I found a similar closed post from 2010 "Classification of Analyzer Bldg - Group B or Not?". I have a 24' x 12' building with forced ventilation (1 scfm/ft^2). There is an analyzer that uses pure hydrogen. The building is classified CL1 Div. 2 Group C and D. Similar to the referenced post, I am questioning whether or not the Group B rating is required. Responses to the referenced posts state there is no volumetric or rate criteria for a hydrogen leak that will affect the group classification. However, in NFPA 70 Article 625 Electric Vehicle Charging System, it is mentioned that batteries off-gas hydrogen during charging, but there is no mention of requiring these locations to be classified group B. This seems to imply that there is an acceptable level of hydrogen concentration before an area has to be classified Group B. What is this magical number? Where can this requirement be found? The problem is that this analyzer is being added to an existing installation that is only Class 1 Div.2 Group C and D rated. A classification of Group B would require a costly upgrade to the existing equipment. Any help is appreciated.