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NEC Six Duct Configuration Example, (Page 5 of 8)

Performing a Multiple Type Ampacity Calculation

In summary, some observations from the previous cases include:

  • The symmetry of the duct and the manner in which both cables of each circuit are placed in "thermally equivalent" positions would cause one to anticipate that the cables in each row will have the same temperatures. The results confirm this.

  • Based on the temperatures of the six cables, it appears that the relative current carrying capability at each position pair will be in order from highest to lowest: Top row, Bottom row, Middle row.

  • There should be a level of reduced C2 current loading and increased C1 and C3 loading where the temperatures of all three circuits will be equal to rated. Neither of the two cases looked at thus far meet this condition.

A calculation can easily be performed in response to the latter observation. Under Type of Calculation, select "Multiple" and under Parameter to Calculate select "Ampacities". Click on "Go" to obtain the following results:

With this calculation, all conductors are operated at their rated 75 degree temperature. As expected, since the middle row runs the hottest, these cables have the lowest ampacity rating of 258.9, the bottom row is next with a rating of 276.8, and the top row has the highest rating of 295.3 amps. If this ductbank system consisted of three circuits of two 3-1/c triplexed cables each, i.e. each row of cables a separate circuit, the ampacities of circuits C1, C2, and C3 would be 590, 518, and 554 amps per phase respectively. If each circuit is operated within these ampacities, none of the cables will operate beyond rated temperature. With all else being equal, the heating of a cable is a function of its underground depth, and the interference effects of heat from other cables in the same installation. In this example, the middle row has the lowest ampacity rating because of the combination of being lower in the earth than the top row, together with the "thermal blanketing" effect of the first and last rows of cables.


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