Minimum Ampacity table for termination temperature limit for DUCT BANK MV feeders

[Cherry2335]

Hello, If we are feeding MV switchgear (34.5kV) through Duct bank which table shall be used to check minimum ampacity regarding termination temperature limit. There will be a vault under MV switchgear. Duct bank will open in the vault (16'x8') and MV cables will pass through that vault before terminating in the MV switchgear.
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[Isaiah]

Hello, If we are feeding MV switchgear (34.5kV) through Duct bank which table shall be used to check minimum ampacity regarding termination temperature limit. There will be a vault under MV switchgear. Duct bank will open in the vault (16'x8') and MV cables will pass through that vault before terminating in the MV switchgear.
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MV metal-clad switchgear is designed and tested to IEEE Std C37.20.2 and is most suited for cable temperature ratings of 90°C. The temperature limitations and air surrounding the cables within the switchgear should be taken into consideration when sizing and determining cable temperature ratings.
The switchgear is probably installed in temperature controlled environment so you won’t have to apply temp correction factors at the terminals. However the deeper the duct bank and proximity to other feeders within the duct bank may affect cable ampacity. This is usually assessed through ETAP study


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[JoeStillman]

For conductors in duct banks, you would use tables 310.60(C)(77) thru 310.60(C)(80), depending on the number of circuits and duct bank configuration as shown in figure 310.60(C)(3). The different tables are for copper vs. aluminum and 1/C cables vs. 3/C. As Isaiah says, use the 90°C column when the termination is on MV switchgear. The temperature rating you use for ampacity has to be that of the termination or the cable, whichever is lower per 110.40.