Transfer switches and the '96 NEC: what you need to know

New NEC changes allow for increased design flexibility in hospital essential electrical systems through the use of more transfer switches.Not all Code changes are more restrictive; in fact, some open up increased design flexibility by permitting alternate approaches. For example, the 1996 NEC now allows multiple transfer switches to supply patient bed locations in critical care areas. In addition,

New NEC changes allow for increased design flexibility in hospital essential electrical systems through the use of more transfer switches.

Not all Code changes are more restrictive; in fact, some open up increased design flexibility by permitting alternate approaches. For example, the 1996 NEC now allows multiple transfer switches to supply patient bed locations in critical care areas. In addition, now you can use the standby generator to supply nonessential loads. (However, an additional transfer switch is involved because those loads cannot share transfer facilities with essential loads. See Sidebar "What Are Essential Loads Anyway?") You need to be aware of the intent behind the rules and about changes and conflicts in other NEC articles that may affect how an inspector applies these rules.

How many transfer switches?

In general, a transfer switch supplying emergency loads can't be used to supply nonemergency loads. This is clarified in the 1996 NEC through a new sentence added to Sec. 700-9(b) Ex. 1 ([ILLUSTRATION FOR FIGURE 1 OMITTED], on page 40.)

The main rule calls for a complete separation between the emergency system wiring and all other wiring, but the first exception simply says "in transfer equipment enclosures." The perennial question has been whether an emergency system transfer switch could be used to supply both emergency and normal standby loads, and this exception has been the focus of that debate. Clearly, the exception allows both normal and emergency conductors within it. The question is where and for what purpose? The intent always has been obvious in part: You can't have a transfer switch without both systems present on the line terminals to which the switch transfers load.

The real question is whether both load branches can originate on the load terminals of the same switch. Many distribution systems have been designed with the load terminals of a transfer switch arranged in just this way. The engineer allows for complete emergency/nonemergency system separation from the transfer switch out, meaning the only potential issue is within the switch itself. This exception is then cited as the basis for allowing both loads on the single switch. This has now been clarified as improper. The reason is to increase the reliability of the emergency system by preventing faults on optional standby systems from affecting it. As we shall see; however, this provision directly conflicts with rules in Art. 517 for small hospitals (and for small nursing homes as well).

The number of transfer switches is a design decision, but "each branch of the essential electrical system shall be served by one or more transfer switches ..." Sec. 517-33(b) specifically provides for the subdivision of the critical branch into multiple branches. The FPN that follows notes there are significant risks involved in feeding an area using only one branch of the critical power system. The note goes on to advise feeding such areas with a mix of normal and critical branch power, so a failure between a critical load and its transfer switch won't lead to catastrophe. The note also recognizes the equivalent benefit of feeding such areas with critical power from separate transfer switches. With this background, we can look at what happened in the 1996 NEC.

General care areas

You'll need to serve every patient bed location in general care areas with power from the emergency system, along with at least one circuit from the normal system (still a two-circuit minimum).

Previously, these locations simply required at least one of the two required branch circuits to originate in a normal panelboard, which obviously meant that both could originate from the normal system if the facility so elected. The only restriction was in Sec. 517-33(a)(8)a, which we previously mentioned, where at least one receptacle in the bedroom (serving as many beds as are in the room) had to be on the critical branch. That provision has been effectively superseded by this change. If your institution had planned to use a normal distribution as the exclusive supply to patient bed locations, that will need to be rethought and may involve additional transfer capacity.

This major change wasn't really substantiated, although there was no adverse comment, which suggests that most hospitals may be doing it anyway. It isn't consistent with Sec. 3- in NFPA 99, The Health Care Facilities Standard. Note that Sec. 3- of NFPA 99 requires a distinctive color for these receptacles, or their faceplates, or some other effective identification for such receptacles installed on the emergency system. This applies in critical care areas as well. If your jurisdiction enforces NFPA 99, watch out for this requirement.

Critical care areas

The patient bed locations in these areas have required the presence of both normal and critical branch power for many years. It is important to note that a "patient bed location" includes not just the normal sleeping room, but also "the bed or procedure table [emphasis supplied] used in a critical patient care area." This means that an operating table is a patient bed location and must follow these requirements, even if the facility is choosing to rely mainly on isolated power.

There is a major change in the 1996 NEC, however [ILLUSTRATION FOR FIGURE 2 OMITTED]. A new additional exception in Sec. 517-19(a) (No. 2) allows all branch circuits in a critical care location to originate from the emergency system, provided two different transfer switches supply each such location. Remember, the reason for the normal power requirement is to make sure there is some power available in the event of a failure in a transfer switch or the feeder it supplies. If there are two independent transfer switches serving each patient bed area, the result is even higher reliability. This change allows for, but does not require, that possibility.

Note that although the concept is well accepted, there are several correlation problems that weren't properly addressed by the panel. First, you can't, on the literal text, use this arrangement in a general care area. Secondly, and inexplicably, when the panel made this change it did the opposite in the very next subsection!

Sec. 517-19(b) covers receptacles at the critical care patient bed locations. Now express language spells out that at least one of the six required receptacles must be connected to a normal system branch circuit. This change clarifies that the normal circuit ordinarily required to supply critical care patient bed locations must supply at least one receptacle at each of those locations. Remember, a receptacle is a single contact point, and a duplex receptacle is, therefore, two receptacles. However, if the facility is using the new exception, there may well not be a normal system branch circuit at the bed location.

Since the actual wording is "the normal system branch circuit required in Section 517-19(a)" and since that subsection now includes the exception allowing two emergency system circuits from different transfer switches, it would be reasonable to apply that new exception to this provision as well. However, you'll need to sort this out with the inspector until this gets correlated.

These patient bed location requirements operationalize the general advice given in Sec. 517-33(b) (FPN) referred to earlier. The note advised against putting all your eggs in one basket (i.e. transfer switch) and instead urged the use of multiple sources. The rules now provide even more effective ways to do just that.

Small hospitals

Nevertheless, Sec. 517-30(b)(4) specifically allows (does not require) a small facility with a load not more than 150kVA to use a single transfer switch for the entire essential electrical system. Remember, the essential electrical system of a hospital includes equipment loads that aren't included in the emergency system. In such a facility, a single transfer switch can supply, therefore, both emergency and nonemergency loads.

Go back to Fig. 1. Sec. 700-9(b) Ex. 1, which expressly prohibits supplying both emergency and nonemergency loads from a single transfer switch. This is a direct conflict within the 1996 NEC that was overlooked by the Correlating Committee. There isn't any easy way around this one, since the provisions of Chapter 7 have equal weight to those in Chapter 5 of the Code.

There is an old principle of statutory interpretation that says when presented with an apparent conflict, interpret so as to give maximum effect to both provisions. In this case, that would mean following the express permission for hospitals (also nursing homes) in Art. 517, and then rigorously following the revision in Sec. 700-9(b) Ex. 1 everywhere else. You definitely want the inspector involved in this one early in the plan review process.

Transfer switches for additional loads

In addition to providing desirable flexibility to the design of critical branch circuiting in hospitals, transfer switches now provide additional options in the allowable loading of the standby generation facilities. A standby generator used as the source of supply for a hospital emergency system can now be permitted to supply other hospital loads not part of that system, provided they have their own transfer switch.

A new paragraph in Sec. 517-30(b) (5) is added to cover "loads served by the generating equipment not specifically named in Secs. 517-33 and 517-34." These loads require their own transfer switch(es). And, there must be provisions so the transfer won't take place if the generator will be overloaded, or the loads must be shed if the generator becomes overloaded. [ILLUSTRATION FOR FIGURE 3 OMITTED].

Be careful, however. The literal text of this new provision includes a potentially dangerous blunder (see drawing). The problem is that Secs. 517-33 and 517-34 cover the critical branch and the equipment system, respectively. Sec. 517-32, on the life safety branch, was inadvertently omitted. It is now a literal requirement of the 1996 NEC to shed the life safety branch if the generator approaches overload from nonessential load! This, of course, is in direct conflict with Sec. 700-5(b), and should never be allowed.

Sec. 3- Ex. of NFPA 99 recognizes on-site generation as a source of power to both essential system and other loads for load peak or internal voltage control, load relief for the utility, or cogeneration, with conditions. If a single generator is used, it must be in such a way that the mean service interval between overhauls must not be reduced to less than 3 yrs. Otherwise, two generators (minimum) must be installed, with the essential demand capable of still being carried with the largest generator out of service.

Many facilities do use their generating equipment for other loads. If done properly, there is no problem. Again, the creative use of additional transfer switches can enhance system reliability and add flexibility in these areas.


In this context, an essential load in a hospital is one that is required to be on the essential electrical system. Note that at this point, we haven't used the word "emergency." Essential electrical systems in hospitals aren't the same as emergency systems. They include both emergency and nonemergency loads, although all their loads have a high priority. Many Code users offhandedly think any loads on a standby generator are emergency loads. This is probably because the lay public commonly refers to the generating equipment as emergency generators.

In hospitals, the true emergency system joins with the equipment system, which includes priority power loads but not emergency loads, to form the essential system. The equipment system supplies "major electric equipment necessary for patient care and basic hospital operation." The emergency system, on the other hand, supplies (and is limited to) "circuits essential to life safety and critical patient care."

The life safety circuits are for egress lighting, exit signs, critical alarm systems, communication systems if needed to relay instructions during an emergency, elevator cab lighting and communications, elevator signaling and control functions, battery chargers for standby lighting used in emergencies, and selected receptacles at the generator set location. Many of these loads are building Code-type issues and are also covered in the Life Safety Code, NFPA 101.

The critical branch includes loads that are directly involved with critical patient care, and, as such, have more of a medical focus. For example, in operating rooms and other areas that use anesthetizing gases, task illumination, fixed equipment, and selected receptacles must be part of this branch. If isolated power is used, those power supply systems must be part of this branch as well. Sec. 517-33(a) provides the complete list, including Sec. 517-33(a)(8)a, which calls for at least one duplex receptacle per patient bedroom to be on the critical branch. We will come back to this particular provision later because it is now inconsistent with a change in a different section.

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