Light-Hazard Tenant Fit-Out With a Marginal Static Supply
A 4,200 sq ft mercantile tenant space in a single-story building is being fitted out. The existing 6-inch ductile underground main has a static pressure of 58 psi and a residual pressure of 41 psi at a measured flow of 620 gpm (the test was taken at a hydrant near the riser). The new layout uses 1/2-inch pendent sprinklers, K=5.6, on a wet system. The contractor proposes a design density of 0.10 gpm/sq ft over 1,500 sq ft, with a 250 gpm hose allowance. The building official has flagged the plan because the tenant is a furniture showroom with foam-filled upholstery stacked to 6 ft. The contractor argues the existing main is "plenty" because static pressure is under 60 psi and no pump was ever needed before.
1. Evaluate whether the proposed density and remote area are defensible for this occupancy, and identify the controlling code logic.
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The key issue is occupancy classification, not pressure. Stacked foam-filled upholstery in a showroom is a classic Ordinary Hazard Group 2 exposure (combustible goods of moderate quantity and height), not Light Hazard. Under NFPA 13, Ordinary Hazard Group 2 requires roughly 0.20 gpm/sq ft over 1,500 sq ft - double the proposed density and a larger remote area than the contractor's 1,500 sq ft claim for light hazard. The contractor's argument that "static pressure was always fine" conflates supply adequacy with hazard classification. The correct path is to reclassify, recompute the demand at OH-2, and then test the supply. If the recomputed demand exceeds what the main can deliver, the design must change (larger pipe, added pump, or a redesign) - the official is right to flag it.
2. Using the test data, explain how you would determine whether the existing supply can meet the OH-2 demand, and what the outcome likely means.
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Plot the two test points on semi-log (flow vs. pressure) paper, or use P = P_static − k·Q^1.85. With 58 psi static and 41 psi at 620 gpm, the drop is 17 psi at 620 gpm, giving a friction coefficient that rises steeply at higher flows. Extrapolating to an OH-2 demand (typically 700–900 gpm plus 250 gpm hose), the residual will fall well below the 20 psi minimum and likely below the pressure needed to drive the sprinklers. The margin is thin at best. Conclusion: the supply cannot support OH-2 without a pump or a main upgrade, and the plan should not be approved on the current basis. Document the extrapolation and require a hydraulic calculation with the safety margin (typically 10% flow or 5 psi) applied.