Which of these practices is recommended for cabling runs without spacing?

Applying derating factors is a critical practice for cabling runs without spacing because it helps to manage and mitigate the heat generated by electrical cables when they are bundled or located in proximity to one another. When cables are run close together, their heat dissipation capabilities are reduced, which can lead to thermal overload and a potential risk of failure or fire. Derating factors allow for the adjustment of the current-carrying capacity of these cables to account for this increased heat, ensuring that they operate within safe limits.

This practice enhances the safety and reliability of the electrical installation, ensuring that even when cables are installed closely, they can function effectively without overheating. Other options, while they might have relevance in certain contexts, do not specifically address the direct implications of cabling without spacing in terms of thermal management, which is why applying derating factors is the most pertinent practice in this scenario.