How to Select a Submersible Pump
A step-by-step sizing method for deep well, borehole and agricultural irrigation applications. This guide explains which measurements you need, how to calculate total dynamic head, how to match pump diameter to your well casing, and how to read the FANPO series tables to reach a shortlist of suitable models.
Step 1 — Collect five measurements before you size anything
Static water level
The distance from ground level down to the water surface when the well is at rest and no pump is running.
Measured in metres. This is the starting point, but it is never the value you size the pump on.
Dynamic water level and drawdown
When pumping starts, the water level in the well drops. The level it settles at during continuous pumping is the dynamic water level.
The difference between static and dynamic level is the drawdown. A well test at the intended flow rate is the only reliable way to obtain it.
Sizing on static level instead of dynamic level is the single most common cause of an undersized pump.
Required flow rate
How much water the system must deliver, expressed in m³/h. For irrigation this follows from the irrigated area, the crop water requirement and the daily operating hours.
The well yield sets the ceiling: a pump can never sustainably deliver more than the borehole produces.
Discharge height and distance
The vertical rise from ground level to the highest delivery point — a tank, a pivot inlet or a reservoir — plus the horizontal pipe run to it.
Record the pipe diameter and material as well, because friction loss depends on both.
Borehole casing diameter
The internal diameter of the well casing decides the largest pump and motor that will physically fit and still allow cooling flow past the motor.
Always allow clearance for the rising main, the power cable and the cable guards.
Water quality
Sand content, chloride level and abrasiveness determine whether a stainless steel or a cast iron pump is the better choice.
High sand content shortens the life of any submersible pump and may require a sand separator or a repositioned intake.
Step 2 — Calculate total dynamic head
Total dynamic head is what the pump must actually overcome
A pump curve is read against total dynamic head, not against well depth. Two wells of identical depth can require completely different pumps once discharge height and pipe friction are included.
Worked example
Dynamic water level 120 m. Water is lifted a further 15 m to a tank, through 250 m of pipe, and 10 m of pressure is needed at the outlet.
If the pipe chart gives 4 m of friction loss per 100 m at the design flow, friction loss is 10 m.
Total dynamic head = 120 + 15 + 10 + 10 = 155 m. The pump is selected for 155 m at the required flow, not for 120 m.
Friction loss in practice
Friction loss rises steeply with flow velocity. Keeping velocity in the rising main in the region of 1.5–2.5 m/s is normal design practice.
Going one pipe size up is almost always cheaper over the life of the installation than paying for the extra motor power that a narrow pipe demands.
Use the manufacturer chart for your specific pipe material and diameter rather than a generic figure.
Where to set the pump
The pump must stay comfortably below the dynamic water level at all times, with margin for seasonal decline.
It should also sit clear of the well bottom so that settled sand is not drawn into the intake.
If the pump has to be installed below the screen or inside an oversized casing, a flow sleeve is required so that water still passes the motor.
Step 3 — Match the pump diameter to the borehole
Pump diameter follows the well, not the other way round
The nominal size of a submersible pump — 5″, 6″, 7″, 8″ or 10″ — refers to the smallest casing it is designed to run in. A larger casing is always acceptable, provided motor cooling flow is maintained; a smaller one is not.
| Borehole casing | Suitable FANPO pump size | Typical application |
|---|---|---|
| 6″ and larger | 5″ cast iron | Small farms, garden and orchard irrigation, domestic supply |
| 6″ and larger | 6″ stainless steel or cast iron | Field irrigation, village water supply, medium deep wells |
| 8″ and larger | 7″ cast iron | High-flow irrigation where an 8″ pump will not fit |
| 8″ and larger | 8″ stainless steel or cast iron | Large irrigation schemes, industrial water transfer |
| 10″ and larger | 10″ stainless steel | Very high flow with high head, municipal and industrial supply |
Casing diameters are internal. Allow clearance for the rising main, power cable and cable guards before finalising the pump size.
Step 4 — Read the FANPO series tables
Stainless steel submersible pump series
AISI 304 stainless steel body, impeller and diffuser. Preferred where corrosion resistance and a long service life matter more than initial cost.
| Series | Size | Max stages | Power | Max flow | Max head |
|---|---|---|---|---|---|
| 6017 | 6″ | 40 | 0.75–30 HP | 20 m³/h | 460 m |
| 6024 | 6″ | 33 | 1.5–40 HP | 32 m³/h | 442 m |
| 6030 | 6″ | 40 | 1.5–70 HP | 43 m³/h | 633 m |
| 6046 | 6″ | 25 | 3–60 HP | 59 m³/h | 372 m |
| 6060 | 6″ | 25 | 3–60 HP | 72 m³/h | 370 m |
| 8075 | 8″ | 22 | 5.5–100 HP | 96 m³/h | 423 m |
| 8095 | 8″ | 22 | 7.5–175 HP | 110 m³/h | 551 m |
| 8096 | 8″ | 22 | 10–200 HP | 125 m³/h | 467 m |
| 10110 | 10″ | 20 | 10–200 HP | 130 m³/h | 549 m |
Cast iron submersible pump series
Cast iron body with a robust hydraulic design, built for demanding duty in agricultural and industrial deep wells.
| Series | Size | Max stages | Power | Max flow | Max head |
|---|---|---|---|---|---|
| 5004 | 5″ | 6 | 2–10 HP | 40 m³/h | 85 m |
| 6012 | 6″ | 10 | 5.5–50 HP | 48 m³/h | 295 m |
| 6013 | 6″ | 25 | 5.5–75 HP | 60 m³/h | 420 m |
| 6019 | 6″ | 18 | 7.5–60 HP | 84 m³/h | 270 m |
| 7021 | 7″ | 25 | 12.5–150 HP | 120 m³/h | 435 m |
| 8420 | 8″ | 18 | 15–175 HP | 117 m³/h | 436 m |
| 8530 | 8″ | 14 | 25–175 HP | 144 m³/h | 414 m |
| 8535 | 8″ | 14 | 30–175 HP | 162 m³/h | 405 m |
Maximum flow and maximum head are series limits and do not occur at the same operating point. Use the stage-by-stage performance table on each product page to find the exact stage count for your duty point.
Step 5 — Size the submersible motor
Power must exceed the pump shaft demand
Read the absorbed power at your duty point from the pump table, then select the next motor rating above it. A motor loaded to its limit has no reserve for voltage drop or a falling water level.
Motor diameter follows pump diameter
FANPO supplies 6″, 7″, 8″ and 10″ water-cooled submersible motors. See the submersible motor range for ratings and dimensions.
Cooling flow is not optional
Water-filled submersible motors are cooled by the water flowing past them. If the casing is much wider than the motor, or the pump sits below the screen, fit a flow sleeve so the flow is forced along the motor surface.
Cable size affects motor life
Voltage drop over a long drop cable reduces available torque and raises current. Size the cable for the full installed depth, not for the pump rating alone.
Starting method
Direct-on-line starting is common at lower ratings. Star-delta, soft starters or variable frequency drives reduce mechanical and electrical stress on larger units and on long pipelines.
Protection
Overload protection, dry-run protection and phase failure protection are the three devices that prevent the majority of submersible motor failures.
Frequently asked questions
Can I size a pump from well depth alone?
No. Well depth tells you how deep the pump can hang, not what head it must produce. You need the dynamic water level, the discharge height, the friction loss and the required outlet pressure.
Stainless steel or cast iron?
Stainless steel resists corrosion and aggressive water better and keeps its hydraulic efficiency longer. Cast iron offers a robust, cost-effective solution for conventional agricultural wells. FANPO produces both in overlapping flow and head ranges.
What happens if the pump is oversized?
An oversized pump draws the well down faster than it recovers, which leads to dry running, air entrainment and premature wear. It also consumes more energy than the duty requires.
How many stages do I need?
The stage count sets the head. Divide the required total dynamic head by the head per stage at your design flow, then round up to an available stage configuration in the series table.
Does a longer pipe run change the pump selection?
Yes. Every additional metre of pipe adds friction loss, which the pump must overcome as extra head. Long horizontal runs frequently make the difference between two adjacent models.
Can FANPO help with the selection?
Yes. Send the dynamic water level, required flow, discharge height, pipe details and casing diameter through the contact page and our engineering team will propose suitable models.