Fire Pump Supply and Circuits – Reliability Is
Fire Pump Supply and Circuits – Reliability Is Paramount
The reliability of the power supply circuit to an electrically driven fire pump is a major focus of requirements in Article 695 of the NEC®, as well as those found in Chapter 9 of NFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection. Acceptable power sources for fire pumps are identified in 695.3. Because the power source directly relates to performance of the fire pump, the responsibility for development of these requirements falls under the purview of the NFPA 20 technical committee. The ultimate decision on what constitutes a reliable power source rests in the hands of the authority having jurisdiction (AHJ).
In the 2007 edition of NFPA 20, there is support material provided in Annex A to guide the AHJ in deciding whether a power source is reliable. Because it is in an annex, this material is informational only. However, it provides some valuable direction for the authority or authorities having jurisdiction in approving a fire pump installation. The following steps for determining a reliable power source are found in A.9.3.2 of NFPA 20.
A reliable power source possesses the following characteristics:
(1) The source power plant has not experienced any shutdowns of longer than 4 continuous hours in the year prior to plan submittal. NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems, begins to require special undertakings (i.e., fire watches) when a water-based fire protection system is taken out of service for longer than 4 hours. If the normal source power plant has been intentionally shut down for longer than 4 hours in the past, it is reasonable to require a back-up source of power.
(2) No power outages have been experienced in the area of the protected facility caused by failures in the power grid that were not due to natural disasters or electric grid management failure. The standard does not require that the normal source of power is infallible. NFPA 20 does not intend to require a back-up source of power for every installation using an electric motor–driven fire pump. Should the normal source of power fail due to a natural disaster (hurricane) or due to a problem with electric grid management (regional blackout), the fire protection system could be supplied through the fire department connection. However, if the power grid is known to have had problems in the past (i.e., switch failures or animals shorting a substation), it is reasonable to require a back-up source of power.
(3) The normal source of power is not supplied by overhead conductors outside the protected facility. Fire departments responding to an incident at the protected facility will not operate aerial apparatus near live overhead power lines, without exception. A back-up source of power is required in case this scenario occurs and the normal source of power must be shut off. Additionally, many utility providers will remove power to the protected facility by physically cutting the overhead conductors. If the normal source of power is provided by overhead conductors, which will not be identified, the utility provider could mistakenly cut the overhead conductor supplying the fire pump.
(4) Only the disconnect switches and overcurrent protection devices permitted by 9.2.3 are installed in the normal source of power. Power disconnection and activated overcurrent protection should only occur in the fire pump controller. The provisions of 9.2.2 for the disconnect switch and overcurrent protection essentially require disconnection and overcurrent protection to occur in the fire pump controller. If unanticipated disconnect switches or overcurrent protection devices are installed in the normal source of power that do not meet the requirements of 9.2.2, the normal source of power must be considered not reliable and a back-up source of power is necessary.
Once a reliable power source has been established, the supply conductors are required by 695.4 of the NEC to be connected to the fire pump controller or the combination fire pump controller and power transfer switch using one of the two methods specified in 695.4. The methods listed in695.4 are intended to minimize the possibility of interruption of power to the listed fire pump control equipment.
The first (and most direct) method is to run conductors directly from the source to the fire pump equipment. With this arrangement, if the fire pump circuit is supplied from a service point, the conductors are service conductors and must comply with all of the applicable installation requirements for service conductors. One of the most important considerations is the location of the service disconnecting means relative to where the conductors enter the building or structure. If the fire pump controller is the service disconnecting means and is located inside the building, the controller has to be installed at the nearest readily accessible location relative to where the conductors enter the building. This requirement applies to all service installations and limits the amount of conductor inside a building to only that length necessary to enter the building and terminate in the equipment. Because these conductors do not have an NEC-type short-circuit, ground-fault protective device at the point they receive their supply, these conductors are considered to be “unprotected” from an NEC standpoint, and limiting the length minimizes the potential for these conductors to be damaged, which in turn limits the fire initiation threat that these conductors pose.
Running service conductors directly to the fire pump control equipment is not always possible, particularly in existing buildings. The second option of supplying fire control equipment is to install a single “supervised” disconnecting means and associated overcurrent protective device. This overcurrent device provides short-circuit and ground-fault (not GFPE) protection of the supply conductors that run through a building, but unlike the Article 430 rules for sizing motor circuit short-circuit, ground-fault protective devices, the Article 695 device has to be sized to allow a motor to remain in a locked-rotor state indefinitely. In other words, the motor can remain in locked-rotor condition to failure, and the protective device in the supervised switch has to be sized to allow current to the motor until it starts or fails. There are protective devices in the listed fire pump controllers that will respond within a specified time frame, but the device ahead of the fire pump controller cannot be the link that opens under locked-rotor conditions. A change to the 2005 NEC clarified that conductors connected to this supervised disconnecting means do not have to be sized based on the rating or setting of the overcurrent protective device.
In addition to the reliability provided by the locked-rotor current rating or setting of the overcurrent protective device installed in or as a part of the supervised disconnecting means, the circuit conductors run through the building from the supervised switch to the fire pump control equipment have to be provided with two-hour fire protection. This protection can be afforded through the use of a listed 2-hour electrical circuit protective system, encasement in not less than 2 inches of concrete, or by installation of the wiring method in a fire-rated assembly or enclosure that affords not less than a 2-hour rating. This assembly or enclosure has to be dedicated to the fire pump circuit(s) and no other electrical equipment or circuits can be located in the assembly or enclosure.
Because of their critical fire protection function, circuits for electric fire pump motors are covered by a set of requirements that modify and amend the general motor circuit rules in Article 430. Reliability and operation of the fire protection systems takes precedence over the protection of the motor and motor circuit conductors.
An electric fire pump is to be installed in an existing building. The location of the fire pump equipment room necessitates running the supply circuit through the interior of the building. Therefore a supervised connection/switch and overcurrent protective device are required. The following nameplate information is from an electric fire pump motor:
60 amperes FLC
Locked rotor code letter G
The authority having jurisdiction has determined that the utility supplied 480Y/277-volt, 3-phase, 4-wire service is acceptable as a single reliable power source, and the service runs underground from a utility transformer to the supervised disconnecting means installed on the outside of the building.
1. Size the supervised disconnecting means and overcurrent protective device rating or setting.
695.4(A)(1): Overcurrent Device Selection. The overcurrent protective device(s) shall be selected or set to carry indefinitely the sum of the locked-rotor current of the fire pump motor(s) and the pressure maintenance pump motor(s) and the full-load current of the associated fire pump accessory equipment when connected to this power supply. The next standard overcurrent device shall be used in accordance with 240.6. The requirement to carry the locked-rotor currents indefinitely shall not apply to conductors or devices other than overcurrent devices in the fire pump motor circuit(s).
Fire pump motors are required to be listed. This requirement is specified in 695.10, which is extracted from 22.214.171.124 of NFPA 20. Section 126.96.36.199 in NFPA 20 also requires the fire pump motor(s) to comply with NEMA standard MG-1, Motors and Generators, and to be constructed as a NEMA Design B motor. Further, Table 188.8.131.52 establishes the locked-rotor characteristics of electric fire pump motors and specifies that a 50 horsepower motor is required to be constructed as either a code letter “F” or “G” motor. (See Table 430.7(B) in the NEC for information on how the code letter marking on a motor relates to its locked-rotor current.) Table 184.108.40.206 provides the allowable locked-rotor current (LRC) for 460-volt, 3-phase, ac motors. For a 50 hp motor, the LRC is 362 amperes. Based on this LRC value, the minimum continuous current rating of the overcurrent protective device in the supervised disconnecting means can be determined.
362 amperes LRC = 400 ampere circuit breaker or a 400 ampere switch with 400 ampere fuses
This switch is a second service disconnecting means and is permitted by 230.2(A)(1). The disconnecting means for the fire pump service is required to be remote from the normal building service disconnecting means in accordance with 230.72(B). Labeling of this service disconnecting means must be provided in accordance with 230.2(E), and the switch is to be bear a manufacturer’s marking indicating it is “suitable for use as service equipment” per 695.4(B)(2)(1). The switch is required to be supervised using one of the methods specified in 695.4(B)(5).
2. Size the service conductors supplying the supervised disconnecting means and the branch-circuit conductors between the supervised switch and the fire pump controller.
The minimum ampacity for these conductors is determined using 695.6(C)(2), which simply references the single motor conductor sizing requirement in 430.22. The general requirement in 430.22(A) is to size the motor circuit conductors at 125 percent of the full-load current rating in Table 430.247 through Table 430.250. In addition, because the operation of this motor is critical to life safety and property protection, allowable voltage drop is a mandatory requirement. The allowable voltage drop, measured at the motor terminals, is 5 percent of the rated motor voltage with the motor operating at 115 percent of full-load current. Under starting conditions the allowable voltage drop at the controller line terminals is 15 percent below the controller-rated voltage. These requirements are expressed in 695.7.
50 HP, 460 volt, 3-phase ac motor
FLC from Table 430.250 = 65 amperes
65 amperes × 1.25 = 81.25 amperes
Minimum size copper conductor from the 75° C column of Table 310.16 is 4 AWG
This minimum conductor size has to meet both of the voltage drop performance requirements of 695.7.
Running (measured at motor terminals): 460 volts/1.05 = 438.0 volts
Starting (measured at fire pump controller line terminals): 480 volts/1.15 = 417.0 volts
Because the utility supply is grounded, a grounded (in this case, neutral) circuit conductor is required to be installed from the utility transformer to the fire pump service disconnecting means per 250.24(C). As specified in 250.24(C)(1), the neutral conductor size is determined by selecting from Table 250.66, based on the size of the ungrounded conductors. If the 3-phase motor is the only load being supplied, the circuit from the supervised disconnecting means to the fire pump controller and then on to the fire pump motor can be three ungrounded conductors and an equipment grounding conductor. The equipment grounding conductor is sized from Table 250.122, based on the rating or setting of the overcurrent protective device (400 amperes). In this case, however, the equipment grounding conductor sized from Table 250.122 is 3 AWG copper, and the circuit conductors are 4 AWG copper. The equipment grounding conductor is not required to be larger than the ungrounded circuit conductors per 250.122(A).
3. Install the circuit conductors from the supervised disconnecting means to the fire pump controller location.
Now that the overcurrent protective device has been sized to accommodate the locked-rotor current indefinitely (in reality only as long as the motor remains in that condition before it burns up or the protective device in the fire pump controller opens), the next key element in providing a reliable circuit from the disconnecting means to the fire pump location is to ensure that the circuit is protected against exposure to fire. Section 695.6(B) provides three acceptable means of routing the circuit conductor through the building, and any one or a combination of those methods has to be used from the disconnecting means to the fire pump controller. From the controller to the fire pump, the circuit conductors can be installed using any of the methods covered in 695.6(E). Encasement in concrete or 2-hour fire protection of the conductors is not required in the fire pump room.