- Find out voltage and horsepower of motor
- Look up this value on Table 430.250 in NFPA 70
- Multiply number shown by 1.25 to get # amps
- Find # amps on Table 310.16 in NFPA 70 under the 75C (typical) column
to get conductor size
- For 480V 100HP, correct conductor size is 2/0
a) A fire pump motor should be in accordance with NFPA 20 184.108.40.206 which states that it must comply with NEMA Design B standards and be listed for fire pump service.
b) Fire Pump applications are discussed in Article 695 of NFPA 70: National Electrical Code
c) NFPA 70 695.6(C)(2) states that conductors must have a minimum ampacity in accordance with Article 430.22.
d) NFPA 70 430.22 says that a continuous duty application requires conductors rated for not less than 125% of the motor’s full load current as determined by Article 430.6(A)(1).
e) NFPA 20 220.127.116.11 states that all motors shall be rated for continuous duty.
f) NFPA 70 460.6(A)(1) states that “Tables 430.247 through 430.250 shall be used to determine the ampacity of conductors or ampere rating of switches, branch-circuit and ground fault protections instead of the actual current rating marked on the motor nameplate.”
g) For our example of a 100HP 480V motor, Table 430.250 applies as this table is for three phase alternating current motors. There it shows that 124 amps is the correct full-load current for a 480V, 100HP motor. Note that the table gives nominal voltage ranges, such as 440-480 and not specific voltages such as 473V. So, for example, if the incoming voltage happens to be 492V for instance, the 460V chart is still used.
h) Then, for determining the correct conductors for a 124 amp motor, again NFPA 70 430.22 says that a continuous duty application requires conductors rated for not less than 125% of the motor’s full load current. So, 124 amps times 1.25 equals 155 amps.
i) NFPA 430.6 states that “The size of conductors supplying equipment covered by Article 430 shall be selected from the allowable ampacity tables in accordance with (section) 310.15”.
j) NFPA 70 Table 430.5 shows that Article 430 covers fire pumps.
k) NFPA 310.15(A)(1) and 310.15(B) state that tables 310.16 through 310.19 show the ampacities for conductors.
l) Table 310.16 applies to our example 480V, 100HP motor. There it shows that to provide 155 amps, 2/0 conductors are required. The 1/0 size would not be large enough as it provides up to 150 amps and the 3/0 size would be okay as it provides up to 200 amps but is larger than necessary.
m) Also, if the power source will supply equipment other than the fire pump only, NFPA 70 695.6(C)(1) states that 125% of the full load current of the pressure maintenance pump and 100% of the associated accessories (such as a remote alarm panel) must be added to the value found in Table 430.250 to determine conductor sizing.