Conductors must be derated when installed in conduit _____.

• A. In below-grade installations
• B. On or above rooftops
• C. In dry conditions
• D. When bundled together

Answer: (B)

The correct choice involves the derating of conductors installed in conduit under specific conditions concerning temperature and heat dissipation. Conductors must be derated when they are bundled together because bundling can restrict airflow around the conductors, causing heat to build up. This accumulation of heat can lead to higher temperatures than the conductors are rated for, which may result in insulation damage or reduced performance.

In the context of other options, while installations in below-grade locations and on rooftops can influence temperature, they do not inherently require derating based solely on the nature of the installation unless other factors like excessive heat are present. Dry conditions typically indicate a safer environment for conductors and do not necessitate derating. Thus, the specific action of derating when conductors are bundled is critical for maintaining electrical safety and performance standards.