Residential and small commercial systems utilize 1.5” Headers for arrays up to 10 panels. Larger residential and commercial systems utilize 2” headers where arrays contain up to 16 panels, or where overall flow is critical and must be kept below the total flow rate of the commercial pool. Friction loss for water flow through plastic schedule 40 PVC pipe is based on the Hazen-Williams Equation with a Hazen-Williams coefficient c = 140. Velocities between 7 to 10 feet per sec