Is there a reason to oversize medium voltage cables to motors other than temp and ampacity?

[bdn2004]

I'm on a project that is to replace a number of medium voltage cables to some motors. The motors operate at 4160V. The biggest motor is 500HP, 75Amps or so.
This is an emergency job. The existing cabling is #2/0, the engineering firm specified #2. It seems like they used the 1.25 rule per the NEC which doesn't apply to our situation.
They have the coordination study which I would assume they used to size this...but I'm telling our guy not to get something smaller that what was there - as its no sweat on us, but could if that's too small.
Is this being over cautious? This is at a power plant.

 

[krisys]

Other reasons could be:-
a) Excessive voltage drop during starting
b) Excessive voltage drop during running
c) Excessive de-rating
d) Mechanical strength
c) Minimum available size for the MV cables

 

[mgtrp]

f) Possibly relevant for a powerplant - short circuit withstand.

 

[jraef]

On the other hand, it might be 2/0 just because at one time, the motors were 1000HP, or someone thought that might happen in a future expansion and did not want to have to pull new wires if it did. In that case the new consultant may have vetted the issue thoroughly and investigated the future considerations with the owner, but for the sake of the people footing the bill, decided against over killing the size. Is there any harm in just asking?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

Power plant auxiliaries are frequently required to operate for under severe overload for prolonged periods, and being able to start and run with very low supply voltages (0.7 p.u.) with correspondingly high line currents. In the UK this sort of requirement is more common in the ex-CEGB nuclear and coal stations rather than the newer privately-operated CCGT stations.

 

[7anoter4]

I agree with mgtrp. In my opinion, the required copper conductor XLPE insulated cross-sectional area,to withstand 10 kA in 0.93 sec it is about 2/0 [67.42 mm^2].

 

[bdn2004]

the engineering firm does have the electronic model of the power system. Isn't the short circuit withstand value and the cable damage curve essential parts? Size the cable too small and the software tells you? After I posted this come to find out, the original cable in the plant to these motors was a #4/0 aluminum - and still in the cable trays. The plant is likely 50+ years old. But this had been changed out some where along the line to a #2 copper, but the single line wasn't updated, but the model is. After I found we aren't going back with smaller I dropped the subject.