One question that can be raised about the above calculations
is, why is the calculated temperature of the middle row cables not exactly
75 degrees C, as in the first "Single" calculation case? The "Multiple"
calculation case with the answer of 74.71 is actual more precise. The
calculation procedures for the "Single" option utilize the same cable
parameter values at all positions based on the "Specified Temp". Some
of these parameters, most notably the conductor DC resistance and therefore
the AC resistance, vary with temperature.
If a cable's temperature is being calculated, given a fixed
current, an iterative solution must be used if the Rac at the actual
temperature is to be used in the calculation. This is all the more true when
a cable is in an underground system with other cables is operated with the
same current, but at different temperatures. Consequently, in the above
Multiple Type calculation, which is iterative, the actual Rac based on
conductor temperature was calculated. This resulted in slightly lower losses
for the top and bottom rows of cables and slightly lower operating
Go the the Main form menu item "Options | General Options"
and click on the "Ampacity Calcs" tab. In the section on "Method for
Iterative Solution" check the option button for "Calculate Specified Number
of Iterations" and set the "Number of Iterations" to 1. Now go back to the
Calculation Options form. Under Type of Calculation, select "Multiple" and
under Parameter to Calculate select "Temperatures". Click on "Go" to obtain
the following results:
Since there was only one calculation iteration, cable data
for all positions was based on a temperature value of 75 degrees C. The Rac
for each position could not be modified for the actual conductor temperature
with only one iteration which corresponds to the procedure in the "Single"
type calculation. Consequently, the hottest middle row cables are at exactly
75.00 degrees C.
In any event, the difference between the Single and Multiple
iterative type calculation is so small as to not be significant. Boosting
the Specified Amps by only 0.6 amps to 272.2 will result in the middle row
cables operating at 75.00 degrees C.
Be sure to return to the General Options tab and reset the
Calculation Parameters to "Iterative Solution based on Tolerance", with the
"Set Tolerance in Number of Significant figures to" 3, or just click on the
Reset to Defaults button.