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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
31/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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