Driver Sizing & Best Practices
The pump is sized from the duty point; the driver is then sized from the pump curve at that duty point. Most mistakes on these orders come from sizing the driver off the pump size instead of the curve.
Step 1: Total dynamic head
TDH = static lift + friction losses + required pressure × 2.31 ft/psi. Going the other way, 1 ft of water column equals 0.433 psi. Check suction first: these pumps are rated for a vertical distance of up to 25 ft from pump to water level, including drawdown, and less at altitude. Keep suction lines short, one size larger than the pump inlet on long runs, and sloping up to the pump. Confirm available NPSH exceeds the NPSH required on the curve, especially on the XS at high flow.
Worked example (engine pump). Transfer from a pond with 10 ft of suction lift, up 40 ft of elevation, through piping with 25 ft of friction loss, delivering 30 psi at the outlet. Pressure head = 30 × 2.31 = 69.3 ft. TDH = 10 + 40 + 25 + 69.3 = 144.3 ft. The 40 ft rise alone is 40 × 0.433 = 17.3 psi of backpressure. Step-up rule: keep the duty point at or below about 70% of the pump’s maximum head, so look for a maximum head of at least 144.3 ÷ 0.70 = 206 ft. That points to the FACGF family (to 275 ft) rather than the FNSGF (to 185 ft). We then confirm the flow at 144.3 ft on the FACGF curve for the engine size you choose.
Step 2: Match the driver
Engine HP
On engine-driven models the engine is matched by Franklin; pick the model whose curve covers your duty point. Larger engines move more water: in the FBSGF table, the 9 HP FBSGF-12H moves 310 GPM at 30 ft against 135 GPM for the 3.5 HP FBSGF-8. Rotation is clockwise viewed from the engine side. Run at the throttle setting that holds the duty point; if the pump will not catch prime, low engine speed is one of the first checks in the FACGF manual.
Hydraulic flow, pressure, rotation
Motor speed (RPM) = hydraulic flow (GPM) × 231 ÷ motor displacement (cu in/rev). Hydraulic HP = GPM × psi ÷ 1714. Torque (lb-in) = psi × displacement ÷ 6.28. Example: to turn the pump at 2,200 RPM with a 2.5 cu in/rev motor needs 2,200 × 2.5 ÷ 231 = 23.8 GPM. If the curve shows 20 HP at your duty point and the motor is 85% efficient, you need 23.5 hydraulic HP, or 23.5 × 1714 ÷ 23.8 = about 1,690 psi. Then match the spline (B fit: SAE B, 13 tooth, 16/32 pitch; C fit: SAE C, 14 tooth, 12/24 pitch) and the rotation, CW or CCW.
Bare-shaft coupling
Match rotation (CW viewed from the drive end on FAOT, FACT, FBSTF), shaft diameter and keyway, and speed. The FAOT is published for 1750 to 3450 RPM; operation outside that range is not recommended. Size motor HP from the curve at the actual speed: FAOT-6 needs 3 motor HP at 3450 RPM. On direct drive, lay a straightedge across both coupling halves at 3 or 4 positions; on belt drive, keep the pulleys in line. Coupling, guard and baseplate are not included.
Solar array and power
The Fhoton 1.1 kW controller accepts 115 to 420 V DC and up to 2,000 W, with 420 V open circuit as the absolute maximum and no more than 50 A DC short-circuit current. Size the string so cold-morning open-circuit voltage stays under 420 V, and treat 115 V as a floor, not a design point. Output is 200 V AC three-phase at 20 to 60 Hz to a 208-230 V motor. Flow follows sunlight, so size storage for night and cloudy-day demand.
What to send us
Flow (GPM), TDH or discharge pressure, suction lift and water temperature; the driver: engine preference and HP, hydraulic motor make, displacement, spline, rotation and circuit flow and pressure, or motor HP, frame, RPM and voltage; connection type (NPT, grooved or flanged); and for solar, your location, daily water demand and panel details if you already have them. See our guide to reading a pump curve for how we check the duty point.
