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The required derating in Table 310-15(b)(2)(a) is for heat. Every current carrying conductor produces heat and the more you put in the same raceway the more heat you have. When you limit the current that is permitted to flow in each conductor you reduce the heat produced. In many cases you also have to apply the correction factors for high ambient temperature at the bottom of Table 310-16 in addition to the derating required by 310-15(b)(2)(a).
Don
