What percentage of ampacity must be applied to conductors when housed together in a raceway?

When conductors are housed together in a raceway, the National Electrical Code (NEC) stipulates that the ampacity of these conductors must be derated to account for the heat generated by multiple conductors running in close proximity. This is necessary to ensure safe operation and prevent overheating, which can lead to insulation damage and fire hazards.

The requirement for reducing ampacity is typically tied to the number of conductors present in the raceway. For a situation with more than three conductors in a raceway, the NEC states that the ampacity must be derated to a certain percentage of the conductors' maximum rated ampacity. Specifically, for four to six conductors, the reduction factor is 70 percent. Therefore, if the total number of conductors is four to six, you must calculate only 70 percent of the full ampacity to determine the safe carrying capacity of the conductors in the raceway.

Applying this understanding to the question clarifies why the correct answer indicates that 70 percent of ampacity should be applied when conductors are housed together in a raceway.