NFPA 13 explicitly states you can do that. If your compartment meets the definition of a small room, then you take the room area divided by the number of sprinklers in the room to get the area per sprinkler to apply the 0.10 gpm / sq ft density. You then have to compare that to the manufacturer data sheet for the spacing you are using to determine the calculated flow for the sprinkler.
Ex... Room size is 400 sq ft. You have 3 sprinklers that are spaced 9' off one wall and no greater than 18' between sprinklers. The are per sprinkler is 133 sq ft. The minimum flow at 0.1 is 13.3 gpm. However, if using a typical K4.9 residential pendent, the minimum flow at 18' spacing is 17 gpm. Therefore, you calculate at 17 gpm per sprinkler.
It has been this way since the 2007 edition of NFPA 13. The 2002 didn't give that option. It was corrected/revised by the tech committee for the 2007 edition and has been that way since.
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