Heavy-Duty Stack : A smoke or vent stack with a flue that has a cross-sectional area of the flue greater than 0. 3 m2 (500 in. 2) and a height greater than 23 m (75 ft).
NFPA 780 Standard for the Installation of Lightning Protection Systems, 2008 ed. http://www.uscg.mil/Petaluma/TPF/ET_SMS/Manuals/NF PA_780.pdf, (Std: Instll of Lightning Prot Sys - Amer) {347 NFC}