Written by Mark Delle Bovi
WHAT IS A FIRE PUMP?
In most new buildings being built today, building codes will require a fire protection sprinkler system to be installed throughout the entire building. The goal of the sprinkler system is to protect the building/structure from building and to suppress the fire from spreading to allow occupants to evacuate. In order for the sprinkler system to work properly and flow enough water, a certain pressure throughout the system must be maintained. This is where a fire pump comes into play.
When fire protection engineers start to design the system, they will often start with the incoming water pressure from the water service coming into the building. Usually, there is a dedicated water service just for the sprinkler system. Building codes require a certain amount of pressure to be maintained at all times at the furthest point in the system. If this pressure is too low to meet the standards, a pump must be added (Fire pump). During sprinkler operation, the fire pump will kick on when a certain pressure drop is reached to boost up the pressure to a certain point. The fire pump shall never turn off until the pressure is reached. Essentially, fire pumps are designed to run indefinity. This is an important thing to remember when looking at the electrical side of things for fire pumps.
NEC Article 695
Everything we need to know about fire pumps from an electrical standpoint can be found in article 695 of the 2020 NEC. There are 2 areas of NEC 695 that we will focus on; Fire Pump Power sources, Continuity of power, and Conductor requirements. We will also discuss overcurrent protection and conductor sizing for fire pumps which are also covered in 695.
Fire Pump Power Sources
NEC 695.3(A) Individual Sources require the power source to be reliable as well as being capable of carrying indefinably the sum of the locked-rotor current of the fire pump motor(s) and pressure maintenance pump motor(s) and the full-load current of any associated equipment to this supply. The important thing to take away from this is the reliable part. Determining if a single utility power source is reliable is determined by the authority having jurisdiction or AHJ. It is unusual or unlikely an AHJ will consider a single utility connection reliable. In order for the power source to be considered is has to be one of the following:
- NEC 695.3(A)(1) Electric Utility Service Connection
- NEC 695.3(A)(2) On-Site Power Production Facility (On-Site Generator)
- NEC 695.3(A)(3) A dedicate feeder from a service connection as described by NEC 695.3(A)(1).
As per NEC 695.3(A)(1), a separate service may be provided directly to the fire pump. This could be a service lateral directly from a utility transformer to the fire pump controller. The other option mentioned is a connection from a shared utility service but connected ahead of any disconnecting means. It also mentions that this dedicated fire pump disconnecting means shall not share an enclosure, switchgear or switchboard section with any service disconnecting means. There are 2 options to meet this, provide a dedicated service connection from the utility transformer to the fire pump controller, or provide a tap within the service switchboard/switchgear ahead of any service disconnecting mains. The Tap method would require a separate service switch for the fire pump which can be in the form a fused disconnect switch, enclosed circuit breaker, or integral to the fire pump controller if is service rated.
NEC 695.3(B) Multiple Sources: If reliable power cannot be obtained from one of the sources mentioned in NEC 695.3(A)/above, power shall be supplied by one of the following:
- NEC 695.3(B)(1) Individual sources: any combination of two or more sources from NEC 695.3(A)(1).
- NEC 695.3(B)(2) Individual sources and on-site generator: any combination of one or more sources from NEC 695.3(A)(1) and an On-site standby generator.
- NEC 695.3(B) Multiple Sources: If reliable powered cannot be obtained from one of the sources mentioned in _______________
There are two exceptions to NEC 695.3(B) that allows eliminates the need for an alternate power source if an engine-driven fire pump (Diesel fire pump) or steam-powered turbine-driven fire pump is installed.
NEC 695.3(D) On-site Standby Generator as an Alternate Source: The standby generator must be sized to have sufficient capacity to allow normal starting and running of the fire pump while supplying all other loads connected to the generator simultaneously. Load shedding of optional standby loads for increased capacity on the generator is allowed. Load shedding is a technique that allows the generator controller to shed or disconnect the load from the generator during certain times to gain extra capacity.
Continuity of power
NEC 695.4 Covers all the requirements of how fire pumps are connected to the power and source and the disconnecting means allowed and permitted. The goal of 695.4 is to prevent inadvertent disconnection of the fire pump from a power source. As stated, early fire pumps are designed to run indefinitely which means they are set up to run until they fail.
NEC 695.4(A) allows the supply conductors to directly connect the power source to a listed fire pump controller or fire pump transfer switch. If the fire pump transfer switch and/or controller is service rated with an integral service switch, it can be directly connected to the utility power source without any additional disconnecting means.
If using the fire pump controller/transfer switch as the service disconnecting means and overcurrent protection it must meet NEC 695.4(B)(2)(a)(2) which states the following:
- Overcurrent protection device shall not open within 2 minutes at 600% of full load current.
- Overcurrent protection device shall not open within a re-start transient of 24 times the full-load current.
- Overcurrent protection device shall not open within 10 minutes at 300% of full-load current.
- The trip point for circuit breakers shall not be field adjustable.
If we have an application where we can not go directly from the utility transformer and need a tap within the service switchboard, a single service disconnecting means with overcurrent protection is permitted by NEC 654(B)(1) between the power source and the following:
- A listed fire pump Controller
- A listed fire pump power transfer switch
- A listed combination fire pump controller and transfer switch
NEC 695.4(B)(2)(a)(1) describes the overcurrent protection requirements for the fire pump utility connection. The overcurrent protection device must be sized to carry the locked-rotor current of the largest fire pump motor and the full load current of all other loads being served. Locked Rotor current can be found by using NEC Table 430.251(A). One trick that also works is to multiply the Full-Load current of the motor by 450%. This will get very close to the LRC.
NEC 695.4(B)(3) covers all the requirements for disconnecting means between the fire pump utility connection and the fire pump controller and/or transfer switch. Those requirements are:
overcurrent protection requirements for the fire pump utility connection. The overcurrent protection device must be sized to
- Be identified as suitable for use as service equipment.
- B lockable in the closed position.
- Not be located within the same equipment (switchboards/panelboard/cabinets etc)as other loads.
- Be located remote from other building service and other fire pump disconnecting means.
- This disconnecting means must be marked “fire pump disconnecting means”.
For On-Site generator disconnecting means NEC 695.4(B)(3)(c) points us to NEC 700.10(B)(5). In summary, the on-site generator disconnecting means (usually a generator mounted circuit breaker) is allowed to be installed in equipment that supplies other loads but it must meet certain criteria in NEC700.10(B)(5). For single generator applications, the most common way to achieve this is to provide a dedicated circuit breaker mounted on the generator output bus to feed the fire pump directly.
Multiple generators with paralleling switchgear: When using paralleling switchgear, NEC 700.10(B)(5) permits the fire pump circuit breaker to share a common bus with non-emergency loads as long as it is in a separate vertical section of switchboard or switchgear. An interesting point here is the word EMERGENCY. The fire pump is considered an emergency load, not a standby load hence it can be grouped with other emergency load breakers in the same section.
NEC 695.4(B)(2)(b): Overcurrent protection devices between on-site standby generators and fire pump controller/transfer switch shall be sized to pick up full pump load and comply with 430.62. We size the fire pump OCPD on the generator as we would for any other motor using NEC Table 430.52. This would be the full-load current of the motor multiplied by a maximum of up to 250%.
Service conductors and Conductors from on-site generator(s) serving fire pumps must be physically routed on the outside of the building (NEC 659.6(A)(1)). If conductors cannot be run outside the building, conductors need to be encased with 2 inches of concrete as per NEC 230.6.
Feeders from the load side of the disconnecting means (Service and On-Site generator) to the fire pump shall meet all the following requirements as per NEC 695.6(A)(2):
- Conductors kept independent all of other wiring (No sharing pull boxes)
- Conductors shall supply only loads associated with the fire pump system.
- Conductors shall be protected by damage by fire structural failure or operational accident (routing them outside the building)
- Where routed inside the building conductors need to be 2-hour fire rated. There are 3 methods for this:
- 2 inches of concrete encasement of feeders
- Mineral insulated cable
- UL recognized 2 hour rated MC cable (Vitalink)
Fire pump conductors supplying fire pump motor(s) and other loads shall have a rating of 125% of the sum of the fire Match OCDP type (fuse or Circuit breaker) with what is listed on the nameplate of the equipment.