Figure 310.60

[sparkyguy545]

I recognize Figure 310.60 references Tables 310.77 through 310.86 which are for > 2kV. My question is in regards to the Note on the lower left of the page regarding maximum burial depth. Can someone clarify what I do if I need to exceed "Maximum depth to the top of electrical ductbank shall be 750 mm (30 in) . . ."? This seems like a shallow amount of cover for a Medium Voltage ductbank. What if field conditions require 42" or 48" cover? Is it stating that the Ampacities only apply at the published conditions, or that I cannot increase the amount of cover beyond 30"?

 

[walton]

Increasing the depth of duct banks changes the heat dissipation of the cables. There are equations that take into acount the soil type and depth for derating cable amperage in duct banks.

 

[Smart $]

I recognize Figure 310.60 references Tables 310.77 through 310.86 which are for > 2kV. My question is in regards to the Note on the lower left of the page regarding maximum burial depth. Can someone clarify what I do if I need to exceed "Maximum depth to the top of electrical ductbank shall be 750 mm (30 in) . . ."? This seems like a shallow amount of cover for a Medium Voltage ductbank. What if field conditions require 42" or 48" cover? Is it stating that the Ampacities only apply at the published conditions, or that I cannot increase the amount of cover beyond 30"?

Well the note states maximum burial depth, but it is inferred that is the maximum without adjusting for heat dissipation being worse at greater depths. Esssentially, it's saying you have to have an EE involved if you go deeper.