ampacity of cables 1

[rmullin]

I am sizing MV/HV cables for direct burial based on NEC Standards. It does not seem to mention in detail within article 310 of NFPA 70, the ampacity adjustment factors for changes in temperature, thermal soil resistivity and grouping from the 'base' values. What I have done thus far is take the ampacity value from article 310 and use the derating factors from IEEE Std 399-1997. For example where I have Three Triplexed Single Insulated Copper Conductors directly buried in earth, 5001-35000V, 90degC, I have taken ampacities from table 310.85 of NFPA 70. In this case the cable size is 750kcmil therefore the ampacity is 685A for one circuit. I require 3 circuits in one trench therefore current carrying capacity is 2055A. Now, according to IEEE Std 399-1997, table 13.11, the grouping adjustment factor is 0.71 when a total of 9 horizontal cables are in this formation. Therefore, I calculated the total current carrying capacity for this scenario to be 2055 x 0.71 = 1459A (assuming no derating for temperature and soil thermal resisitivity). If someone could confirm that my method is correct, or propose an alternative, that would be appreciated.

 

[wareagle]

Suggest you go to Okonite Cable Co site. They have this information. In their catalog the cable is rated for 90C
RHO 90. 1 ckt of 750CU 15kv = 683 with LF = 100 and 768 with
LF = 75. If you separate the cables 24" the rating is 613 and 712 amps.