Technical Corner
This month’s article will continue with reliable fire pump circuits and will focus on wire sizing and voltage drop requirements. If you can recall, from the previous articles, the Fire Pump motor is 200HP and the Jockey Pump motor is 3HP. These motors are on the same circuit on a 480V system. The combined Locked Rotor Current (LRC) is 1618.5A. When do you use (LRC) or motor Full Load Currents (FLC) to determine necessary wire sizing which accounts for voltage drop? Wire Sizing Calc: LRC is not taken into account for wire sizing calcs for motors. The FLC’s listed in NEC Table 430.250 are used with a 125% safety factor (Otherwise called Minimum Circuit Ampacity (MCA)). Note: nameplate currents are only used to size motor overloads, see NEC 430.6(A)(1 and 2). Using values from NEC Table 430.250 Full Load Currents (FLC) of both the 200HP fire pump and 3HP Jockey Pump can be obtained at the 460V Utilization Voltage (UV) column. 1.25(240+4.8) = 306 Amps @460V. This should be the minimum rating that the specified wire can handle which equates to the minimum wire size. Using table 310.15(B)(16) and the 75 degree column we can see that #500 kcmil wire is adequate. Technically #350 kcmil wiring would suffice, however, I suspect we will need to use #500kcmil to help with any potential voltage drop issues and the rating of 310 amps is quite close to the MCA. Therefore the #500kcmil cabling is adequate and recommended to use as a starting point for calculations. But what about the Voltage Drop..? Voltage Drop Calc: NEC 695.7(A) indicates that a 15% voltage drop is permitted under fire pump motor starting conditions. LRC current needs to be utilized here to reflect manually starting across the line even if any soft starters are being utilized to start the motor. This ensures circuit operation in the case of a starter malfunction. res - technical corner
I will demonstrate below. R= resistance of conductor in (ohms/1000ft) Look up in NEC Chapter 9 Table 9 using power factor of .85 (Effective Z in PVC conduit) ***With a unity power factor you should be using Table 9 still but use the Alternating Current Resistance column*** L=Conductor length 1-way (ft) I=Current (Amps) R= .043 (ohms/1000ft) L= 325ft I=1618.5A (LRC value calculated in previous article) VD=[(2*R*L*I)/1000] * .866 =39.11 Voltage Drop (Theoretical) Conclusion: If we take this divided by the voltage of 480V we end up with an 8.2% voltage drop, which is in comfortable range below the 15%. There are also different variations of the Voltage Drop formula above which yield similar results, however this formula allows calculating with a more realistic power factor of .85. Hopefully this article/sample calculation finds you well and can be used as a reference for your project needs. If anyone would like to contribute to the RES magazine and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.
Brett Eliasz, P.E., LEED AP BD+C RES Director
FEBRUARY 2019 The ROCHESTER ENGINEER | 5
