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 220.127.116.11 of NFPA 20. Section 18.104.22.168 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 22.214.171.124 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 126.96.36.199 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.