Aerial Ampacity Calculations and the NEC Code

The NEC Code provides multipliers for application to the table ampacities in order to address changes in the following application parameters:

• Ambient Temperature - All Code tables are based on either a 30 or a 40 Degree C ambient air temperature. Correction factor tables are supplied with many tables but provide a fixed multiplier for all temperatures within a 5 Degree C range, so that the ampacity for a 26 Degree C ambient is the same as for a 30 Degree C ambient. There is a formula in the Code for a more precise calculation included in Section 310.60 for Conductors Rated 2001 to 35,000 Volts.

• More than three conductors in a conduit - As indicated above, Table 310.15(B)(2)(a) is provided for ampacity derating, in the event more than three current carrying conductors in a conduit are utilized.

• Cable Tray Applications - Code Sections 392.11, 12 and 13 specify various base table references and derate factors for cable installed in tray.

Even if it is decided that the Code tables will be utilized as the basis for cable ampacities, a computer program is very useful in organizing the application, applying the proper multiplier and providing a definitive organized report in electronic or printed format.

[Last]   Page 5 of 6   [Next]