How many down conductors are generally required for a structure?

The requirement for down conductors is based on the size and configuration of the structure, as well as safety standards aimed at ensuring effective lightning protection. For larger structures, having at least two down conductors is recommended, with an additional down conductor added for every 250 feet of horizontal run of the roof, as this helps to minimize the potential for lightning current to cause damage by providing multiple pathways for the current to enter the grounding system safely.

This approach distributes the electrical load and reduces the risk of failure in the lightning protection system. It also decreases the need for excessive length of any single down conductor, which could pose risks related to heat generated by the lightning strike or possible induction effects. For smaller structures, having one down conductor might suffice in certain situations, but the guidelines support the additional conductors for larger or more complex structures, thus ensuring comprehensive protection against lightning strikes.