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Ref- NFPA 780 : the spacing between down conductors should be 30 mtr for roof perimeter exceeding 76 mtr, subject to minimum of two
As per NFPA, there will be one down conductor after every 30 m distance. So for calculating the total no of down conductor, measure the total perimeter of the building and divide it by 30.
But for any building minimum two no of down conductors are required as per NFPA 780.
Check which kind of building in the following link:http://www.para-rayos.com/protec/docs/pdf/17_Sistemas_de_proteccion_en_jaula_Faraday.pdf
The number of down conductors depends on the perimeter of the external edges of the roof
