Calculation of Higher Ampacities for 0-2000
 Does the NEC Code Allow Alternate Ampacity
 Determining the Effects of Application Factors
Not Accounted For in the Code
 Organization and Reporting of Circuit
 Calculated Ampacities and Local Governing
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.
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
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.
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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