Lightning Protection Calculator – Rolling Sphere & Risk (NFPA 780)

Lightning Protection Calculator – Rolling Sphere & Risk Assessment (NFPA 780)

Free lightning protection calculator: rolling-sphere protection radius, down-conductor count and NFPA 780 risk assessment. No sign-up.

📐 Standard: IS/NFPA 780

✅ Free to use

📄 PDF export

📱 Mobile friendly

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Lightning Protection Calculator

Reference: IS/NFPA 780

🌩️ ELECTRICAL

Free lightning protection calculator: rolling-sphere protection radius, down-conductor count and NFPA 780 risk assessment. No sign-up.

Inputs

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Calculation confidence: Planning-levelSuitable for early design, estimating and feasibility checks. Verify the final design against the governing code and a licensed engineer before construction or procurement.

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ℹ About This Calculator

A lightning protection system (LPS) intercepts strikes with air terminals and channels the current safely to earth through down conductors. This calculator gives the rolling-sphere protection radius for an air terminal and the number of down conductors required, based on the protection level you select under IS/NFPA 780.

The rolling-sphere method imagines a sphere - 20 m radius for the highest protection level (LPL I) up to 60 m for LPL IV - rolled over the structure; anything it cannot touch is protected. The protection level also fixes the maximum spacing between down conductors, from 10 m (LPL I-II) to 20 m (LPL IV). A full design also needs a risk assessment per NFPA 780-2.

Rolling Sphere Formula (IS/NFPA 780)

IS/NFPA 780

rp = √(2 R h − h²) (for h ≤ R)
N = ceil(perimeter / spacing), min 2 rp = protection radius at the reference plane, m
R = rolling-sphere radius (20/30/45/60 m by LPL)
h = air-terminal height above the plane, m
N = number of down conductors
spacing = 10/10/15/20 m by protection level

Worked Example

Example: For protection level LPL II the rolling sphere radius is 30 m and the down-conductor spacing is 10 m. A 5 m air terminal gives a protection radius rp = √(2 × 30 × 5 − 5²) = √275 ≈ 16.6 m at roof level. For an 80 m building perimeter the number of down conductors is ⌈80 / 10⌉ = 8.

Frequently Asked Questions

What is the rolling sphere method in lightning protection? +

It is a geometric design method from NFPA 780. A sphere of a defined radius (set by the protection level) is rolled over the structure; wherever it touches is exposed and needs an air terminal, and any volume it cannot reach is considered protected.

How many down conductors does a building need? +

Divide the building perimeter by the maximum down-conductor spacing for your protection level - 10 m for LPL I-II, 15 m for LPL III, 20 m for LPL IV - and round up, with a minimum of two conductors placed as symmetrically as possible.

What are the lightning protection levels (LPL)? +

NFPA 780 defines four levels, LPL I (highest protection) to LPL IV. Each fixes the rolling-sphere radius (20, 30, 45, 60 m) and the down-conductor spacing. The required level comes from a risk assessment based on the structure, its contents and strike frequency.

How high should a lightning air terminal be? +

Taller air terminals protect a larger radius but must remain within the rolling-sphere geometry. The protection radius grows with height up to the sphere radius; beyond that, additional terminals are needed rather than a single taller mast.

Does this replace a full NFPA 780 risk assessment? +

No. This tool sizes the air-terminal reach and down-conductor count for a chosen protection level, which is useful for preliminary design. Selecting the correct protection level itself requires the NFPA 780-2 risk assessment for the specific structure.