How to Size a Borehole Pump

Sizing a borehole pump is not a matter of picking the biggest unit the budget allows. It is an arithmetic problem with six inputs, and getting any one of them wrong costs either water or a motor. This guide works through the method we use on every borehole we specify, from the yield test to the final cable size.

Step 1: Test the borehole before you shop

No pump can be sized without a yield test. The driller’s air-lift figure taken on the day of drilling is a rough indication, not a design figure – it is usually optimistic because the hole has not yet stabilised. What you need is a proper pumping test.

1. Step test. Pump at three or four increasing rates, an hour each, recording water level at every step. This shows how drawdown responds to demand.
2. Constant-rate test. Pump at the chosen sustainable rate for at least four hours – twelve to twenty-four on a hole that will carry irrigation – and record the water level every few minutes at first, then every half hour.
3. Recovery test. Stop the pump and record how fast the level returns. A hole that recovers to 90 per cent within an hour is healthy; one that takes a day is telling you to pump gently and store water.

From the test you want three numbers: rest water level, stabilised pumping water level at your design flow, and sustainable yield in litres per hour. Write them down. Everything else follows from these.

Step 2: Fix the design flow

Design flow is the smaller of what you need and what the hole will give. Work out demand honestly.

Divide daily demand by realistic running hours. If a household needs 3 000 litres a day and you are willing to run the pump ten hours, the design flow is 300 litres per hour – a very gentle duty that most boreholes handle without complaint. Chasing a high flow so the tank fills in twenty minutes is what turns a 30-year borehole into a five-year one.

Step 3: Calculate total dynamic head

Total dynamic head is the real work the pump must do. It has three parts:

• Pumping water level – measured from the wellhead down to the water while pumping at design flow, not the rest level.
• Static discharge height – the vertical rise from the wellhead to the tank inlet or the highest delivery point. Add the tank stand height, not just the ground level difference.
• Friction loss – pressure lost to the pipe walls in the rising main and the horizontal delivery run, plus fittings.

Expressed simply: TDH = pumping water level + discharge height + friction loss. Our article on calculating pump head covers the friction arithmetic and the Hazen-Williams approach in detail. For quick work, the table below is close enough for HDPE class 10.

Two things fall out of that table immediately. Friction rises roughly with the square of flow, and it falls fast as pipe diameter grows. On long runs the cheapest head you will ever buy is one pipe size up.

Step 4: Pick the duty point on a real curve

With design flow and TDH you have a duty point – say 2 400 litres per hour at 63 metres. Now look at manufacturer curves rather than catalogue headline figures. Two rules apply.

• Your duty point should sit near the middle of the curve, not at either end. A pump running far left of its best efficiency point suffers radial load and bearing wear; far right it risks motor overload and cavitation at the inlet.
• Check the efficiency value at the duty point, not the peak. A pump that hits 72 per cent at its sweet spot but 48 per cent at your duty will cost real money over a decade.

Remember that stages set head and the impeller family sets flow. If the curve gives the right flow but too little head, add stages. If the flow is wrong, you need a different pump end entirely.

Step 5: Size the motor and the cable

Hydraulic power in kilowatts is flow in cubic metres per second multiplied by head in metres multiplied by 9.81. Divide by pump efficiency to get shaft power, then allow motor efficiency and a service margin. Most suppliers publish the required motor rating against each stage count, and you should not go below it – but equally, a deliberately oversized motor does not protect anything, it just draws more magnetising current.

Cable sizing is where good installations separate from bad ones. Voltage drop over a long drop cable raises current, and current is heat. Keep drop under 3 per cent at full load.

Step 6: Set the pump at the right depth

Setting depth is a compromise between staying submerged and staying out of the silt. The working rule:

• At least 5 m below the stabilised pumping water level, so that a seasonal drop does not expose the inlet.
• At least 3 m above the bottom of the hole, so the set does not draw sand and so the pump is not buried if the hole silts up.
• Above the main water-bearing fracture where possible, so that inflow passes the motor and cools it.
• In a wide casing, fit a flow sleeve to force water over the motor skin at the required 0.15 m per second.

Worked example: a Limpopo irrigation borehole

Two hectares of citrus under drip, summer peak demand 90 000 litres per day. The borehole tests at 6 500 litres per hour sustainable, rest level 22 m, pumping water level at 6 000 l/h settles at 41 m. The reservoir is 320 m away, 11 m higher than the wellhead, fed through 63 mm HDPE.

1. Design flow. 90 000 litres over 16 running hours is 5 625 l/h, inside the 6 500 l/h sustainable yield. Round to 5 600 l/h.
2. Pumping water level. Interpolating the step test, roughly 40 m at 5 600 l/h.
3. Discharge height. 11 m to the reservoir inlet.
4. Friction. 63 mm at 5 600 l/h is about 1.3 m per 100 m; over 320 m that is 4.2 m, plus roughly 6 m in the 70 m rising main, plus 10 per cent for fittings – call it 11 m.
5. Total dynamic head. 40 + 11 + 11 = 62 m.
6. Duty point. 5 600 l/h at 62 m – a 6-inch pump end, nine stages, on a 3 kW 380 V motor.
7. Setting depth. 48 m: eight metres below the pumping water level, well clear of the 76 m hole bottom.
8. Cable. 4 mmsq four-core submersible, 48 m down plus 25 m to the board.

Because irrigation demand varies through the season, this installation is a good candidate for a variable speed drive: it lets one pump serve both the summer peak and the low winter duty without throttling. Our VSD and control range covers the options.

The four mistakes that cost the most

• Sizing on rest water level. Always use the pumping level. This alone accounts for most premature motor failures we are called out to.
• Ignoring the horizontal run. A 400 m delivery line in undersized pipe can add more head than the borehole depth.
• Buying on flow alone. A pump that delivers your litres at 40 m will deliver almost nothing at 70 m.
• Oversizing to be safe. A pump bigger than the hole simply empties it faster and trips the cut-out. The hole, not the catalogue, sets the ceiling.

If you are choosing between pump types before you get to the sizing stage, start with choosing the right water pump, and see how borehole pumps work for the mechanics behind these numbers.