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.