Ampacity in Buried Conduit vs Above Grade

[dball]

I am running 25 ft of 3-500MCM/phase from Padmount Tfmr to a Crypto mining container that has a cable entry point 8 ft above grade. So, 17 ft of the conductors will be in buried conduit and the remaining 8 ft will extend vertically above grade to the cable entry point. Since more than 10% of the Total circuit length is in above-grade conduit, outdoors, and therefore different ambient temperature, NEC requires the use of the lower ampacity. Question; does the NEC distinguish between above and below-grade conduits?
We ran some calculations in SKM and it appears that the heat doesn't dissipate in conduit above grade like it does 3 ft in the Earth. But do i really need to de-rate for 8 ft of above-grade conduit?

 

[infinity]

What current level are you trying to achieve with the 3 sets 500kcmil?

 

[dball]

The load is around 314 amps, continuous so X 125% = 393 amps. Also a mostly nonlinear load, so we are having to consider the neutral a "current-carrying conductor".

 

[dball]

I'm not sure I understand the 10% or 10 ft rule. Because technically I could make the first part of the UG circuit route longer so that the second part of route would be less than 10% of circuit length. Not that I would.

 

[infinity]

500 kcmil THWN-2 is good for 430 amps before applying any derating. So with 3 sets your ampacity is 430*3=1290 amps before derating.

 

[dball]

Sorry, yes that's correct. My question is really regarding the distinction between above grade and below grade conduits and how it affects ampacity. NEC Table doesn't distinctively say it only applies to buried conduits, but after looking Annex B, the various values of Rho change with burial depth, and therefor calculated ampacity.

 

[steve66]

I am running 25 ft of 3-500MCM/phase from Padmount Tfmr to a Crypto mining container that has a cable entry point 8 ft above grade. So, 17 ft of the conductors will be in buried conduit and the remaining 8 ft will extend vertically above grade to the cable entry point. Since more than 10% of the Total circuit length is in above-grade conduit, outdoors, and therefore different ambient temperature, NEC requires the use of the lower ampacity. Question; does the NEC distinguish between above and below-grade conduits?
We ran some calculations in SKM and it appears that the heat doesn't dissipate in conduit above grade like it does 3 ft in the Earth. But do i really need to de-rate for 8 ft of above-grade conduit?

Actually, it should be the other way around. For conduit in air, once the heat is in the air, it is basically dissipated.
Underground, the heat is passed to the dirt, but then it much reach the air to be dissipated. So current carrying capacity is theoretically higher for a conduit in air (unless you are also counting solar radiation, or a rooftop installation).

But the NEC lets you use the same ampacity straight out of table 310 for either installation. This generally is OK since the NEC load calcs are usually very conservative.