Lightning Protection Calculator
Air terminal and conductor layout per NFPA 780 from building dimensions and roof type.
About this calculator
This calculator lays out air terminals, down conductors, and ground rods per NFPA 780, the Standard for the Installation of Lightning Protection Systems, and its spacing rules from roof dimensions and roof type. Air terminal spacing is a step, not a smooth curve: a flat roof (Roof type 1) allows terminals up to 25 ft apart, while a gable or hip roof (Roof type 2 or 3) tightens that to 20 ft along the ridge. Roof length (ft) moves Total air terminals further than any other input, since it feeds directly into Perimeter (ft) (used for edge terminals on every roof type) and, on a flat roof, into the interior grid terminal count as well -- but the two contributions scale differently: doubling roof length alone (holding width fixed) scales edge terminals by only about 1.6x, since perimeter grows by less than 2x when only one of its two dimensions doubles, while it scales the flat roof's interior grid terminal count by roughly 2.3x, since the interior grid is a 2-D count that grows faster than the 1-D perimeter.
Building height (ft) has no effect on Total air terminals, Perimeter terminals, or Interior/ridge terminals at all; it only affects Total conductor (ft) through the down-conductor run length (down conductors x building height), since taller buildings need longer vertical conductor runs to reach grade even though they need the same number of rooftop terminals. Class (1=Standard, 2=Enhanced) changes only which conductor gauge is specified (Conductor Spec) -- 2 AWG copper for Class I, upgraded to 3/0 AWG copper for Class II -- and has no effect on the terminal count, down conductor count, or ground rod count in this calculator, even though real NFPA 780 Class II designs can also call for tighter terminal spacing than this simplified layout applies. Down conductors is the LARGER of a 2-conductor minimum or one conductor per 100 ft of perimeter, rounded up -- not the two added together, so a compact building (this calculator's 320 ft default perimeter) still lands on 4 conductors (ceil(320/100) = 4), not 6, and Ground rods match the down conductor count one-for-one.
Inputs
Results
Total air terminals
19
Figures current as of 2017. Source: National Fire Protection Association, NFPA 780-2017, Standard for the Installation of Lightning Protection Systems
How to Use This Calculator
- Enter the roof length and width in feet from the building's floor plan.
- Enter the building height in feet from grade to the roof peak or parapet.
- Select the roof type: flat (1), gable/ridge (2), or hip (3) to determine air terminal placement.
- Select the protection class: standard Class I (1) for ordinary structures or enhanced Class II (2) for high-value buildings.
- Read the total air terminal count, down conductor count, ground rod count, and total conductor footage needed.
How the result changes with Roof length (ft)
| Roof length (ft) | Total air terminals |
|---|---|
| 50 | 11 |
| 75 | 15 |
| 150 | 27 |
| 250 | 43 |
What each input means
- Roof length (ft)
- Building length along the longest roof dimension.
- Roof width (ft)
- Building width along the shorter roof dimension.
- Building height (ft)
- Height from grade to roof peak or parapet.
- Roof type (1=Flat, 2=Gable, 3=Hip)
- 1 = Flat/low-slope, 2 = Gable/ridge, 3 = Hip roof.
- Class (1=Standard, 2=Enhanced)
- Class I for ordinary structures, Class II for high-value or high-risk.
What each result means
- Total air terminals
- Total number of lightning rods (air terminals) required.
- Perimeter terminals
- Air terminals along the roof perimeter.
- Interior/ridge terminals
- Air terminals on roof interior grid or along ridges.
- Down conductors
- Vertical conductors from roof to ground -- the larger of a 2-conductor minimum or one conductor per 100 ft of perimeter (rounded up), not the two added together.
- Ground rods
- Ground electrodes required (one per down conductor).
- Total conductor (ft)
- Total linear feet of main conductor needed.
- Roof area (sq ft)
- Calculated roof plan area.
- Perimeter (ft)
- Building perimeter in feet.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersRoof length (ft) = 100, Roof width (ft) = 60, Building height (ft) = 30, Roof type (1=Flat, 2=Gable, 3=Hip) = 1 = 5 input(s) provided
- Calculate Total air terminalsTotal air terminals19 = 19
- Calculate Perimeter terminalsPerimeter terminals13 = 13
- Calculate Interior/ridge terminalsInterior/ridge terminals = interiorTerminals + ridgeTerminals6 = 6
Figures and sources
- Air terminal spacing and down-conductor/ground-rod count rules (2017) — National Fire Protection Association, NFPA 780-2017, Standard for the Installation of Lightning Protection Systems
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Does building height affect how many air terminals I need?
No -- Total air terminals, Perimeter terminals, and Interior/ridge terminals all depend on roof length, roof width, and roof type, never on Building height (ft). A taller building needs the same rooftop terminal layout as a shorter one with the same roof footprint; height only lengthens the down conductor runs that carry the strike current to ground, which shows up in Total conductor (ft).
Why does terminal spacing change with roof type?
NFPA 780 allows wider spacing on a flat roof (up to 25 ft apart) than along a ridge line, so this calculator uses 25 ft for Roof type 1 (flat) and tightens to 20 ft for Roof type 2 (gable) or 3 (hip). But tighter ridge spacing doesn't mean a ridged roof always needs MORE terminals overall -- it only tightens the ridge-line and edge terminal spacing, while a flat roof's interior grid (which a ridged roof doesn't have) grows with roof area, not just perimeter. At small roofs the tighter ridge spacing dominates and a ridged roof needs slightly more terminals than a flat roof of the same footprint (21 vs 19 at this calculator's 100x60 ft defaults), but as the roof grows, the flat roof's interior grid quickly overtakes it: at 500x300 ft, a flat roof needs 273 terminals against only 105 for a gable roof of the same footprint.
What's the difference between Class I and Class II protection?
In this calculator, Class (1=Standard, 2=Enhanced) only changes Conductor Spec -- Class I uses 2 AWG copper, Class II upgrades to heavier 3/0 AWG copper for higher-value or higher-risk structures. It does not change the number of air terminals, down conductors, or ground rods calculated here, though a full NFPA 780 Class II design in practice can also call for tighter terminal spacing than this simplified layout uses.
How is the number of down conductors and ground rods determined?
Down conductors takes the LARGER of a 2-conductor minimum or one conductor per 100 ft of building perimeter (rounded up), per the minimum conductor path requirements in NFPA 780, Standard for the Installation of Lightning Protection Systems -- the two are not added together, so at this calculator's defaults (320 ft perimeter) the answer is ceil(320/100) = 4 conductors, not 2 + 4 = 6. Ground rods is set equal to the down conductor count, one grounding electrode per down conductor, so a larger or more elongated building perimeter increases both figures together.
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