NFPA 13 (2016 Edition) Section 126.96.36.199 requires sprinklers to be installed underneath fixed obstructions over 4ft (48") in width and provides no further information on how that 4ft width is determined to be an obstruction. For most situations this is rather straightforward, but the open ended 4ft wide requirement leaves quite a few obstructions up to interpretation.
With that being said, how is it determined that an obstruction such as a duct, can be considered unobstructed if it is 40" wide by 6ft long, but if it is 48" wide by 3ft long, it is considered an obstruction? How is the obstruction rule only applied to the width of a piece of equipment and in turn is not applied to the length?
Is there a ratio or rule of thumb that is widely used to determine whether an item hanging below the ceiling is considered to violate the 4ft rule?
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