Rule 26 of 40 · Chapter IV — Conductors and Loads
Derate for heat and bundling
Why this rule exists
The ampacity tables assume specific conditions, a certain ambient temperature and a limited number of current-carrying conductors bundled together. Reality often differs. Pack many conductors in one conduit and they cannot shed heat, so each must be derated. Run cable through a hot attic or against a chimney and the high ambient eats into its capacity. Ignore derating and a wire rated for the load still overheats, because the table's assumptions did not hold. Ampacity is not a fixed property of the wire; it is a property of the wire in its actual conditions.
In practice
When more than three current-carrying conductors share a raceway or cable, apply the conductor-count adjustment factors and reduce allowable ampacity accordingly. Apply the ambient temperature correction for hot locations, attics, boiler rooms, rooftops in sun, which can run far above the 30-degree-C table baseline. Stack the factors when both apply. Then confirm the derated ampacity still covers your load, and upsize the conductor if it does not. Count only current-carrying conductors; a grounded neutral in a balanced circuit may or may not count.
When it doesn't apply
Neutrals that carry only unbalanced current usually do not count toward the conductor total, but a neutral on a circuit with significant harmonic load does. Rooftop conduit in sunlight gets a specific ambient adder. These details come from the code tables; when conductor fill or ambient is significant, work from the tables rather than estimating.