NEC Six Duct Configuration Example, (Page 4 of 8)

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 temperatures.

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.