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[1]  Introduction

[2]  Calculation of Higher Ampacities for 0-2000 Volt Cables

[3]  Does the NEC Code Allow Alternate Ampacity Calculations?

[4]  Determining the Effects of Application Factors Not Accounted For in the Code

[5]  Organization and Reporting of Circuit Ampacities

[6]  Calculated Ampacities and Local Governing Authorities


Aerial Ampacity Calculations and the NEC Code

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The deficiencies of the Code in trying to provide ampacities for what is essentially an infinite number of possible underground cable system arrangements are apparent. However, one would think that the limited number of possible aerial cable applications are well covered by the Code Tables. While in some ways this is true, there are still some significant issues with the Code treatment of ampacity ratings for aerial systems, in particular for 0-2000 Volt systems.

  1. It is possible using the industry accepted Neher-McGrath calculation procedures to derive significantly higher ampacities for 0-2000 Volt cables than those listed in Code Tables 310.16 and 310.17. These tables are based on research performed in the 1930s and are very conservative. In fact, almost all the other ampacity tables in the Code are based on the Neher-McGrath calculation procedures.

  2. The Code includes a number of derate/uprate multipliers on the table ampacity values in order to provide for variations in ambient air temperature, quantity of conductors in a raceway or cable, and various cable tray configurations. However, even if the NEC table ampacity values are utilized, a program is very useful in organizing the application, applying the proper multiplier and providing a definitive organized report in electronic or printed format.

  3. There are necessary aerial cable modifications to the NEC table values for which no derate/uprate multipliers exist in the Code.

The remainder of this paper examines the above points in more detail.


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