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NEC Six Duct Configuration Example, (Page 6 of 8)

Ampacity Calculations with Six 1/c Cables per Duct

Data file ‘ex1c.ugs’ illustrates the use of two sets of triplexed 1/c cables per duct position, or 6-1/c cables per duct. The results, based on a Single Ampacity calculation, are shown below.

Note the derate of the conductor current carrying capability from 272 to 196 amps. Putting in more copper into the same underground duct configuration will yield a higher overall system capacity, but not in proportion to the additional cross section of copper due to mutual heating effects. If the entire ductbank in this example is utilized for a single three phase circuit, with 3 cables per duct, the circuit consists of a total of 6 conductors per phase with a total ampacity of 6 x 272 = 1,632 amps per phase. With two sets of triplexed 1/c cables at each position, the amount of copper in the installation has increased 100%, but the total ampacity has only increased to 12 x 196 = 2,352 amps per phase, a 44% increase.


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