Duct Bank installations

[phoenix68]

According to NEC, does the duct bank run needs to be installed at a constant depth throughout the duct length? Or are there any provisions specifying certain length or percentage of total run can be deeper/shallower than the specified depth?

Can you please reference the article that specifies the same. Thanks for help.

 

[tom baker]

Burial depths are in table 300.5, which is cover to top. There are some special provisions for duct banks that I am not familiar with. No provision for percentages that you asked about.

 

[phoenix68]

Burial depths are in table 300.5, which is cover to top. There are some special provisions for duct banks that I am not familiar with. No provision for percentages that you asked about.

I see that 300.5 specifies the minimum depth required.

But for example, if I design ducts to be installed 36" deep for a total run of 150'. If during the run, for 10' length the duct is 48" below grade due to existing foundations. In such a case, while doing Neher Mcgrath calculations, should I be considering depth of ducts as 36" or 48"? I could not find anything related to this in NEC.