Underground Ampacities

[faresos]

Hello Every,

I'm working on a project (Data Center) which we are running 4000A feeders. typically we use 10 sets of 600Kcmil for this application and much more for derating. However, I'm using a software (Etap) to run the underground ampacity calculations. we will be using THWN2 conductor (90 deg) for this application but we will be using the 75 deg ampacity listed in the NEC since we are using 100% rated breakers. My question, as we all know, the THWN2 is rated for 90 deg but can I still choose the conductor temperature rise to be set at 90 deg since its rated for that or I need to set it for 75 deg since we using the 75 deg ampacity?

Anythoughts?

Thanks,

 

[don_resqcapt19]

You cannot exceed the 75°C ampacity at the terminations, but I think you could use a maximum temperature of 90°C to calculate the duct bank. (not 100% sure, so wait for some more comments)

 

[JoeStillman]

I always apply any derating factors to the 90° ampacity of THWN-2. That is what the examples do in my NFPA 70 handbook.

 

[Julius Right]

In my opinion, I don't think it is possible to run 10*3*600 MCM single-core copper cables in ducts for 400 A/cable[even one cable per duct it is not possible].You may use10 groups of 3*600 MCM directly buried in 90 RHO 25oC ground at one foot distance. In my opinion too, you may use 90oC conductor temperature for the underground part of the run but the part above the ground-in air-has to be 75oC or less. If this part it is more than 10 ft. long you may neglect the axial thermic flow from underground part.

 

[charlie b]

It is most likely that the ampacity calculated for an underground duct bank by a software package such as ETAP (or my personal favorite, Amp Calc) is going to be lower than the values that appear in NEC Table 310.15(B)(16). Even if you set the temperature rise limit at 90C, I predict that ETAP will give you an ampacity that is lower than 420, the value that the table gives us for 600 MCM cable in the 75C column. If I prove to be right in this belief, I would say that it is acceptable to use the 90C value in the calculation. My reasoning is that the current flowing in the wire will be lower than the 75C limit, and therefore the terminations would not be at risk of overheating.

 

[faresos]

Thank you all for the responses...

I was leaning toward that i can use the 90 Deg C since the conductor insulation is rated for that temperature.

Thanks again great informations!