De-rating is when the capacity of the conductors are reduced due to effects like heat and other factors which reduce the current carry capacity of a conductor based on a certain condition.
For example, A conductor is rated to have an ampacity at a certain temp. rating but if it is installed in a condition that is above or below the listed temp. rating of 310.16 in the NEC for example a correction factor is needed and will be applied…this “derates” the current carry value of the conductor.
Basically derating is an adjustment factor placed on the conductor based on certain conditions.
Another condition is when you have more than 3 current carry conductors in a raceway, you have to derate ( reduce ) the conductors current carry capacity based on the sec 310.15(B)(2) because remember the more wires…less air space…more heat…this creates resistance(heat) for the conductor…so you thus would have a lower ampacity rating on the conductors within the raceway…thus you derate the wires (lower) their capacity or amps they can handle.
Wow…did I make that confusing or what…way better to explain in a classroom…lol