the perceived reliability of on-site fuel. NEC 700.12(B)(3) requires that the generator(s) shall not be solely dependent on the public gas utility unless the probability of concurrent gas and electric failures is shown to be acceptably low to the satisfaction of the authority having jurisdiction (AHJ). Faced with this strong code positioning, most system designers and facility managers may get a false sense of security in the reliability of on-site diesel and an overly pessimistic perception of the reliability of natural gas generators.
National Fire Protection Association (NFPA) 110, which covers emergency standby systems, has a more balanced view of the reliability advantages and disadvantages of both diesel and natural gas. NFPA 110 accepts natural gas as an acceptable fuel unless the installation location has been shown to have high probability of gas failure. The NFPA 110 standard understands that for diesel to be reliable it must be maintained.
• “Fuel system design shall provide for a supply of clean fuel.” (110 188.8.131.52)
• “Tanks shall be sized so the fuel is consumed within the storage life, or provision shall be made to remediate fuel that is stale or contaminated…” (110 184.108.40.206)
• “Where fuel is stored for extended periods of time (e.g. more than 12 months), it is recommended that fuels be periodically pumped out and used in other services and replaced with fresh fuel.” (110 A5.5.3)
• “A fuel quality test shall be performed at least annually using appropriate ASTM standards.” (110 220.127.116.11)
• “Fuel maintenance and testing should begin the day of installation and first fill in order to establish a benchmark…” (110 A18.104.22.168)
System designers will typically design the amount of on-site diesel to allow the hospital to defend-in-place for the duration of a major event. In recognition of this need to defend-in-place, the Joint Commission created a 96-hour rule. The requirements contained in EM.02.01.01 EP 3 (the 96-hour element of performance) is often misunderstood. Hospitals seem to feel that this rule requires them to stockpile or make provisions to sustain operations for a full 96 hours. On the contrary, this requires hospitals to plan their actions for the first 96 hours after a disaster and does not imply that they should prepare to stay in operation regardless of the circumstances.
From a generator standpoint, some system designers misunderstand the 96-hour rule and design the fuel system for 96 hours of full load operation. This creates a significant challenge in managing the reliability of that much on-site diesel. Sizing the emergency system for 96 hours of operation at 50% load may create a more readily manageable solution and better align with generator typical load levels. Extending that run time could be supported with refueling planning or on-site load management strategies.
Another interesting strategy to consider is adding some natural gas generators (30 to 50% of desired total capacity) into the diesel generator parallel line-up. The diesel generators could meet the needs for the code required emergency system and the natural gas generators could be sized to further support system capacity robustness and non-essential equipment like air conditioning. Having the benefit of both fuels in the operational mix creates many more contingency options in managing through difficult situations.
The implementation of generators to hospitals applications involves many design considerations that will continue to be discussed as this multipart series continues.
If you would like more information about emergency power design considerations, contact your local Generac Industrial Power Distributor. You may also call 1-844-ASK-GNRC or email ASKGNRC@generac.com.