NEC Derating
The NEC’s grouping/bundling adjustment table has its own specific structure, distinct from ambient temperature correction.
Beyond base ampacity (Table 310.15(B)(16)) and ambient temperature correction (Table 310.15(B)(1)), the NEC has a third relevant table: Table 310.15(C)(1), covering adjustment factors for more than three current-carrying conductors bundled together in a raceway or cable.
Final usable ampacity = Base ampacity × Temperature correction factor × Bundling adjustment factor
Not every conductor in a bundle counts toward this adjustment — a properly balanced three-phase system's neutral, for instance, may not count as current-carrying under specific conditions defined elsewhere in the code. Getting this count wrong in either direction skews the adjustment factor applied.
The same physical principle covered in the general grouping derating article: more current-carrying conductors sharing a raceway means more mutual heating and less individual heat-dissipation capacity per conductor — the NEC's specific numeric table quantifies exactly how much adjustment applies for a given conductor count.
All three NEC ampacity tables are applied together automatically in the NEC Cable Sizing calculator.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks