National Electrical Installation Standards

Standards as High as Your Own

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Monday, November 13, 2017

Question:

Thanks for  the service. Are fire pump ground conductor sizes based on table 250.66? I have a fire pump rated at 250HP @ 480V/3ph. The fire pump service is achieved through a fire pump tap ahead of the service disconnecting means as allowed by NEC 695.3 (A)(1). The fire pump conductors are sized based on 250HP F.L.A. of 302 x 1.25% = 377.5 amperes. 377.2/2 = 188.75amps. NEC table 310.15(B)(16) ampacity for #3/0 at 75Deg C equals 200amps.

As an example the fire pump controller lugs will not accept any conductors larger than #3/0. In essence the electrician elected to run two (2) parallel #3/0 conductors to the fire pump controller. If I were to apply table 250.66 for sum of the cross sectional area for parallel conductors I used table 8 Properties of Conductors in the NEC. #3/0 has a circular mil area of 167800. I then multiplied this valve by two (2) for the number of parallel sets which is equal to 335,600 total cross-sectional area. NEC table 250.66 for copper conductors over #3/0 through 350 kcmil would require a #2 copper GEC in each parallel set.

Dan Craven

A

Answer:

Hey Dan thanks for your question and you are welcome. Taps for fire pump equipment are allowed ahead of service equipment if provided with their own service equipment as stated in 230.82(5) and by 695.3 (A)(1). In addition to having a grounding electrode conductor as stated in 250.24(A) a grounded conductor (neutral in your example) must also be brought to that service equipment as stated in 250.24(C). The grounding electrode conductor is sized based on Table 250.66, using the equivalent area for paralleled conductors, but the grounded conductor(s) is sized based on Table 250.102(C)(1) as stated in 250.24(C)(1). The 3/0 copper conductors could be used if no adjustment factor from 310.15(B)(3)(a) applies such as the parallel sets being in separate raceways. If in a single raceway even if you have 90 Degree C rated conductors and start with the 225 ampere value in Table 310.15(B)(16) applying the 80 % adjustment factor from 310.15(B)(3)(a) results in an ampacity of 180 X 2 [conductors per phase]= 360 amperes - not enough for the 377.5 ampere minimum ampacity in your example. If installed in separate raceways the grounded conductor must also be installed in each raceway and cannot be smaller than 1/0 as stated in 250.24(C)(2). Keep in mind that if those conductors also supply pressure maintenance pump motors they must be included at 125 % plus 100% of any accessory equipment as stated in 695.6(B).

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