Cables installed in Ductbanks

CEC suggests specific arrangements for underground cable installations in section B4. I am working on a design which specifies 6C/phase cables in 6 conduits.
Our preferred arrangement to accommodate installation flexibility is to have all phase-A, B and C cables installed in one duct bank; however this does not comply with section B4- requirements which mandate A-B-C cables to be installed in each conduit. Conduit run is less than 100'.
I am planning to run cable ampacity study and prove that certain percentage of imbalance will not cause any overheat and then apply for exemption. I appreciate any other feedback on this issue.

 

[dpc]

From an engineering standpoint, you can separate conductors by phase ONLY if there are no metallic components to the conduit system. It is common to use steel elbows even when PVC is used for the main duct bank runs. Any metal encircling the phase conductors can overheat due to induced currents. It isn't a question of imbalance. This will occur even if all phases are perfectly balanced. (Assuming I understand your question).

 

[pwrtran]

Use CYMCAP can do the analysis for you

 

[fx007]

Just look at the diagram B4-3, page 372, I think you are misinterpreting the requirement. You are probably looking at Diagram B4-4, and applying your interpretation...
Good luck.

 

[7anoter4]

I don't know what CEC requires but NEC Table 310.60(C)(77) shows the Ampacities of Three Single [core] Insulated Copper Conductors in Underground Electrical Ducts (Three Conductors in An Electrical Duct).For one single-core cable in each duct you may refer to Annex B[for information]. Informative Annex B Application Information for Ampacity Calculation.