A bore pump that is too small leaves taps weak, sprinklers uneven and tanks slow to fill. One that is too large can cycle excessively, draw a bore down too hard and create avoidable power and repair costs. Knowing how to size a bore pump means matching the pump to the water source, pipework and demand - not selecting solely by the depth of the bore.
For a house, livestock supply, irrigation system or commercial site, the correct answer comes from four figures: required flow, pumping water level, required pressure and pipe friction. Once these are clear, a pump curve can be used to select a model that will perform reliably in the field.
Start with the water you need to deliver
Flow rate is the volume of water required at the point of use, normally measured in litres per minute (L/min) or litres per second (L/s). This is the first part of pump selection, but it needs to reflect realistic simultaneous use rather than every outlet running at once.
A typical home supplied from a bore may need 30 to 50 L/min for comfortable domestic use, depending on the number of bathrooms, appliances and outdoor taps. A property with irrigation may require substantially more. For example, six sprinklers using 12 L/min each need 72 L/min before allowing for any domestic demand. Livestock systems are often lower-flow but may need dependable delivery over long distances to troughs.
Where a bore feeds a storage tank, consider the refill period. A 22,500-litre tank does not necessarily need to be filled quickly. If the bore yield is limited, a lower-flow pump running for longer may be the better system. Trying to force high output from a low-yield bore can lead to dry running, sand ingress and pump damage.
Measure the bore, not just its drilled depth
The total drilled depth shown on a bore report is useful, but it is not the lift the pump works against. The key measurement is the dynamic water level - the level of water in the bore while pumping at the required flow.
Start with the static water level, which is the distance from ground level to the water surface when the bore has rested. Then account for drawdown. Drawdown is how far the water level falls once the pump is operating. If static water is 20 metres below ground and testing shows it drops to 45 metres while pumping, the pumping water level is 45 metres below ground.
The pump should be installed below the expected dynamic water level, with enough submergence to keep the motor cooled and reduce the risk of vortexing. It should also remain clear of the bottom of the bore, particularly where sediment may be present. Installation depth depends on the bore construction, pump manufacturer requirements and water level behaviour. Deeper is not always better: placing a pump unnecessarily low can increase cable and pipe costs without improving performance.
A bore driller's completion report or a proper pump test provides the best information. If these records are unavailable, measure the static level and arrange a yield test before committing to a pump. Bore conditions can also change through dry periods, so size for the lower water level likely to occur, not only the level measured after rain.
Calculate total dynamic head
Total dynamic head, often shortened to TDH, is the pressure load the pump must overcome at the selected flow. It combines vertical lift, pressure required at the outlet and friction losses through pipework and fittings.
A practical calculation is:
TDH = pumping water level + height above ground + required pressure head + friction loss
If the pumping water level is 45 metres below ground, the tank inlet or highest outlet is 8 metres above ground, the system needs 300 kPa pressure and pipe losses are 12 metres, the calculation is:
45 m + 8 m + 30 m + 12 m = 95 m TDH
As a working conversion, 100 kPa equals approximately 10 metres of head. A household pressure system requiring 300 kPa therefore adds about 30 metres to the pump duty. For irrigation, the pressure required at the sprinklers or valves needs to be included, not just the height of the water.
Friction losses are frequently underestimated. Long runs of undersized poly pipe can consume a large portion of the pump's available head, especially at higher flows. Elbows, check valves, filters, isolation valves and pressure control equipment add resistance as well. Larger pipe can cost more upfront but often reduces energy use and improves pressure at the far end of the line.
Match the duty point to the pump curve
With flow and TDH established, the required duty point can be expressed simply: for example, 50 L/min at 95 metres head. This is the point to locate on a pump performance curve.
A pump curve shows how much flow a particular pump can produce at different heads. As head increases, available flow falls. A pump should be selected to operate near its efficient working range at the required duty point, rather than at either extreme of its curve.
Do not select a pump based on its maximum flow or maximum head listed on a product page. Those figures are usually measured separately. A pump capable of 120 L/min at low head may deliver far less at 95 metres head. Similarly, a model with a maximum head of 100 metres will have very little useful flow close to that limit.
Allowing a sensible margin is worthwhile, particularly where bore water levels vary or future demand may increase. However, excessive margin can create a system that runs at too high a pressure or flow. Throttling a significantly oversized bore pump wastes energy and can increase wear. The aim is a stable operating point, not the largest pump available.
Check bore yield and water quality
Pump capacity cannot exceed the sustainable yield of the bore. A yield test identifies the rate at which water can be extracted without the water level falling to a point that threatens the pump or causes the bore to recover too slowly.
If the property requires more water than the bore can sustainably provide at peak times, use a storage tank as a buffer. The bore pump can replenish the tank at a controlled rate, while a separate pressure pump supplies the home, irrigation or stock system at higher short-term demand. This arrangement is common on rural properties and gives more protection against low water levels.
Water quality also influences selection. Sand, iron, minerals and aggressive water can shorten pump life if the wrong materials are used. A bore with sand production may need a lower-flow setup, careful pump placement and appropriate filtration. For mineralised or corrosive water, check the pump's material compatibility before installation.
Size the pipe, power supply and controls with the pump
A correctly selected pump can still perform poorly if the supporting system is undersized. Rising main diameter should be selected for the planned flow and pipe length, with friction losses included in the TDH calculation. A narrow pipe may make a pump appear underpowered when the real issue is excessive resistance in the line.
For submersible bore pumps, confirm the bore casing diameter, pump outside diameter and cable arrangement. The pump needs sufficient clearance for installation and water flow around the motor. Electrical cable sizing is equally critical, particularly on deep bores or long runs from the switchboard. Voltage drop can reduce motor performance and cause premature failure.
Controls should suit the application. A pressure switch and pressure tank can work well for domestic supply, while constant-pressure control may suit variable household demand. Tank filling systems commonly use float switches or level controls. Dry-run protection is strongly recommended where yield is uncertain or water levels fluctuate, as a submersible pump relies on water flow for cooling.
Common bore pump sizing mistakes
Several problems recur in bore pump installations:
- Selecting from bore depth alone instead of calculating dynamic water level and total dynamic head.
- Ignoring pressure requirements at taps, irrigation valves or elevated tanks.
- Using small pipe over long distances and overlooking friction losses.
- Choosing a pump with more output than the bore can sustain.
- Installing without dry-run protection, suitable controls or correctly sized cable.
When to get technical advice
Simple domestic replacements can often be sized from an existing pump's performance, provided the bore, pipework and water use have not changed. For a new bore, long-distance transfer line, irrigation system, multiple dwellings or variable water levels, a full duty calculation is the safer approach.
Have the bore depth, static level, test yield, expected pumping level, pipe diameter, pipe length, elevation changes and required flow available when seeking advice. This allows a pump specialist to compare suitable models properly and identify whether a tank-and-pressure-pump arrangement would better suit the site.
The right bore pump is one that delivers the required water where it is needed, without overworking the bore or the equipment. Take the time to establish the duty point before purchase, and the system is far more likely to provide dependable water through changing seasons.