When performing a fire hydrant flow test, a minimum static pressure drop is required to ensure that the test provides accurate and meaningful results. According toNFPA 291, Recommended Practice for Fire Flow Testing and Marking of Hydrants:

A10 percent pressure dropis the minimum acceptable difference between the static pressure (no flow) and residual pressure (while flowing).

This drop ensures that water flow is sufficient for an accurate test and reflects the system’s capacity under actual fire conditions.

5 percent (A):Too low to provide reliable results.

15 percent (C) and 20 percent (D):Higher drops are acceptable but not the minimum requirement.