Is there any reason to oversize power cables to motors? 4

[bdn2004]

We have a project to replace a bunch of large cables feeding a significant number of 250Hp motors. The cables are being run in cable tray single file. As far as I can tell the engineering firm did the calculation right per the NEC and calculates the ampacity of the 500kmil cables after all the various temperature deratings to be around 382 Amps. Per the Code the motor leads must be sized with an ampacity of 302 x 125% = 378 Amps.

Because this is close, they say to bump it to 750kcmil. Now in my mind the 125% requirement already covered the safety factor. Is there any reason or any data anyone can provide that would justify spending this extra cash? Like will we get twice the life out of the cables?

 

[rbulsara]

bdn:
No that is not atypical. NEC tables are still applicable. To verify cable thermal withstand capacity, you need to compare the cable damage curves with the breaker/fues TCC. Similar to those posted by mayanees. NEC cable sizing is minimum requirement, if you have special needs you need to address those without violating NEC. This assumes that NEC is applicable in your jurisdiction.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Also prolonged starting (acceleration) time may justify upsizing the cable to minimize voltage drop at the motor terminals to provide adequate torque during starting period. Motor torque varies in proportion to square of the voltage.

Rafiq Bulsara

http://www.srengineersct.com