Calculate the rating for multiple circuits in the same duct

Modified on Wed, 21 Jul 2021 at 10:56 AM

This problem is not covered directly by the IEC Standards (i.e. they only cover the rating for 1 circuit in a duct).  To solve this problem using the IEC equations is a multi-step process using Cable HV Software.


This method has the following assumptions:

  • Cables are the same type.
  • Circuits are equally loaded.



Example calculation - 3 circuits installed in the same buried duct

In this example we will be calculating the current rating for multiple circuits (3 circuits) installed inside the same buried duct.


Step 1: Calculate the current rating for a single circuit (3 cables) buried in the same duct.


Make sure the duct size being modelled is the same duct size which will house the multiple circuits.


Rating 1 in this example = 517.38 A

This is the current rating for a single circuit installed in the same duct.

Next we will derive a custom derating factor for multiple circuits.



Step 2: Calculate the current rating for a single circuit installed in air.


Use the installation method called "Three cables in trefoil, clipped direct" (as shown) to include that while installed inside a duct the cables will be touching the inside walls of the duct.


Rating 2 in this example = 605.96 A

This is the current rating for a single circuit installed in air.



Step 3: Calculate the current rating for multiple circuit installed in air In Groups.


Use the installation method called "Side by side_3 trefoils" (as shown) and make sure the Clearance, e = 0 m to include the thermal proximity effect (mutual heating between the circuits in air).


Rating 3 in this example = 578.8 A

This is the current rating for 3 circuits installed (touching) in air in a group as they would be inside the duct together.




Final step: Calculate the derating factor for the groups in air and apply it to the buried case rating.


Use the installation method called "Side by side_3 trefoils" (as shown) and make sure the Clearance, e = 0 m to include the thermal proximity effect (mutual heating between the circuits in air).


This is the current rating for 3 circuits installed (touching) in air in a group as they would be inside the duct together.


The final rating for multiple circuits installed in one duct = (Rating 3/ Rating 2) x Rating 1  


Where Rating 3 / Rating 2 (578.8 A/605.96 A=0.9552) is the derating factor to be applied to Rating 1.


Final current rating of a circuit installed inside the same duct (3 circuits in total) = 494.94 A


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