Electrical Duct Banks - NEC 310.60 - Details 1 - 3

[DMG_1]

Hello.

I have a question regarding "Electrical Duct Banks - NEC 310.60 - Details 1 - 3". In Detail 1, (1) Conduit is shown, in Detail 2 (3) & (4) Conduits are shown. If you have an electrical bank with (2) Conduits, what classification would it fall under, Detail 1 or Detail 2?

Thanks.

 

[jghrist]

Neither. Either use the lower ampacity or determine under engineering supervision as in 310.60(D). I guess that the AHJ would have to approve using the lower ampacity since it is not strictly according to code, but practically speaking, the ampacity of 3 circuits is lower than that of a single circuit because of mutual heating among circuits. There is more mutual heating with three circuits than with two.