Spec’ing hospital electrical distribution systems


NEC Article 517: Articles 700 and 701 on steroids

As mentioned, NEC Article 517 is dedicated to health care facilities. It should be noted that NFPA 99: Health Care Facilities Code also has specific electrical requirements for hospitals, but as these requirements are very close to those in NEC Article 517, we will focus on these NEC requirements. One other code that affects generator design and installation is NFPA 110: Standard for Emergency and Standby Power Systems. This code is more specific to generator installation requirements (and not the emergency loads that must be connected to them) for the most part, but includes many other important requirements for generators.

NEC Articles 700 (Emergency Systems) and 701 (Legally Required Standby Systems) apply to all facilities (including hospitals); however, the requirements of Article 517 are typically more stringent and apply only to hospitals and other similar type health care facilities (nursing homes, ambulatory surgical centers, etc.). For both Articles 700 and 517, emergency power must be restored within 10 seconds of loss of normal power.

NEC Article 700 requires that a portion of the building’s electrical system be capable of providing emergency power in the event of normal power failure. This would include features such as exit/egress lighting, fire alarm systems, and other similar life safety functions. This can be done via a small generator (for larger buildings) or battery backup power. This amount of power is typically a very small portion of the building’s total power consumption, about 5% to 10%.

NEC Article 517 also requires that hospitals be provided with emergency power. Again, as hospitals must remain open throughout normal power interruption and include patients who rely on emergency power for preservation of life, this article divides the emergency branch (same terminology as Article 700) into two branches: the life safety branch and the critical branch (new terminology just for hospitals).

Figure 3: Two 1250 kW generators paralleled at the Cleveland Clinic in Weston, Fla. Note the size of these generators; the step ladder is needed to allow staff to review the annunciator panel on the rear of the generator in the foreground. Courtesy: CleveIn a hospital, the life safety branch of the emergency power system is very similar to the Article 700 requirement for other building types. It includes only the small amount of power necessary to allow for the safe evacuation of the public from the building in the event of normal power failure. This includes the exit/egress lighting and fire alarm systems—similar to other buildings—as well as other loads unique to the health care environment, such as medical gas alarm systems. It also includes generator set accessories loads (such as battery chargers and block heaters) that are necessary to ensure proper starting and operation of the generator. Again, these loads would include a very small percentage of a hospital’s total electrical system, typically 5% to 10% at most.

As we’ve discussed, unique to the hospital environment is the need to maintain patient safety during the loss of utility power. This is where the second branch of the emergency system, the critical branch, takes over. The hospital’s critical branch is a much larger part of the electrical distribution system and handles loads such as much of the lighting and receptacles in patient rooms, intensive care rooms, operating rooms, post-anesthesia care units (PACUs), nurse stations, pharmacies, labs, blood banks, and other similar types of spaces where patients are either directly cared for or services for these patients are arranged. Further, Article 517 identifies two different types of patient care areas—general care and critical care—depending on the severity of the patient’s needs. A general care area includes rooms such as a “normal” patient room or exam room where critical branch power is needed but the patient’s condition is not severe. A critical care area then is exactly how it sounds: an area where the patient’s care is more dependent on the hospital staff (and their need for more equipment and more emergency power). This includes operating rooms, labor/delivery rooms, intensive care units, trauma areas in emergency rooms, and so on. Article 517 requires even more available power in general and also more emergency power for these types of spaces.

All these requirements for critical branch power further increase the size of a hospital emergency power system. Where life safety power would only be 5% of the building’s power requirement, the critical power system of a hospital could easily account for 25% or more of a hospital’s total power requirement.

In addition to “true” emergency power, other power needs in a building are also very important in the event of normal power failure but not necessarily needed for the preservation of life. This might include heating and refrigeration systems, ventilation and smoke removal systems, sewage disposal, industrial processes, and others whose interruption could create a hazardous condition or hamper rescue or fire-fighting operations. For all buildings, this is covered by NEC Article 701: Legally Required Standby Systems, and these systems must be restored to power within 60 seconds from loss of normal power. For a hospital, Article 517 further defines these systems and calls them the equipment system.

Article 701 doesn’t specifically define which systems must be considered legally required standby but rather states that this designation should be made by the designer and/or authorities having jurisdiction (AHJ). As there are so many different types of buildings with different hazards, the designer and code reviewers must use discretion. However, for a hospital, these equipment systems are much better defined. They include large medical gas suction systems, elevators, kitchen hood supply/exhaust, ventilation systems (supply/return and exhaust) for patient care areas, heating for patient care areas, large sterilizers, and similar type loads. Again, the equipment system of a hospital can be a substantial part of the overall electrical system, especially as much of this system is the larger equipment loads such as elevators, large air handlers, and sterilizers. The equipment system can easily account for 30% or more of the overall hospital electrical system.

Even this equipment system power is further delineated based on the importance of the loads being served—into nondelayed automatic, delayed automatic, and delayed automatic or manual connection. These distinctions can require a minimum of three automatic transfer switches (ATS) for the equipment system. The highest equipment system level is the nondelayed automatic connection and includes only loads such as certain generator accessories. These loads (as the name suggests) must be automatically restored without delay upon loss of utility power (similar to emergency power). Note that these types of systems also may be connected to the life safety branch. (This would be a designer’s choice and may depend largely on the size and complexity of these systems.) Next, equipment such as medical air vacuum pumps and compressors, smoke control systems, ventilation systems for operating and labor/delivery rooms, smoke control systems, and kitchen supply and exhaust systems must be automatically restored upon loss of utility power but are allowed to delay the emergency system restoration (typically under 1-minute delay). The final step of equipment power is allowed to have a delayed automatic connection (which would lag all other ATS) or even a manual connection to the generator system. This includes loads such as elevators, heating to patient care areas, automatic doors, sterilizing equipment, hyperbaric or hypobaric facilities, and other selected loads.

In summary, the total amount of emergency power for most buildings (and therefore the amount of emergency distribution equipment needed) is typically 10% or less and consists of only that minimal amount of power needed to help people safely exit a building within the first few minutes of normal power interruption. For hospitals, emergency power becomes the life blood of a building without utility power and must be maintained throughout a power outage, which could last for days after a storm or other catastrophic event. As a result, it’s not unusual to see the emergency power of a hospital exceed 50% or 60% of the building’s total power needs. Also, as separate transfer switches are need for each type of load (life safety, critical, nondelayed automatic equipment, delayed automatic equipment, and delayed automatic or manual connection equipment loads), multiple ATSs are always needed for hospitals. For a 200,000-sq-ft hospital, eight or more transfer switches could be used. A similarly sized office building would typically have only two ATSs.

BRIAN , UT, United States, 03/21/13 04:36 AM:

This article leaves much to be desired regarding hospital electrical systems. The graphic shown is only applicable for a tiny building covered under NEC 517, or smaller than 150kVA...in accordance with 517.30 B 4. I hope this isn't included in the printed version.
JITAO , NY, United States, 03/21/13 09:08 AM:

Should the three essential branches be fed from three separate ATS's?
Anonymous , 03/22/13 03:57 PM:

Some States even require bypass transfer switches.
PHILIP , PA, United States, 03/22/13 07:53 PM:

How are non-linear loads being addressed and what sizing changing were made to accomodate them?
SPENCE , VA, United States, 03/28/13 08:33 AM:

This article appears to be a broad brush to homogenize the design of hospital systems. But the author used a diagram that is applied only for small hospital facilities,typically less than 150 Kva of load. While the author aknowledged the extensive use of emergency power for various systems, he did not address the need clearer definitions for sizing the emergency systems. For instance, is a demand factor appropriate in the sizing of emergency systems?
J WESLEY , OR, United States, 03/28/13 11:50 AM:

Reads like a first grade primer. Nothing new unless you have never designed a hospital.
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