How to design safe, reliable fire pump power service
- Illustrate the basic design of fire pumps and their power systems.
- Examine the codes and standards that dictate fire pump power system design.
- Explain best practices by reviewing example designs.
As with all life safety systems, maintaining power to the key elements of a building in the event of an emergency is vital to the safety of the building and its occupants. For buildings that do not have an adequate supply pressure from the incoming water source, the fire pump is critical in that protection. Consideration of the power supply to the fire pump and its accessories is based on an evaluation of the codes that govern its installation as well as conversations with the local authority having jurisdiction (AHJ) to determine the best solution for the building’s particular design.
In the event of a fire, it is not unusual for first responders to disconnect the main power supply to the building, especially if there is a risk that the source of the fire may be from the electrical system. It is important that the AHJ and the building operator have a clear understanding of how the fire protection system is powered so that the power supply to the fire pump is not inadvertently disconnected. With proper understanding of the codes and good design practice, engineers can ensure the design will provide a reliable source of power for a building’s fire protection systems.
Fire pump intent
The fire pump plays a critical role in providing for a safe environment, and how it is powered is an important consideration of the design. A reliable source, backup power supply, protected feeders, and properly sized overcurrent protection are the keys to a successful installation.
Keeping the fire pump operational throughout the entire emergency event is the most important goal of each design. This is even more important than protecting the equipment from harm. If the fire pump fails to perform its intended duty, the building and its occupants will be at great risk.
Codes that apply
Multiple codes apply when it comes to powering fire pumps. Two are primary references: NFPA 70: National Electrical Code (NEC) Article 695 (fire pumps), and NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection. The NEC has responsibility for the electrical installation of these systems whereas NFPA 20 holds responsibility for the performance of the system. Other articles of the NEC and other NFPA sections offer guidance on emergency power systems and building requirements for fire protections systems.
A reliable power source is necessary for proper fire pump installation. The reliability of a utility service feed for a fire pump will be determined by the local AHJ. Also, depending on the type of building, occupancy type, and the building operator, the method for serving the power to the fire protection systems should be considered as a part of their overall fire-response plan.
As defined by NFPA 20, the following items should be considered to determine if the source is reliable:
- The source power plant has not been shut down for more than 4 continuous hours within the past year.
- There have been no routine power outages in the area that have occurred within the past year.
- The building cannot be served by an overhead utility line. The overhead service line will become a risk because fire department personnel will not work around energized lines. If this overhead line is the source of power, it will become disconnected while fighting the fire and render the fire pump inoperable.
If the utility source is determined to be unreliable, then multiple sources will be needed for the installation.
A single power-supply source for a fire pump can be provided from either a reliable utility supply or from onsite power production. The important consideration for a single supply service is that the incoming supply power is not interrupted. Therefore, special consideration should be given to how the service enters into the fire pump controller and what disconnecting means are located upstream from the pump.
If a utility power connection is used, there are two methods that can provide power to the fire pump:
1. The first method is a direct connection from the utility source where a power feed is brought straight from the utility into the fire pump controller with no interruptions. This feed is then treated as a service entrance and needs to meet all of the code-required functions as defined by NEC Article 230 (services).
2. The second method is a tap ahead of the service main disconnecting means. When using this method, the following items should be considered.
- The service feeders shall terminate in a dedicated cabinet or vertical switchboard section located ahead of and not within the same cabinet as the disconnecting means.
- The service feeder shall be in a location separate from the fire pump so as to minimize the risk of damage to the fire pump in the event of a fire at the main service area.
If onsite power production is available to the building, it can be considered as a service to the fire pump if it is deemed reliable and meets the fire protection requirements as outlined in NFPA 850: Recommended Practice for Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations.
With a single source of power to the pump, if the source that is providing power to the building fails, then a fire watch or other approved procedure needs to be put in place to verify that the building and its occupants are not put into harm’s way.
Multiple sources of power
When the single supply source is deemed unreliable, you then need to provide multiple supply sources. The NEC allows for several options for this type of supply:
- Combine two individual sources. If the site has access to two utility feeds from different substations, this may be an acceptable method. The local AHJ would need to determine if the combination of these two services would then be considered reliable.
- Another option is to have a backup engine- or steam-driven fire pump.
- Most commonly, the second source is an onsite generator to supply the secondary source of power to the fire pump.
Whereas a primary supply source is arranged to allow for the fire pump to run under any adverse situation, this is not the case with the supply source from the generator. When a generator provides the secondary supply source to the fire pump, it is important to arrange the connection to the fire pump so that any fault or malfunction of the pump does not interrupt service from the emergency generator to the other life safety loads within the building. The code does not require that the generator is able to carry the locked-rotor current indefinitely, as is the case with the primary source. If a fault would occur at the fire pump, it could take out the generator and lose other life safety functions like lighting.
When designing the generator service to the fire pump, the following items should be considered:
- The size of the generator shall be capable of carrying the normal starting and operation of the fire pump, jockey pump, and other accessory loads. The generator is able to automatically shed nonessential loads to make room for the fire pump capacity.
- If the fire pump is the majority of the load on the generator, the designer should review the generator characteristics so that it can support the starting of the pump.
- A variable frequency drive (VFD) can be provided on the fire pump as a tool to regulate different incoming water pressures (for example, pulling from a tank and a city supply line). The size of the generator should still be able to account for full start-up of the fire pump in the event the VFD goes into bypass.
- When providing the secondary supply from the generator, you may have a disconnecting means located ahead of the fire pump. This is in place to automatically disconnect the fire pump if an adverse condition would jeopardize the emergency power system.
- The arrangement of equipment and feeders should be oriented such that a fire at one source would not impact the other source.
- A transfer switch listed for fire pump service shall be provided and located in the same room as the fire pump.
Overcurrent protection and disconnecting means
The best method for delivering a primary power supply source to a fire pump is by direct connection to the power source. With a utility transformer serving the building, a direct connection from that transformer with no disconnecting means will provide a high level of reliability and minimize the potential for inadvertent disconnection of the supply feeder. If a disconnecting means is needed, the following should be considered:
- Overcurrent protection shall be sized to carry the pump’s locked-rotor current indefinitely. This is different than sizing characteristics for another type of pump and will result in a much larger breaker rating.
- The disconnecting means shall be supervised to alert personnel if the device is opened.
- Overcurrent protection is provided as a part of a listed assembly for fire pump service.
The disconnecting means for the fire pump should be clearly identified to prevent the inadvertent disconnecting of that supply source. It should also be located separately so that in the event of a fire, multiple supply sources are not disabled, rendering the fire pump inoperable.
Again, it is important to note that the code does allow for overcurrent protection on the emergency power feed and it does not require that it be sized for locked-rotor current.
It is important for the electrical engineer to design the feeder system delivering that power to be protected against harm or damage. Keep in mind: If the fire pump is operating, there is most likely a fire within the building. Thus, protecting the feeder from harm by the fire is important to ensure the building does not lose power to the pump that is protecting it and its occupants.
The method of protection is similar to that spelled out in NEC Article 230 (services), which requires a feeder to be routed outside of the building or to provide an equivalency of protection against harm. This can be accomplished through concrete encasement or a protected system with a minimum of 2-hour fire protection. It also is important to segregate the power wiring that serves the fire pump and its accessories from other loads so a failure in another feeder would not induce harm to the fire pump feed.
The sizing criteria for the feeder supplying the fire pump are provided by NEC Article 695 (fire pumps), which indicates that the minimum size is 125% of the sum of the full-load amps of the fire pump and pressure maintenance pumps plus 100% of the remaining accessory equipment. Installation, voltage drop, and other derating factors should be considered when determining the final selected wire size.
Grounding for the fire pump feed is dependent upon what method of connection is used. If the system is designed with a single source and the power is essentially being treated as a service, include a grounding electrode conductor as sized per NEC Table 250.66 and based upon the size of the incoming ungrounded conductor. If there is a secondary source, like a feed from a generator, this is treated like a feeder; provide an equipment-grounding conductor sized per NEC Table 250.122.
The fire pump power feed is a critical item to consider as a part of the overall design. Consideration for this should be done when determining the systems that will be used to serve the normal and emergency power supply to the building. Engaging the local AHJ as well as the building operator in the key design decisions is important to ensure a safe and operable fire protection system for the building.
Sarah Kuchera is senior vice president and electrical engineer at ccrd + WSP. Her duties have included project management, production coordination, and project engineering on a variety of different projects including hospital, retail, hospitality, and office buildings.