Final answer:
The current-carrying conductors in the conduit need to be derated due to the generated heat. To calculate the derating, the Ampacity Correction Factor (ACF) based on the number of conductors and the ambient temperature is needed.
Step-by-step explanation:
The 20" length of conduit's current-carrying conductors must be derated because placing several conductors close to one another can cause overheating by creating resistance that generates heat. Finding the Ampacity Correction Factor (ACF) based on the number of conductors and ambient temperature is necessary to compute the derating. The derated ampacity can be obtained by multiplying the ACF value by the original ampacity rating of the conductors using the NEC (National Electrical Code) table.