When multiple wires are packed into a single conduit or raceway, they trap each other’s heat.
There are two primary scenarios where you must derate a wire: and Bundling (Conduit Fill). wire derating
However, you are allowed to use the 90°C rating for the derating calculation itself (to get a higher starting number before applying the penalties), provided the final result does not exceed the termination temperature rating. When multiple wires are packed into a single