How many down conductors are required for structures with a roof perimeter exceeding 250 feet?

For structures with a roof perimeter exceeding 250 feet, the requirement is to install additional down conductors to ensure effective lightning protection. The principle behind this requirement is to minimize the electrical resistance and provide multiple paths for the lightning current to safely dissipate into the ground.

The recommendation to have at least one additional down conductor for every 100 feet of perimeter beyond the initial 250 feet is based on the concept of redundancy and coverage. This helps to manage the large potential voltage differences that can occur across long distances if there are insufficient conductive paths for the lightning strike's energy. By adding conductors incrementally for each segment of 100 feet beyond the first 250, the system becomes more robust and effective in channeling lightning safely away from the building structure.

This guideline is in line with industry standards for lightning protection systems, which aim to ensure the structural integrity and safety of buildings exposed to potential lightning strikes.