Interpolation chart for underground calc

[Isaiah]

Not quite sure how this works. Trying to determine the ampacity of 12 per phase 500 kCMIL, 480V transformer secondary conductors routed in underground concrete encased duct bank, 40 feet long and 36” deep with a RHO of 60 and LF of 50. they termibate at the MCB of a LV switchgear. I figure the temp correction factor of 77 degrees F for earth, so how does the interpolation chart fit into all this?


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[Isaiah]

Not quite sure how this works. Trying to determine the ampacity of 12 per phase 500 kCMIL, 480V transformer secondary conductors routed in underground concrete encased duct bank, 40 feet long and 36” deep with a RHO of 60 and LF of 50. they termibate at the MCB of a LV switchgear. I figure the temp correction factor of 77 degrees F for earth, so how does the interpolation chart fit into all this?


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Let me try this another way: Table B.2(7) shows 3, 500 kCMIL in a single duct as 503A for a RHO of 60 and LF of 50….with 6 ducts the ampacity reduces to 412A which is a difference of 91A.
Is it safe to assume the ampacity would be reduced by another 91A to 321A (412-91=321)
if the ducts were increased to 12?


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