Under which circumstance would you decrease the ampacity of a conductor?

Decreasing the ampacity of a conductor when it is grouped with multiple conductors is based on the principle of heat dissipation. When several conductors are bundled together, they are unable to dissipate heat as effectively as a single conductor would in open air. This can lead to higher operating temperatures, which can adversely affect the insulation and overall performance of the conductors.

Because of this increased thermal buildup, the National Electrical Code (NEC) provides guidelines to reduce the ampacity of conductors when they are installed closely together. This involves applying correction factors to account for decreased heat dissipation, ensuring that the conductors can operate safely without overheating. Thus, the proper adjustment in ampacity is necessary to maintain the safety and reliability of an electrical system.