Answer / sanu chaudhary

armature current of alternator depends upon the power
factor of machine .AT THE UNITY POWER FACTOR armature
current is minimum ,armature current of machine will
increse for same power if power factor of machine is
lagging as well as leading so it is desired to run the
machine near the unity power factor for optimum utilisation
of capacity of machine .in the leadind power factor machine
take reactive power from the infinte bus or grid and
machine is under excitation so it may be the cause of
panelty from power corporation due to reaciving of reactive
power hence we operate the machine near .9 lagging power
factor

We have drive feeding an output contactor for a 5000 hp motor @ 5kV application. The motor FLA is 629amps x 1.25% service factor = 786.25 amps The cable that we current have in place between the drive and the output contactor is 350kcmil- MV-105 (parallel cables). Per the NEC table 310.60 (C) (69). This show the cable ampacity is at 615 amps per cable for a rating of 1230 amps. We have derated the cable for the cable tray and the multiple cables application and our calculations show that after the correction factor factor we are at 922amps. 1230 amps x .75% correction factor factor = 922.5 amps. The customer is driving us to then use another safety factor of .25% off of the already derated cable. Can you please provide a longhand calculation for your solution on this application?

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