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The lightning air terminal's height above the roof or surface it is protecting.
Measured vertically from the base of the terminal (at the protected surface) to its tip.
The half-angle of the protective cone, set by NFPA 780's protection class and terminal height tables.
Read from the table, not chosen. The counter-intuitive part is that the angle NARROWS as the terminal gets higher — a taller mast does not protect a proportionally wider footprint, so doubling the height buys much less than doubling the covered area. Above the heights the cone method covers, NFPA 780 stops offering an angle at all and the rolling-sphere method takes over.
Protected radius
33 ft
The classical cone method's protection angle depends on the lightning protection class and terminal height per NFPA 780 tables, and this simplified single-cone method does not replace NFPA 780's more comprehensive rolling sphere method, which is required for a complete and code-compliant lightning protection system design by a certified LPI installer/designer.
They open the calculator with your figures already in it
Lightning Rod (Air Terminal) Cone of Protection Calculator: 33 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Classical cone-of-protection method: protected radius = rod height × tan(protection angle), where the protection angle depends on the terminal's height and the desired protection class per NFPA 780 (this simplified cone method is a traditional approach; NFPA 780 also provides the more comprehensive rolling sphere method for modern lightning protection system design)
Inputs used
- Air Terminal Height Above Protected Surface
- 33 ft
- Protection Angle (°, Per NFPA 780 Class/Height Table)
- 45
Confidence note: The classical cone method's protection angle depends on the lightning protection class and terminal height per NFPA 780 tables, and this simplified single-cone method does not replace NFPA 780's more comprehensive rolling sphere method, which is required for a complete and code-compliant lightning protection system design by a certified LPI installer/designer.
What this calculation does not cover
- A protected zone is not a path to earth. The terminal only helps if the strike can get to ground down low-impedance conductors — at least two from each air terminal system, run down separate faces without tight bends, into grounding electrodes and bonded to the building's other earthing. A terminal sitting on a roof with one long lead full of sharp corners concentrates the strike rather than dispersing it.
- Being inside the cone does not protect metal — it puts metal in the way. Ductwork, handrails, pipework, roof plant and anything else conductive within the zone has to be bonded to the system or held beyond a computed separation distance, because the surge travelling down the conductor will jump sideways to any grounded metal closer than that distance. Side flash inside a roof void is what starts the fire.
- Most lightning damage never touches the roof at all. It arrives as a surge induced onto incoming power, telecoms and data lines by a strike some distance away, and a cone of protection does nothing whatsoever about it — that needs surge protective devices at the service entrance and again in front of sensitive equipment. Buildings with a textbook air terminal layout still lose every controller on site to it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Classical cone-of-protection method: protected radius = rod height × tan(protection angle), where the protection angle depends on the terminal's height and the desired protection class per NFPA 780 (this simplified cone method is a traditional approach; NFPA 780 also provides the more comprehensive rolling sphere method for modern lightning protection system design)
Which documents these citations point at
- Standard for the Installation of Lightning Protection Systems (NFPA 780) (United States)Lightning protection — air terminal placement, the rolling sphere and protection angle methods, bonding and grounding.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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