Neher-McGrath calcs 3x2 duct bank non-identical circuits

[morgan3520]

Has anyone seen or can point me to an example where the Neher-McGrath equations are used on a 3x2 duct bank with 480V service non-identical circuits. What I mean 1 circuit will take up 3 ducts and be 500kcmil copper and the other circuit will take up the other 3 ducts and be 350kcmil. Each duct will carry 3 cables. I am checking to see if I have the problem and equations set up correctly.

Thanks for any help.

 

[ron]

We have used Ampcalc to model that type of arrangement

 

[Julius Right]

At first you have to calculate the resistance of each cable and to choose a current for each cable -let's say according to NEC if all 4 are the same cable. Then calculate the temperature according to IEC 60287-2-1 -for instance, see attachment. After then reduce the current in one cable -or in both- up to the temperature is as required.

 

[Julius Right]

If we keep the conditions as per NEC Informative Annex B Application Information for Ampacity Calculation the ampacity of 3 cables of 350 mcm copper conductor 90oC rated insulation and 3*500 mcm conductor 90oC insulation each one in a pvc duct of 4” will be close to NEC Table B.310.15(B)(2)(5) Ampacities of Single Insulated Conductors, Rated 0 Through 2000 Volts, in Nonmagnetic Underground except for 500 mcm only 529 A instead of 531 as per this table.

 

[Julius Right]

In my opinion, on Table B.310.15(B)(2)(5) the ampacity is calculated for a conductor temperature of 90oC and not for 75oC as noted. However, the ampacity has to be compared with the temperature limits as per art. 110.14. If the ampacity for 75oC at termination has to be limited as per B.310.15(B)(16) the maximum for 350 mcm has to be 310 and not 355 and for 500 mcm 380 A and not 431A.