Change Summary

• An 83 percent multiplier is provided for calculating ampacity for feeders and service conductors supplying dwelling units, if the conditions of this section are met
• Table 310.15(B)(7) has been deleted and Section 310.15(B)(7) has been revised and restructured into a list format
• A new informational note references a new Annex (D7) where an example calculation is provided

Expert Analysis

This section has been restructured into a list format and an 83 percent multiplier has replaced Table 310.15(B)(7). It was determined during the Sixteenth NFPA Annual Meeting that 84 percent was used to establish the aluminum residential service conductor size. However, the 84 percent factor resulted in larger sizes for some conductors compared to the sizes in the 2011 NEC. Therefore an 83 percent multiplier was selected to maintain consistency with the sizes in the 2011 NEC Table 310.15(B)(7). Code Making Panel-6 also analyzed the existing Table 310.15(B)(7) values and determined that the conductor sizes provided were equivalent to those if a 0.83 multiplier was applied to each service ampere rating. The resulting conductor size ends up the same as existing values in Table 310.15(B)(7), if the same conductor types and installation conditions are applied. Informational Note No. 1 clarifies that adjustment and correction factors apply depending on conditions of use. Informational Note No. 2 references a useful Annex (D7) that provides an example of applying the 83 percent factor. This revision eliminates the need for defining the term “main power feeder” in 310.15(B)(7), which reduces confusion considerably. These revisions enhance usability and clarity of this section and provide for more consistent application for qualifying dwelling service and feeder conductors.