In a nine-duct conduit run that is completely empty and four power cables, should the cables be placed in the middle ducts to reduce heat radiation?

• A. Yes
• B. No
• C. It depends
• D. Not sure

Answer: (B)

Cooling in a duct bank mainly happens through convection with the surrounding air, so where you place cables affects how easily heat is carried away. If four cables sit in the middle ducts, they’re surrounded on both sides by duct walls and other spaces, which restricts air flow and creates a hotter, more stagnant zone around them. This increases the temperature rise because the heat has to travel through more thermal resistance to reach ambient air, and nearby cables can mutually heat each other. Conversely, the outer ducts have better access to cooler room air and more surface area for heat to escape, so placing cables toward the ends promotes better cooling. So, placing them in the middle does not reduce heat radiation; it tends to make cooling harder.