U/G Feeder ampacity after derate

[Dale001289]

Is there anyone out there who can run a rough calculation using ETAP, or similar software to determine the ampacity of a 480 VAC, 3 phase feeder in underground concrete encased duct bank with 17 per phase in parallel (A,B,C plus EGC in each of 17 Ducts) 500kCMIL, 600V 90Degree C XHHW-2, Cable buried at 26 feet to top of concrete? And also buried at 4 feet to TOC?
The spacing is 7.5” between ducts, 4” PVC, with two rows of 6 ducts and one row of 5 ducts. The overall duct bank thickness is around 30”. Earth temp is around 68 degrees F, Rho is 55, or 60 (for concrete) and LF load factor is around 50.
I tried using the NEC calculation, but the feeder ampacity was so low it didn't seem to make sense.

 

[ron]

Like this?

 

[Julius Right]

I got 289 A and 90oC induct no.9 and 10-instead of 85.5 in you image.

 

[Dale001289]

Like this?
View attachment 2572305

Yes that correct.
The firm used 312” for their calc when in reality the depth was only around 48” - these are transformer secondary conductors that terminate on a SWGR MCB with a trip of 3720A and horizontal buss of 4000A. I’m sure the ampacity is far higher than required.


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[ron]

I got 289 A and 90oC induct no.9 and 10-instead of 85.5 in you image.

If I run the paralleled set at 289A, all duct exceed 90deg

 

[Dale001289]

If I run the paralleled set at 289A, all duct exceed 90deg
View attachment 2572308

Thank you - what happens at 48”?


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