How are electric fire pump motor conductors sized?

Method:

  1. If the motor/controller starting method is:

a)   Across-the-Line (FTA1000)

b)   Primary Resistance (FTA1500)

c)   Autotransformer (FTA1800)

d)   Soft-Starter (FTA1930)

  • Use the same method to determine wire size as shown above for between power source(s) and controller.  (In our example a 480V, 100HP motor uses 2/0 conductors).
  • For these controllers, three conductors (plus ground) are used between the controller and the motor.
  1. If the motor/controller starting method is Part Winding (FTA1250):

a)   Find the motor full load amps from Table 430.250.

b)   Multiply the # amps by 1.25

c)   Divide this number of amps result by 2

d)   Divide this number by .80 and find this result in Table 310.16 in the 75C column *

e)   Connect six conductors (plus ground) between the controller and the motor.

f)     480V,100HP uses 3 AWG

  1. If the motor/controller starting method is Wye-Delta Open (FTA1300) or Wye-Delta Closed (FTA1350):

a)   Find the # amps from Table 430.250.

b)   Multiply this # amps by 1.25

c)   Multiply this number of amps result by 0.58

d)   Divide this number by .80 and find this result in Table 310.16 in the 75C column *

e)   Connect six conductors (plus ground) between the controller and the motor.

f)     480V,100HP uses 2 AWG

      Example:  Part Winding Start & Six conductors ran in common conduit:

                  100hp. 460V = 124 FLA  (Table 430.25)

(124 X 1.25) divided by 2 = 77.50, then 77.50 divided by .80 = 96.88

Answer:  96.88A = 3 AWG per Table 310.16

 

Reference:

Reduced Voltage Motor Wiring: Reference NFPA 70 Article 430.22(A), (C) & (D)

Do not derate 20% if two conduits are usedSix conductors in common conduit: The conductors must be derated by 20% to account for the additional heating of six conductors together in one conduit. Reference NFPA70, Article 310.15(2) and Table 310.15(B) (2) (a).

 

 

 

 

Important Disclaimer

 

Information contained herein was assembled from national electrical and fire protection codes and references to assist engineers, pump distributors and installing contractors.

Flotronix Corporation disclaims liability for any personal injury, property or other damages of any nature whatsoever, resulting from the use of this document. No guaranty or warranty is made as to the accuracy or completeness of any information published herein. Anyone using this document should rely on their own research or seek the advice of a competent professional.  

 

Copyright © 2005

Please log in to rate this.
0 people found this helpful.


Category: Fire Pump Controllers -FAQ

← FAQs

Comments are closed.