How to Use the Lightning Rod Protection Radius Calculator
How much a single air terminal actually protects depends not only on how tall it is but on how small a strike you intend to intercept. NFPA 780 defines the zone of protection with the rolling sphere method, using a 150 ft striking distance for ordinary structures and a 100 ft sphere where explosive or munitions occupancies demand tighter coverage.
The geometry is exact. At grade the protected radius is r = √(2Rh − h²), and for equipment that already sits above grade, such as a rooftop condensing unit, the radius shrinks to r = √(2Rh − h²) − √(2Rh₂ − h₂²). The numbers make it obvious how fast coverage disappears as the protected item gets taller.
The equivalent protection angle shown in the results is a reference value converted from the calculated radius. NFPA 780 tabulates the protection angle method separately and limits it by structure height, so verify the angle against the standard before relying on it. A terminal taller than the rolling sphere radius also needs a side flash review, and the full system of air terminals, down conductors, grounding and bonding should be confirmed by a qualified lightning protection designer.
Frequently Asked Questions
NFPA 780 uses a 150 ft striking distance for ordinary structures. A 100 ft sphere applies where the standard requires tighter coverage, such as structures containing explosive materials, because a smaller sphere also intercepts lower-current strikes.
The angle method is convenient for short structures but is limited by height, while the rolling sphere works at any height. This calculator uses the rolling sphere geometry and reports the angle only as a cross-check.
No. The zone only describes where a direct strike is intercepted. Surges traveling through down conductors, grounding and incoming utilities still reach equipment, so surge protective devices and equipotential bonding are required as well.