Wire sizing and ampacities

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Noooorm

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I'm working on a project for the US Government in Afghanistan. The project is supposed to be designed per the NEC. I sized the wiring from distribution panels to buildings per the feeder breaker sizes, taking into account the appropriate derating factors for temperature and number of current carrying conductors. Up to 100A, I derated from the 75 deg C column, and over 100A I derated from the 90 deg C column (Table 310.16). Then I found out the cable that was ordered was what they could find readily available overseas, is Turkish or Italian, and is rated for 70 deg C. Does this mean I can only derate from the 60 deg C column? Another piece to the puzzle is that the manufacturer's cutsheet for the cable shows that its ampacity ratings for 70 deg C are higher than the ampacities shown in the 90 deg C column in Table 310.16. Does this mean I can derate from the 90 deg C column, since the cables won't see the ampacities that will cause them to reach 70 deg C? Thanks.
 
You cannot derate from a rating higher than the cable [insulation] temperature rating. Derating per NEC does not allow derating from manufacturers ampacity (under most conditions). You have to use the appropriate NEC Table for derating. In fact, you are not even permitted to use cable and conductors that the NEC does not recognize. You also have to take into account termination temperature ratings, and if you are 600V or less, the maximum ampacity is per Table 310.15(B)(16). Details of the terminations and connected equipment determine what column to use.
 
After looking at your profile, I see you are an EE. NEC permits ampacity determinations under EE supervision. See 310.15(C).

You will have to decide how to proceed on termination temperature limitations. If 600V or less, see 110.14(C), or 110.40 for over 600V, then come back and ask any remaining questions...
 
Do you mean:
"Table 310.15(B)(16) Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60?C Through 90?C (140?F Through 194?F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30?C (86?F)"?
I don't see any derating factor but only "Allowable Ampacity".
If it is Allowable Ampacity for a Direct Buried cable we don't know the depth and RHO of the Earth -so could be 3 ft. and 250 [dgr.C*cm/W]. The ampacity on NEC is calculated as per Neher and McGrath theory and an Italian [or Turkish] manufactured cable could be calculated as per IEC 60287 but for another depth [0.5 usually], another temperature, another RHO.
My opinion: leave NEC and take IEC 60364-5-52 .See:
Sizing conductors and selecting protection devices - Legrand
www.legrand.com/files/fck/File/pdf/Power_guide/EX29008.pdf
 
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