A borehole pump is the only pump on most South African properties that nobody ever sees running. It hangs somewhere between 30 and 200 metres below your feet, in a 150 mm hole, and the first sign that something is wrong is usually a dry tank. Understanding what the machine is actually doing down there is the difference between a set that lasts eight years and one that is pulled twice in a season.
What a borehole pump is
A borehole pump – also called a submersible or bore pump – is a long, thin pump and motor assembly built to fit inside a drilled hole. Because the hole is narrow, the pump cannot be wide. The engineering answer is to stack many small pumping elements on one shaft rather than use one large impeller, and to bolt a water-filled electric motor directly underneath the whole stack.
Everything is submerged. There is no suction line, no priming and no cavitation risk from lift, because the pump pushes water up rather than sucking it. That single fact is why a submersible can deliver 150 m of head where a surface pump on the same borehole cannot lift water past about seven metres. If you are weighing the two options, our comparison of surface and submersible pumps sets out where each one belongs.
The four parts of a borehole set
1. The wet end
The wet end is the pumping section: a series of identical stages, each made up of an impeller and a diffuser. The impeller spins with the shaft and throws water outward; the diffuser is a stationary bowl that catches that fast-moving water, slows it down and converts the speed into pressure before handing it to the next impeller. Stages are stacked in series, so their pressures add.
2. The motor
Below the wet end sits a submersible motor, typically a water-filled or oil-filled 4-inch or 6-inch unit running on 220 V single phase or 380 V three phase. It is cooled entirely by the water flowing past its outer skin on its way into the pump inlet. No flow means no cooling, which is why dry running destroys a submersible motor in minutes rather than hours.
3. Rising main and cable
The rising main is the column that carries water to surface – usually HDPE for shallower installations or galvanised steel column pipe for deep, heavy sets. The submersible drop cable runs alongside it, strapped every two to three metres, and a stainless safety rope carries the dead weight so the pipe thread is never the only thing holding the pump.
4. Control gear
At surface you will find a control box with overload protection, a dry-run or low-water cut-out, and on better installations a variable speed drive. VSDs are increasingly the default on boreholes because they soft-start the motor and match delivery to demand instead of cycling the set on and off. We cover the detail in how VSDs improve pump efficiency.
Where the pressure comes from
Each stage produces a roughly fixed amount of head at a given flow, and stages in series add up. A 4-inch pump end rated at 6 m per stage delivers approximately 60 m of head across ten stages, 120 m across twenty. Flow, by contrast, is set by the size of the impeller family, not by the number of stages – adding stages buys you height, never litres.
| Stages | Approximate head at duty point | Typical application |
|---|---|---|
| 8 | 48 m | Shallow farm borehole feeding a nearby tank |
| 14 | 84 m | Household borehole, 60 m water level plus tank height |
| 22 | 132 m | Deep Karoo or Limpopo borehole with long delivery run |
| 33 | 198 m | High-lift transfer from a deep hole to an elevated reservoir |
To pick the stage count you need the total head, not just the depth. That means pumping water level plus tank or discharge height plus friction loss in the rising main and delivery line. Our guide to calculating pump head works through the arithmetic in full.
Submergence, drawdown and the number that catches people out
A borehole has a rest water level – where the water sits when nothing is pumping – and a pumping water level, which is lower because drawing water out draws the level down. The gap between the two is drawdown, and it grows with flow rate. Pull harder and the level drops further.
The pump must stay submerged at the pumping water level, not the rest level. This is the single most common sizing mistake we see on South African installations: a pump set at 45 m in a hole with a 40 m rest level, which then draws down to 52 m within twenty minutes of running and burns out the motor.
- Have the driller or a pump technician run a proper yield test, ideally a step test followed by a constant-rate test of at least four hours.
- Record the pumping water level at the sustainable yield, not the maximum the hole will give for ten minutes.
- Set the pump at least 5 m below the stabilised pumping water level, and at least 3 m above the bottom of the hole so it does not draw sand and silt.
- Confirm the motor gets at least 0.15 m per second of flow past it; on an oversized casing, fit a flow sleeve to force water over the motor skin.
Matching the pump to the borehole yield
A borehole is a supply, not a tap. Fitting a 5 000 litre per hour pump to a hole that recharges at 1 200 litres per hour does not give you more water – it gives you twenty minutes of water and then a dry hole, a tripped cut-out and a shortened motor life.
| Tested yield | Sensible pump duty | Storage strategy |
|---|---|---|
| Under 1 000 l/h | 800 l/h | Pump continuously to a large tank; draw from the tank |
| 1 000 – 2 500 l/h | 2 000 l/h | Float-switched tank filling, 5 000 – 10 000 l storage |
| 2 500 – 6 000 l/h | 5 000 l/h | Direct irrigation blocks or pressurised supply with buffer tank |
| Above 6 000 l/h | Match to demand | Consider a larger 6-inch set and a VSD to trim to demand |
A worked example
A smallholding outside Brits has a borehole with a 38 m rest level. A four-hour constant-rate test at 2 400 litres per hour settles the pumping water level at 54 m. The storage tank stands on a 4 m stand 180 m from the hole, fed through 40 mm HDPE.
- Pumping water level: 54 m
- Tank inlet above ground: 4 m
- Friction loss, 180 m of 40 mm HDPE at 2 400 l/h: approximately 5 m
- Total dynamic head: 54 + 4 + 5 = 63 m
- Design duty: 2 400 l/h at 63 m
- Pump depth: 60 m – six metres below the pumping water level, comfortably above the 78 m hole bottom
From the stage table, that duty lands on an 11-stage 4-inch pump on a 1.1 kW motor. Because the tank is the buffer, a simple float switch and a dry-run probe are enough; a VSD would add little on a fixed-duty tank-filling application.
Why borehole pumps fail early
- Dry running. No flow, no cooling. A low-water probe or electrode set costs a fraction of a new motor.
- Sand. Abrasive water erodes impellers and wear rings. Raise the pump, fit a sand separator, or have the hole redeveloped.
- Voltage problems. Undersized drop cable causes voltage drop at the motor terminals, which raises current and cooks the windings. Size the cable for the run length, not the motor plate.
- Rapid cycling. Each start is a thermal and mechanical shock. Bigger pressure tanks, correct float differentials or a VSD all cut starts per hour.
- Lightning and surges. Highveld summers are hard on control gear. Surge arrestors at the board are cheap insurance.
Most of these show up as symptoms long before they become failures. Our checklist of signs your pump needs maintenance covers what to listen and look for.
Sensible maintenance intervals
| Interval | Check |
|---|---|
| Monthly | Running current against nameplate; delivery pressure and flow at the tank |
| Quarterly | Control box contactors, cut-out operation, earth leakage test |
| Annually | Insulation resistance of motor and cable, water level check, water quality sample |
| Every 3 – 5 years | Pull the set, inspect wear rings and impellers, replace the rising main straps and rope |
Everything above applies to grid-powered sets. If your borehole is off-grid or you are fighting load shedding, the same hydraulics apply but the energy side changes completely – see how solar water pumping systems work, or browse our solar pumping solutions.
Need a borehole pump sized properly? Send us your yield test and tank details and we will specify the pump, motor, cable and control gear as a package. Browse our borehole pump range, request a quote, or call +27 (15) 516-4991.