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.
By Neal Boothe, exp US Services Inc., Maitland, Fla. March 18, 2013

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.

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.

All receptacles are not created equal

Many different types of electrical receptacles are available today. Common types are general use, residential grade, commercial grade, specification grade, and hospital grade. Many of these designations have been developed by manufacturers to define the level of quality of these receptacles. Typically, a residential grade is lower quality than a commercial grade, a commercial grade is lower quality than a specification grade, and so on. The highest level of quality for receptacles is hospital grade. Hospital grade receptacles are manufactured to the highest standards to ensure grounding reliability, assembly integrity, overall strength, and durability. All patient care areas in a hospital are required to use hospital grade receptacles per NEC Article 517. Further, it requires that hospital grade receptacles shall be marked to identify them as such. U.S. manufacturers typically mark a green dot on the front of the receptacle (see Figure 4).

Figure 4: A red and an ivory hospital grade receptacle are shown. Note the green dot on each to indicate that these are hospital grade. Red receptacles typically indicate emergency power connections. Courtesy: LegrandHowever, hospital grade receptacles are not required throughout a hospital; they are required only in patient care areas (such as operating rooms, intensive care unit rooms, patient rooms, emergency department exam rooms, labor/delivery rooms, etc.). They are not required in offices, nurse stations, labs, pharmacies, or other areas in the hospital, but they are commonly used in all rooms of a hospital to provide the highest quality of receptacles with the longest life and durability.

A different color (typically red) is marked on emergency receptacles within the hospital to help identify that these receptacles are on emergency power and will continue to operate during utility power outages. This is required by NEC Article 517 in critical care patient areas but historically has been provided in all areas of the hospital for critical receptacles and even the few life safety receptacles in a hospital (such as at the generator set location or by the automatic transfer switches).

Grounding is twice as important

Grounding is an issue that is often misunderstood when discussing electrical distribution systems, and it’s not the intent of this article to define or explain grounding in-depth. From a simplistic point of view, the grounding of an electrical system is needed for many reasons such as establishing the voltage reference point and enhancing the safety of the electrical system by providing a return path for stray voltage/current in the system (and therefore keeping it away from you). For most buildings, every branch circuit (defined as the last wiring from any panel or other source to the final point of use) must be grounded. For the purpose of illustration, think of branch circuit wiring as the wire just behind the electrical receptacle or connected to the light fixture in your house or office. It is the wiring that touches the electrical devices you touch. Poor grounding at this level can lead to the possibility of you being shocked when plugging in (or unplugging) your radio, phone, or other equipment.

Furthermore, even though all ground buses in every panel should theoretically be at the same reference point (or zero voltage point), there is always the possibility of very slight voltage differences between multiple panels grounding reference. As a result, NEC Article 517 requires that any panels that serve the same patient area must have another grounding jumper (wire) connect their ground buses to eliminate the possibility of even the smallest trace of any stray voltage that could be introduced to a patient. This is another requirement distinct to the hospital environment.

There are two acceptable means for providing grounding for branch circuit wiring per NEC. One is by the use of a dedicated grounding wire being run with the other wires in the circuit (the famous green or sometimes bare wire any amateur electrician knows). The second method is the use of metallic boxes and metallic conduits throughout the branch circuit that are rated to provide an effective ground path back to the electrical source.

Anyone who has ever been shocked by an electrical appliance (meaning that stray voltage used them as a grounding path instead of a grounding wire or conduit system) can tell you that it is no fun. Unfortunately, these incidents can sometimes lead to injury or even death, depending on other conditions. Obviously, these concerns are greatly magnified for someone who is already in a weakened state due to illness or injury. As a result, NEC Article 517 requires that all branch circuits serving patient care areas must have both types of grounding installed (grounding wire and the use of metal raceway throughout)—often referred to as redundant grounding. This further enhances the safety of the electrical system for the patient (and the hospital staff).

Protection of the emergency system

Another requirement for hospitals in NEC 517 is the need to provide mechanical protection of the emergency system (this would apply to life safety and critical branch power). This code greatly reduces the available methods for the wiring and conduit systems of emergency power branch circuits. In non-patient-care areas (where redundant grounding is not required), methods include mineral insulated cable (very expensive, fire rated cabling), PVC Schedule 80 conduit, or conduits such as PVC Schedule 40 and some flexible metal conduits where installed in 2 in. of concrete.

Typically, only nonflexible metallic conduit is allowed in patient care areas due to the need to provide redundant grounding and protection of the branch circuit. There are a few exceptions; the NEC does allow flexible conduit for specific applications where nonflexible metallic conduit is not possible due to the need for flexibility. There is even a specific flexible “health care grade” ac cable manufactured just for hospitals (where the flexible metal jacket is still rated as a grounding path and a separate grounding conductor is installed within) for these applications.

Breaker coordination

Another code requirement that adds significant complexity to the design of hospital electrical systems is that of overcurrent coordination of the emergency system. This requirement is actually found in NEC 700, which applies to all building types. However, as mentioned previously, the emergency system for most buildings is a very simple one concerned primarily with small loads such as lighting, fire alarm, and other similar loads. In a hospital, the amount of emergency power and the larger size of the distribution equipment needed further complicate this issue, as close to half (or more) of the panels and breakers in a large hospital may be connected to life safety and critical branch power. The need to provide overcurrent coordination requires much more design attention to ensure that downstream breakers will open (trip) prior to larger upstream breakers so that any disruption of emergency power is minimized. Although this may seem simple enough, the tolerances of breakers is such that often larger electrical distribution equipment is needed to provide coordination than may be required based on the loads being served. This further increases the cost of the electrical system and may also affect the physical size of the equipment. 

Figure 5: A sample one-line diagram for a hospital. Note the arrangement among the 4000 A, 800 A, 250 A, and 20 A circuit breakers. The figure shows the coordination among these breakers such that the lowest breaker should always trip first to prevent higSee Figure 5 for a sample overcurrent coordination curve on a hospital project. This figure shows four breakers that properly coordinate such that in each case the smaller breaker will trip before the larger breaker upstream of it. Graphically this is represented by the fact that none of these breaker curvesoverlap. This starts with the lowest level breaker on the left, with each level of distribution shown coordinating with the distribution level above and below. If any of these breakers were to overlap, it would indicate that at a certain current (on the horizontal axis) and time (on the vertical axis) either breaker could trip first. 

Ground fault protection 

Ground fault protection (GFP) is the sensing of current on an electrical system to ensure that there is not a dangerous ground fault occurring downstream in the electrical system. By comparing outgoing current (on the phase conductors) with neutral currents (the “return” current), GFP devices can determine if any current is being lost in the system (i.e., a fault condition). If this is the case, the GFP protection will open a breaker (typically the main breaker) and interrupt power to the system. 

The NEC requires GFP on the system’s main circuit breaker for solidly grounded systems of 1,000 A or more with a line-to-ground voltage of 150 V or more. As most electrical services in US buildings of sufficient size are 480 V line-to-line (which equals 277 V line-to-ground) and 1,000 A or more, this GFP is often required. 

For a hospital, however, NEC Article 517 expands the GFP provisions and requires two levels of GFP protection. So in a hospital, if an electrical service needs GFP due to voltage and size of the system, the main breaker and all the feeder breakers in the electrical service must be protected by GFP. This serves to enhance reliability by removing only one feeder (through which the fault is travelling) instead of removing power from the whole service (by tripping the main breaker for the system). Similar to overcurrent coordination, this is another case where the code recognizes the need to isolate an electrical condition in a hospital electrical system in order to minimize any power disruption.

Electrical systems unique to hospitals

Figure 6: A time-current curve illustrating coordination among breakers in a hospital electrical system. Current is shown on the x-axis and time on the y-axis. Any overlap between breakers would indicate a possible occurrence where a downstream breaker miAlthough most electrical equipment in hospitals is common to other building types, there are some systems unique to hospitals—notably, isolated power systems. Originally introduced in hospitals due to the use of flammable anesthetics (commonly ether) many years ago, these systems were once mandatory in all areas where anesthesia was used. Flammable anesthesia hasn’t been used in hospitals for many years, but isolated power systems remain for some applications. NEC Article 517 requires the use of isolated power systems in “wet” locations where power disruption cannot be tolerated. The NEC code leaves the determination of wet locations to the hospital’s discretion, but areas commonly considered to be wet include some general surgeries, open heart surgery, orthopedic surgeries, and cystoscopy. Please note that the 2012 edition of NFPA 99 recently included in its requirements that all operating rooms are identified as wet locations unless a risk assessment has been provided to ensure that fluids within the space will cause no danger to patient or staff (Section 6.3.2.2.8.4). As a result of this revision, the use of these complex electrical panels will likely increase significantly in future hospital projects.

Isolated power systems serve to reduce the risks of electric shock hazards from patients’ or staff members’ inadvertent contact with stray voltage and allow for the safe continuation of electrical appliance use in the event of a low-level fault condition where loss of power could affect patient safety. These systems are also designed to limit the leakage of electrical current in the system (which is very small but common in any electrical system) that may cause an electrical shock. Though such a shock is normally very small and poses little risk of harm, it becomes magnified in a wet location or with a patient more susceptible to any contact to even very minor stray voltage (such as a patient in a heart surgery where any voltage introduced to the heart could have fatal consequences). Further, these systems are capable of monitoring even these smallest amounts of leakage current (under 5 mA, or five one-thousandths of an amp) and providing alarms of dangerous levels of leakage current. As you may imagine, the use of these complex electrical systems in a hospital adds significant costs and maintenance issues.

The only constant is change

Most buildings experience some level of change throughout the years—offices are renovated, finishes are updated, etc.—but in a hospital this level of change in the building’s functions is much more frequent and can be more drastic. Anyone who lives or works at or near a hospital can tell you that construction seems to be ongoing. As new technologies and treatments come and go, the building’s infrastructure changes frequently. As a result, hospital electrical systems must be designed with extra flexibility and spare capacity to help accommodate the inevitable changes. This, coupled with the fact that the hospital electrical distribution system is a much larger and more complex system than that of other building types, means hospital electrical systems must be designed to be more robust.


Neal Boothe is a principal and electrical engineer at exp, where he specializes in the design of hospital electrical systems. He has over 18 years of experience, including over 200 projects ranging from new, greenfield hospitals to additions and renovations of existing facilities.