Duct Bank Ampacity

[Coco_HueHueHue]

Hello,

I was wondering if when sizing cables going through duct bank conduits, you were using this logic...

For example, 2x 3C-500MCM goes through their own respective conduit within the same duct bank.
I use Canadian Electrical Code, so i would use Table 2 for the allowable ampacity of not more than 3 conductors. Since the spacing between the conduits are spaced by at least 1 conduit overall diameter (and obviously by a cable overall diameter as well), i don't need any derating applied to that table due to having more than 3 conductors...

Chapter 12 on wiring methods covers all installation methods for cables except duct bank... i am 99% sure of what i bring here, i am just trying to confirm...

What confuses me is Appendix D which requires a lower ampacity, where you have proper schematic for multiple conductors per phase, but i believe this is applicable only when more than 1 cable per phase is required? Not for 2x runs feeding 2 different loads...

Thank you!

 

[7anoter4]

I don't know what Canadian Electrical Code says but NEC[USA] gives ampacity for low voltage cable in Annex B informative only. In chapter 310.60 [requirements] are tables only for above 2 kV rated cables.
So, if the conductor is copper, the Earth thermal resistance [RHO] is 90 and Earth temperature is 20oC the closest detail is Detail 2 [3 ducts 3 conductors per duct]
No recommendation for 2 ducts 3 cables per duct, indeed.
However, this ampacity is for all 9 conductors equally loaded.
The distance between cables is not important, only the distance between ducts is taken into consideration.
If you have the cable cross section area details as conductor diameter, insulation thickness and sheath [if any] and overall diameter of the cable ampacity may be calculated. There are details of different type of cables in the chapter 9 in NEC.

 

[7anoter4]

In my opinion the average of ampacities for 1 duct and 3 ducts per duct bank it is very close to 2 ducts duct bank ampacity.