Borehole pump sizing: well diameter decides first
For a deep drilled well, a borehole pump is the right approach. But before choosing power output, the diameter of the well casing sets the limit. Here's the correct order of steps.
A borehole (submersible) pump is fully immersed in the water and pushes it up from below. There's no suction limit, which makes it the only reliable solution for deep wells. However, the selection process doesn't start with power output — it starts with a geometric question: will the pump actually fit in the well?
Well diameter comes first
Borehole pumps are grouped into families by diameter, from 2 inches up to 6 inches. The inner diameter of the well casing must be larger than the pump body, with enough margin for the cable and discharge pipe to fit, and for water to flow past and cool the motor.
If the well is narrow, a larger unit is useless on paper if it simply doesn't fit. That's why the first step in sizing is always knowing the well's outer or casing diameter.
Then the operating point
The required head is the sum of the desired outlet pressure and the operating water level. For irrigation, you also need to account for drawdown: under heavy draw-off the water level drops further, and the pump has to lift from this deeper level too.
Flow rate is determined by the consumers running in parallel. The goal is a healthy margin at the operating point, typically 15 to 25 percent. Too large a margin wastes energy and requires an oversized motor; too little results in falling pressure as the water level drops.
Single-phase or three-phase
- 1x230 V, typically marked with an "m", e.g. 4RSm: lower power, for locations with only single-phase supply.
- 3x400 V, without the "m": higher power, smoother start-up, for industrial and agricultural flow rates.
- Frequency-converter control gives soft start and pressure control on both types, plus dry-running protection.
So the correct order is: well diameter, then operating point, then power supply. The NERO Pumps selector follows exactly this logic, and factors in the well diameter to filter out models that won't fit at all.