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Calcimator

Lightning Protection Calculator

Air terminal and conductor layout per NFPA 780 from building dimensions and roof type.

Inputs

Results

Total air terminals

19

Perimeter terminals13
Interior/ridge terminals6
Down conductors4
Ground rods4
Total conductor (ft)1,120
Roof area (sq ft)6,000
Perimeter (ft)320
Conductor Spec2 AWG copper
Ground Rod Spec5/8" × 10 ft copper-clad steel
How to Use This Calculator
  1. Enter the roof length and width in feet from the building's floor plan.
  2. Enter the building height in feet from grade to the roof peak or parapet.
  3. Select the roof type: flat (1), gable/ridge (2), or hip (3) to determine air terminal placement.
  4. Select the protection class: standard Class I (1) for ordinary structures or enhanced Class II (2) for high-value buildings.
  5. 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
50185
1,751285
3,250523
4,500723

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 (min 2, +1 per 100 ft perimeter).
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

  1. Identify Input Parameters
    4 parameters
    Roof length (ft) = 100, Roof width (ft) = 60, Building height (ft) = 30, Roof type (1=Flat, 2=Gable, 3=Hip) = 1 = 5 input(s) provided
  2. Calculate Total air terminals
    Total air terminals
    19 = 19
  3. Calculate Perimeter terminals
    Perimeter terminals
    13 = 13
  4. Calculate Interior/ridge terminals
    Interior/ridge terminals = interiorTerminals + ridgeTerminals
    6 = 6

Engine last updated . Checked against 3 independently-derived tests — how we verify calculators.

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