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NEC Six Duct Configuration Example, (Page 3 of 8)

Performing a Multiple Type Temperature Calculation

In the case just calculated, a single ampacity value was calculated for the six paralleled cable grouping making up a single circuit. Since the circuit ampacity was based on running the hottest middle cables at the rated operating temperature of 75 degrees C, it stands to reason that the top and bottom rows of cables will run at some lower temperature. This brings to mind some questions:

1.  What is the actual temperature of each conductor when running at 271.6 amps?

2.  What would be the ampacity of each conductor if each were run at rated temperature?

The standards provide no way to make these evaluations. However, for AmpCalc it is an easy task!

The data file for the second case of this example is ‘ex1b.ugs’. Scan through the position data and notice the only difference from the data in file ‘ex1a.ugs’ is that the "Specified Amps" for each position has been made equal to 271.6.

Proceed 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:

These results answer the first question cited above, "What is the actual temperature of each cable when running at 271.6 amps?" AmpCalc has held the current in each conductor constant at the 271.6 amps entered in the "Specified Amps" field of the Cable Position Data Entry form. Under the "Temp" column, the cables at the positions in the top and bottom rows are running at a temperature less than rated, 70.02 and 72.93 degrees C respectively. The cables in the middle row are running slightly below the rated temperature at 74.71 degrees C. Clearly the middle row is running the hottest, and when all cables in the installation must carry the same current, the temperature of the cables in the middle positions limit the maximum ampacity of the installation.

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