Medium Voltage Duct Banks Maxiumn

[mcsnola]

I have an medium voltage concrete encased duct bank with 13.8 kv lines installed however the location going into the building requires it to be quite deep . The note on the table in apendex D states min depth per table 300.5 and a max depth of 30inches . is this correct or am i miss reading this ?

 

[dicklaxt]

It can be deeper but must be so marked or indicated ,,,see the last note under that Table, Note 3d in the 2008 code

dick

 

[mcsnola]

Thank you ...i was told this had something to do with heat disapation ?

 

[dicklaxt]

Thank you ...i was told this had something to do with heat disapation ?

Take a look at Table 310.77 or 78 it will tell you what detail reference number of Table 310.60 installation to use for heat dissipation spacing ..



The deeper the cooler is usually the norm,,,

dick

 

[charlie b]

The deeper the cooler is usually the norm,,,

That may seem logical. But whenever I calculated ampacity of underground ductbanks, the results were that deeper runs had lower ampacities.

 

[dicklaxt]

That may seem logical. But whenever I calculated ampacity of underground ductbanks, the results were that deeper runs had lower ampacities.

I didn't mean to go as deep so as to get a Geothermal influence

seriously what do you attribute that to? inductive heating from adjacents conductors

There was once published a chart of derating factors dependant on the position in the bank and then the banks configuration,,,,did you ever see that.I disposed of all of my old design data when I retired.

dick

dick

 

[charlie b]

seriously what do you attribute that to? inductive heating from adjacents conductors

I don't know what it is, but I doubt that it is inductive heating. You have that same effect no matter how deep the conduits are. The best guess I can make is the possibility that as you go deeper into the soil, it becomes harder for the conduit to reject heat to its surroundings.