Before you can ?derate,? you must first ?rate.? That is, you must find the ampacity of the conductors in the conditions in which they are installed. That begins by selecting a table from which to read the ampacity. There is a table for single conductors in free air, and another table for not more than three current-carrying conductors in a raceway, and several tables for conductors in ductbanks. If you have additional considerations, such as more than three current-carrying conductors or perhaps a high ambient temperature, then you derate from the value given in the applicable table.
You have a situation that is open to some debate. Many will say to use Table 310.16, no matter how many ducts there are in your ductbank. I can only agree with that if the selection of conductor size was based on an Article 220 load calculation.
The only tables that apply to conductors rated under 2000 volts and installed in ductbanks are in Annex B. I have used Table B.310.7 for some applications. One problem with that table is that you have to select a value of RHO that is appropriate to the geographic area of the facility. Another problem is that if you wish to use RHO = 60, you have to find a way to justify a Load Factor (LF) of 50%.
