calculation of MV Cable cross section

General variable that we have to know for calculating cable cross section are : load power, voltage,cable lenght and other variable. From your information there is no information about cable lenght. Because this information is needed to determine voltage drop of cable.This value is kept not more than 5%. But if your cable isn't too long, this calculation can help your problem.

Assume : using multiple core cable (3 core), pf = 0.8 ; derating factor = 0.7

1) Find the current rating, I = P/(sqrt (3)*V*pf) = 900/(sqrt(3)*6.6*0.8) = 98.41 A

2) After getting the current rating, refer to NEC code, you can multiply the current rating by 1.25, so we get 1.25*98.41 = 123 A.

3) See in cable catalogue for MV, current ampacity for this current. First of all, we can use 35mm2 with ampacity 130A, after multiplied with derating factor we get 0.7*130 = 91 A (This value is less than 123 A, refer to code we don't use this), with trial and error, we can find 70mm2 with ampacity 190A. After multiplied 0.7*190A=133A (this value is more than 123A,so we can use this size)

So, for this case we can use N2XSEFGbY 3C x 70 mm2.

Thanks