Spec’ing hospital electrical distribution systems

When specifying electrical distribution systems in hospitals, the engineer must account for the facility’s size, flexibility needs, emergency power needs, and safety requirements.


View the University of California San Diego Jacobs Medical Center electrical system being built from the ground up in these animated renderings.

Figure 1: The University of California San Diego Jacobs Medical Center is a 10-story, 509,000-sq-ft building scheduled to open in 2016. Initially, it will have 161 beds, including 24 intermediate care unit beds, 24 bone marrow transplant beds, and 24 inteFor updated details about the project, visit the official UC San Diego project website.

Image courtesy: UC San Diego Health System, Paul Turang, Photographer. Renderings courtesy: exp US Services Inc.

Learning objectives:

1. Understand which codes govern the design of hospital electrical systems

2. Learn the differences between emergency and essential power

3. Learn and understand the unique electrical system requirements of hospitals. 

On the surface, the electrical distribution system of a hospital may look the same as that for other types of buildings—offices, hotels, etc.—but there are several important distinctions. These distinctions are not so much in the equipment used. Panelboards, switchboards, transformers, circuit breakers, and other products are all common to other types of projects. The differences lie primarily in the size and complexity of the electrical systems: the overall size of the electrical system, its need for a higher level of flexibility, enhanced needs for more emergency power that can remain operational for longer durations, and the need for enhanced safety in the hospital environment.

Size does matter

First, the electrical demands of a hospital far outweigh those for most other building types. One reason is that hospitals have unique equipment with very large power requirements, such as magnetic resonance imaging (MRIs), computer tomography (CT) scanners, and other imaging equipment. Second, even some equipment common to all buildings is larger for hospitals due to their complexities. A good example of this lies in air conditioning and ventilation systems, which can be two or three times larger in hospitals than in other buildings of a similar size due to the heightened air change rate, more stringent temperature requirements, and higher equipment loads. Also, many specialized areas in a hospital have a large number of receptacles to allow for a multitude of equipment to be used. For example, a hospital operating room may be 500 to 600 sq ft and include 30 or more receptacles due to the amount of equipment required in this space. An office building may have 10 receptacles or less in a comparably sized area. Hospital lighting also follows the same thinking and typically has a higher lumen and Watts/sq ft requirement based upon the complex procedures (surgeries, examinations, and other medical procedures) that are performed.

Similarly, the hospital’s emergency system will be significantly larger than that of other building types. Where all buildings require emergency power to help occupants safely exit in the event of normal power failure, hospitals also require emergency power to sustain the life of patients (who often are not capable of self-preservation due to illness or injury), and this emergency power is needed indefinitely (not just long enough to allow the public to safely exit the building). This requires a more robust and larger emergency power system. For example, a 200,000-sq-ft office building might require 50 to 100 kW of generator capacity to provide the emergency power it needs, whereas a 200,000-sq-ft hospital may need 2,500 kW or more of generator capacity.

Figure 2: NFPA 70-2011 Article 517 outlines the way in which the emergency system is divided into the life safety branch and critical branch. Courtesy: NFPAAnother consideration becomes the need for sufficient fuel storage to sustain the emergency generators. For most buildings, fuel storage (usually diesel for generators) requirements are quite small. For a hospital, however, the dual considerations of much larger generators that need to run for longer time periods lead to very large on-site fuel storage needs. It’s not unusual to see a hospital with 30,000 gal or more of diesel fuel storage for its emergency generators.

Emergency versus essential power

Often the term “emergency” power is used to refer to all power needed on a generator in a hospital. A better term for this would be “essential” power. True, emergency power comprises only those loads required to be restored within 10 seconds as required by NFPA 70: National Electrical Code (NEC) Article 700 for all buildings, and as defined as life safety branch and critical branch by NEC Article 517 for hospitals. For a hospital, you have the essential power supply, which would include the generator(s), and this power is divided into emergency system power and equipment system power. Then the emergency system is divided into the life safety branch and critical branch (of the emergency power system, as shown in Figure 2).

The size, complexity, and needs for emergency power in a hospital are only a few of the ways in which its power distribution system differs from that of other building types. Following are other ways hospitals differ from other types of buildings.

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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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