Sizing a Bore Pump

Bore pumps are unforgiving of guesswork. They sit out of sight, they cost more than surface pumps, and pulling one out to swap it is a day’s work. Worse, the two most common sizing errors — oversizing for the bore’s yield, and setting the pump too shallow — both end the same way, with the pump running dry and burning out.

Getting the numbers first is worth the effort.

The measurements you need

Bore depth

Total depth of the hole. Available from your driller’s log, or measured with a weighted tape. This sets the maximum depth the pump can be installed at, but is not the depth you pump from.

Standing water level (SWL)

The depth from ground level down to the water surface when the bore has been at rest. Measure with a water level meter, or a weighted float on a marked line.

This is the figure people mistakenly use for sizing. It is only the starting point.

Drawdown and pumping water level (PWL)

Start pumping and the water level in the bore falls, because water flows in from the aquifer more slowly than the pump removes it. It keeps falling until inflow matches outflow, then stabilises.

Drawdown = how far the level falls.
Pumping water level = standing water level + drawdown. This is the depth your pump is really lifting from.

Drawdown is why a bore with water 40 m down may need a pump sized for 70 m or more. On a tight aquifer the drawdown can exceed the standing level itself.

Your driller’s bore completion report usually gives drawdown at a tested flow rate. If not, run a test: pump at a known rate for an hour or two and measure the level as it stabilises.

Bore yield

The sustainable flow the bore can deliver, in L/min or L/s, usually from the driller’s test.

This is a hard ceiling. A pump that can move 100 L/min installed in a bore that yields 40 L/min will pump the bore down to the pump inlet and run dry. Always choose a pump whose flow at your operating head is at or below the bore’s tested yield.

Casing diameter

Determines what will physically fit. Most Australian domestic and stock bores are cased at 4” (100 mm) or 6” (150 mm). The RM-BORE2 is a 4” pump, so it suits 4” casing and anything larger.

Measure, do not assume. A pump that will not fit is an expensive mistake.

Calculating total dynamic head

For a bore installation:

TDH = pumping water level + rise above the bore head + pressure required + friction loss

Component	What it is
Pumping water level	Standing water level plus drawdown
Rise above bore head	Vertical metres from ground to the tank inlet or highest outlet
Pressure required	0 m if filling a tank; 20–30 m for direct household supply
Friction loss	~5% of total pipe length, including the vertical riser

Worked example

A stock and domestic bore:

Bore depth: 90 m
Standing water level: 45 m
Drawdown at 60 L/min: 15 m → pumping water level 60 m
Tank on a rise, 12 m above the bore head
150 m of pipe from bore to tank
Filling a tank, so no residual pressure needed
Tested bore yield: 70 L/min

Component	Head
Pumping water level	60 m
Rise to tank	12 m
Pressure required	0 m
Friction, ~5% of 150 m + 60 m riser	10.5 m
Total dynamic head	≈ 83 m

The RM-BORE2 produces up to 132 m of head and 67 L/min. At 83 m of head it sits comfortably within its curve, and 67 L/min maximum is below the bore’s 70 L/min yield — so the bore can keep up.

Had the bore yielded only 30 L/min, this pump would be the wrong choice regardless of the head figure. Yield governs.

Setting depth

Where the pump hangs in the bore matters as much as which pump you choose.

Set the pump below the pumping water level, with margin. A common approach is to sit it at least 3–5 m below the stabilised pumping water level, so seasonal drops and heavier-than-expected drawdown do not expose the inlet.

Keep it clear of the bottom. Leave a minimum of 2–3 m of clearance above the base of the bore. Sediment collects there, and a pump drawing it in will wear rapidly. If the bore has a screened section, the pump normally sits above the screen so water flows past the motor and cools it.

Do not simply drop it to the bottom. It is the single most common installation error, and it guarantees a short pump life.

Cooling flow

A submersible motor is cooled by water moving past it. In a wide casing, or where the pump sits in a large chamber, flow past the motor can be too slow to cool it. In those cases a flow sleeve (shroud) forces water along the motor body. In standard 4” casing with a 4” pump, the annular gap is small enough that this is rarely an issue.

Protecting the pump

Bore pumps die from running dry far more often than they wear out. Three defences:

Size to the bore’s yield. The fundamental protection. If the pump cannot outpace the aquifer, it cannot pump itself dry.
Set it deep enough, with margin for seasonal variation.
Fit low-water cut-out protection — a level probe, a flow switch, or a controller with run-dry detection that stops the pump when water is not moving.

The RM-BORE2 ships with a control box containing the motor’s start capacitor and protection circuitry. It is not optional and it is not generic — use the box supplied with the pump, mounted in a dry, ventilated, shaded position.

Cable and electrical

The RM-BORE2 comes with 40 m of insulated SAA-certified cable. If your setting depth plus the run to the control box exceeds that, the cable must be extended with a proper submersible splice kit — heat-shrink or resin-filled, rated for continuous immersion. Ordinary insulation tape will fail, and a failed splice means pulling the pump.

Long cable runs also cause voltage drop, which makes motors run hot and shortens their life. On runs over about 50 m from the switchboard, have your electrician calculate the required conductor size rather than assuming.

All fixed wiring must be carried out by a licensed electrician in accordance with AS/NZS 3000. In Australia, electrical pump equipment must also carry SAA approval and be registered under the national EESS scheme — check both before buying any bore pump.

Sizing checklist

With the numbers settled, Installing a Bore Pump covers getting it down the hole and commissioned.