The Power Behind the Pump
Proper fire pump system design is an essential piece of a building's life-safety system. But don't forget that there's an equally important electrical component to consider for this system. More specifically, in addition to referencing NFPA 20—the standard for stationary fire pump installation—fire pump design also draws on National Electrical Code section 695, NEC section 430 and a number of other NEC sections.
Design of fire pump electrical systems must be coordinated with both the plan review agency and the local electric utility. Beyond safety, a well-planned fire pump system can have a dramatic effect on the building design and life-cycle cost.
For example, take the sizing requirements for the utility transformer that will serve the system. If not carefully planned, larger or even additional transformers might be required, which in turn, requires bigger transformer vaults. This translates into less usable—and less leasable—space for the owner, a big issue in high-rise facilities.
Also, if a single utility transformer feeds the fire pump service and other building loads as well, it would have to be significantly oversized to meet actual running loads. In the vast majority of cases, oversized transformers are lightly loaded, which causes them to become inefficient. Specifically, the no-load losses remain the same throughout the loading curve percentage of the transformers. Therefore, when a transformer is lightly loaded, the no-load losses represent a higher percentage of the entire input current of the transformer. Elsewhere, the dielectric and copper losses that result from the no-load current in the primary winding translates into a significant waste of energy—and dollars—over the life of the building.
Get to know your AHJ
But we're getting ahead of the game. Let's back up and discuss just exactly how an engineer designs an efficient and well-planned fire pump electrical system. It is critical to note that every electrical design for a fire pump service is based on specific design criteria and that each authority having jurisdiction (AHJ) might interpret the various relevant NFPA codes in its own way.
Even so, don't hesitate to employ good engineering analysis, as it can be a reality check to what might be overly conservative standards, such as those utilized for sizing transformers.
A good place to start is coordinating with the local electric power utility. It has been my experience that, like the plan review agencies, different utilities will have various requirements with regard to fire pumps. For instance, NEC 230.82 allows the fire pump to be connected ahead of the main. Typically, the "normal" feed to the fire pump is directly out of the utility vault or tapped ahead of the main service disconnect in the main service gear. If tapped out of the main service gear, the manufacturer must prepare the tap in a separate compartment and shall comply with NEC 230.82(4).
Some AHJs in the Seattle area prefer no overcurrent protection for the normal side of the fire pump automatic transfer switch (ATS). The thought behind this is that the pump needs to operate during a fire—even if it is eventually damaged from an overload or short circuit. At the same time, other jurisdictions, per NEC 695.4(B)(1), require overcurrent protection on the normal side of the fire pump ATS, but for short-circuit protection only—not for overload protection. NFPA 20 and NEC 430.7(C) provide a table for the lock rotor amps of various horsepower ratings and motor letter codes. If the AHJ requires overcurrent protection on the normal side of the fire pump, it must be located remotely from the main service. Most jurisdictions will also require labeling to indicate "Fire Pump Disconnect - Do Not Open." The purpose of the labeling is to reduce the possibility of accidentally disconnecting the fire pump circuit from fire department personnel.
Overcurrent protection for the emergency source—a standby generator, for example—is sized similar to that of a typical motor. NEC 430.152 allows a thermal magnetic breaker to be sized at a maximum of 250%, based on the letter code of the motor. It would make little sense to have an overloaded fire pump pull down the entire emergency generator. In this case, all emergency loads would be lost, including fire pumps, communications systems, egress lighting and other critical loads as defined by NEC 700.1.
Multiple fire pumps
When a design incorporates more than a single fire pump, it is important for electrical designers to coordinate with the fire pump system designer on the basis of operation, which will define when and how many pumps will operate simultaneously.
If more than a single fire pump can operate at a time, the electrical distribution, including the standby generator, must be sized appropriately. If only a single fire pump is intended to operate at any given time, the fire pump system designer must ensure actual lockouts exist that will prohibit an unintentional override and operation of more than one fire pump at a time.
In large high-rise buildings, there can be more than one stage of fire pumps required. A call for a sprinkler at a higher floor level can require the pressure from both a low-level and a high-level fire pump. In this case, the basis of operation of the system becomes even more important. In jurisdictions that require overcurrent protective devices on the normal side of the fire pump ATS—sized per the locked rotor amps per NEC 430.7(C)—the utilities will size their transformers based on the switch size of the overcurrent protective device feeding the fire pumps.
If two pumps can operate at any given time, then both overcurrent protective devices must be added together to determine the utility transformer size. As these overcurrent protective devices are greatly oversized—typically six times full load amps per locked rotor amps—this can have a dramatic effect on the utility transformer and associated vault size as noted earlier.
The intent of sizing the transformers based on the size of the overcurrent protective device is to ensure that even in very conservative worst-case scenarios, the transformers will be able to handle an overcurrent that could be as high as six times full load amps of the motor before the overcurrent protective device trips. Every fire pump configuration must be evaluated on its own merits, but I believe that in a multi-pump scenario—the system incorporated in most tall high-rise buildings—this criteria must be carefully evaluated through the eyes of prudent engineering.
As for the large ampacity overcurrent protective device, its purpose is to allow the fire pump to operate for a short time even under conditions of overload, up to and including lock rotor amps, which as mentioned above is six times full load current.
Again, the idea is to allow the fire pump motor to operate even if it is in an overload condition. It makes no sense to protect and save the fire pump motor if the building around the fire pump burns down. At lock rotor amps, the insulation on the conductors from the fire pump service to the fire pump controller would start to break down, and the conductors could soon go into a short circuit event and trip the overcurrent protective device in a short period of time.
Two examples, based on data from the Insulated Cable Engineers Assn. (ICEA), help express this point. Both cases use the ICEA formula: (I/A)2t = 0.0297 log ((T2+234 / (T1+234)).
Case 1. For a 200-hp fire pump, where I = short-circuit current, which in this case is the maximum allowable ampacity based on the breaker size, 1,600 amps; A = circular mills of a 350 kcmil conductor = 350,000; and T = 134 seconds or 2.2 minutes, the start of insulation failure. The short-circuit current, assumed to be 1,600 amps, is also the maximum amount of current allowed before the overcurrent protective device trips. The fire pump feeder is based on 125% of the full load amps, which is 240, of the fire pump; 240 amps multiplied by 125% is 300 amps. So, the 350 kcmil copper conductor is good for 310 amps at 75°C.
Case 2. For a 125-hp fire pump, where I = 1,000 amps; A = the circular mills of a #3/0 kcmil conductor = 167,800; T = 79 seconds or 1.3 minutes, the start of insulation failure. As in case 1 above, the short-circuit current is assumed to be the maximum amount of current allowed before the overcurrent protective device trips (factoring in that the fire pump has 156 full load amps and a #3/0 copper conductor). The fire pump feeder is based on 125% of the full load amps of the fire pump. The result: 156 amps multiplied by 125% is 195 amps. So, #3/0 copper conductor is good for 200 amps at 75°C.
Where T2 represents the maximum short-circuit temperature (150°C) and T1 represents the maximum operating temperature (75°C), we can see that for a 200-hp motor in a locked rotor current condition, insulation breakdown will begin to occur in approximately 134 seconds (2.2 minutes). For the 125-hp fire pump, insulation breakdown will begin to occur in approximately 79 seconds (1.3 minutes).
Keep in mind that where there are multiple fire pumps, the feeders to the fire pumps are based on 125% of the entire load—not just 125% of the largest motor. Therefore, the feed to the fire pump service in the examples above would have to be at least large enough for 125% of the full load amps of a 200-hp fire pump and a 125-hp fire pump. This can be represented as follows (FLA stands for full load amps):
200 hp = 240 FLA; 125 % of 240 amps = 300 amps
125 hp = 156 FLA; 125 % of 156 amps = 132 amps
Total fire pump service amps = 432 amps
Bus size utilized: 600 amps
Oversizing the fire pump feeders will also help to reduce the total voltage drop at the fire pumps. NEC 695.7 indicates that "the voltage at the motor terminals shall not drop more than 5% below the voltage rating of the motor when the motor is operating at 115% of the full load current rating of the motor."
Considering the alternatives
Those are the hard facts of fire pump electrical requirements, but I believe it is overly conservative to utilize the locked rotor amps of both sets of fire pumps when determining the maximum utility transformer size. Typically, in a multi-staged fire pump scenario, the sets of fire pumps will be located on different floors, remotely located from each other. A locked rotor current event in a fire pump motor during pump operation when both sets of pumps are required is an unlikely occurrence. It would develop into insulation failure followed by a short circuit, and the overcurrent protective device would trip in a time frame from 1.3 minutes to just over 2 minutes.
I believe the more logical approach would be to size the utility transformer based on the lock rotor current and the associated overcurrent protective de-vice feeding the larger of the two fire pumps (200 hp) and the minimum circuit amps (full load amps multiplied by 125%) of the second smaller fire pump (125 hp). This would result in a reduction in the size and/or number of utility transformers required in jurisdictions that size transformers based on the locked rotor amps of all fire pump motors that can run at the same time.
But what about the worst-case scenario? Even if both fire pumps at remote locations did go into a lock rotor current condition during fire pump operation when both sets of fire pumps are required, and in the time frames noted above, the serving utility transformer can probably experience and handle an overload for these short periods of time.
The main issue with overloaded utility transformers is how they deal with and remove internally generated heat. If a utility transformer is overloaded by say approximately 15% to 30% above its kVA rating for a short period of time, it is most likely that any heat developed in the transformer coils will be transferred without complication to the outside of the transformer. Consequently, there's a good probability that an overload for this short time frame will not cause a problem. On the other hand, when an overload condition lasts for a longer time frame, heat can start to build up internally within the transformer, which can cause serious damage.
Evaluate each case
Every fire pump design scenario will be different and should be evaluated independently, but the important issue to note is that the lock rotor current of both fire pumps as noted above would be a very unlikely event and would not be a steady-state occurrence. The possibility of this event occurring should also be evaluated with its potential time frame and the ability of the utility transformers to handle an overload condition for this anticipated time frame.
Choosing a Fire Alarm Annunciator
Firefighters are experts at putting out fires. They usually are not, nor should they have to be, computer scientists. Fortunately, recent advancements in annunciator technology combine intelligence, interactivity and simplicity.
When choosing an annunciator system, there are several factors to consider. For example, consider choosing an annunciator that features a fully-integrated graphic touch screen interface in which options appear and disappear according to the situation.
That way, when firefighters arrive, they see only the information and controls related to the situation, including location of the alarm and two buttons: acknowledge and silence. Once they select a button, it vanishes from the option screen, leaving only the controls and information necessary. Firefighters get key information and control the alarm system in less time.
Also, the computer-driven touch screen allows adjustments to nearly every situation. Firefighters don't need to get into the intricacies of the system; they only need pertinent information. A good annunciator panel should do just this, and on a situational basis. Ultimately, it is the control panel that should do all the footwork, and all research should be performed by the computer behind the scenes. Then, the needed information should be presented in a large, easy-to-read display.
Specifiers should be cautious about specifying traditional fire systems, which often display up to 90 buttons, many of which are unnecessary to first responders.
The needs of the building manager and technician who service the system should also be a primary concern. A panel should provide service professionals with a completely different set of menu options from those used by firefighters. And the ideal panel should be advanced enough for a technician's maintenance and service needs while maintaining the simplicity of operation required by non-technicians.
Finally, a system should also allow users to add custom response instructions tailored to best address a building's safety needs, a feature that's extremely appealing to building and facility managers. An annunciator system that is proficient for a broad range of industries, including schools, prisons, high-rises, facility management and health care, is definitely a sound choice.
In general, the age-old axiom that "less is more" is a good rule to follow when choosing an annunciator. For today's firefighter, less complexity and frustration equals more productivity and increased safety.