What Size Submersible Pump Do You Need for a Deep Well?

A submersible pump for a deep well is chosen by head and flow, not by horsepower. Work out the total head first: the depth to the water while the pump is actually running, plus the height from the well head up to your tank, plus the friction in the pipe. Then decide how many cubic metres an hour you really need. Only after that do you pick a model, and on a 4 inch borehole pump the model number carries both figures. In the DAFU 4SD range, 4SD8/29 means a pump for a 4 inch borehole, in the 8 m3/h flow class, with 29 impeller stages. Power follows from those two choices, it is never the starting point. DAFU publish seven different 4 kW pumps in this range, and the widest one delivers less than a quarter of the head of the narrowest.

In this article

• What do the numbers in 4SD8/29 mean?
• Why is horsepower the wrong thing to shop by?
• How do you work out the head the pump has to produce?
• Why is the head on the spec sheet not the head you get?
• What else do you have to buy with the pump?
• Single phase or three phase?
• When is a deep well pump the wrong choice?
• Frequently asked questions

What do the numbers in 4SD8/29 mean?

Every part of the code tells you something you need before you order.

Part of the code	What it means
4	Fits a 4 inch borehole. DAFU give a minimum well diameter of 4 inches for this motor family.
SD	Submersible deep well, multistage.
8	The flow class, not a maximum. DAFU publish a range of 0 to 10.8 m3/h for the 4SD8 family. The 4SD3 family runs 0 to 4.8 m3/h, the 4SD16 family 0 to 21 m3/h.
/29	29 impeller stages. Stages are what build the head.

The number after the slash is the one people misread most often. It is a stage count, not a depth in metres. A 4SD3/30 is not a 30 metre pump. It is a narrow, high pressure pump with thirty impellers stacked one above the other, and DAFU publish a head of up to 226 metres for it.

That stacking is the whole idea. A 4 inch well leaves almost no width to work with, so the only way to build pressure is to send the water through another impeller. More stages means more head at the same flow. A higher flow class means more water at less head for the same power.

Why is horsepower the wrong thing to shop by?

Because the motor rating tells you what the pump will draw, not what it will do. Here are three DAFU pumps we stock that all run on the same 2.2 kW (3 HP) motor, with DAFU's own published figures.

Pump	Flow range	Head at zero flow	Head at top of range	Outlet	Price
4SD3/30	0 to 4.8 m3/h	226 m	54 m	1.2 in	$75
4SD8/17	0 to 10.8 m3/h	106 m	29 m	2 in	$72
4SD16/9	0 to 21 m3/h	50 m	19 m	2 in	$75

Same motor. Three dollars between them. One of them will lift water more than four times higher than another, and one of them will move more than four times as much water. If you order by horsepower you are choosing at random between those three. If you order by head and flow you cannot go far wrong.

How do you work out the head the pump has to produce?

Total head is a sum of four things, and three of them get forgotten.

• The pumping water level. Not the borehole depth, and not the resting level you measure before you start. Once the pump runs the level drops and settles lower, and that dynamic level is what it lifts from. Your driller has it from the test pumping.
• The vertical rise above the well head. Ground level at the well up to the tank inlet. On a hillside this is often the largest number in the sum.
• Friction in the pipe. It grows sharply with flow and falls sharply as the pipe gets wider. A long thin rising main quietly eats tens of metres of head, so undersized pipe is a false economy.
• Pressure wanted at the far end. Sprinklers and drip lines need real pressure at the outlet, not just water arriving.

Add them up, then check the second number: the yield of the well. A borehole that has been tested at 5 m3/h will not supply a pump asking for 10, no matter what you spend. The well sets the ceiling on flow, and the pump has to sit under it.

Why is the head on the spec sheet not the head you get?

Look closely at how a manufacturer prints the head figure. DAFU list the 4SD8/29 as 181 to 50 metres. That is not a tolerance and it is not a typing error. It is the two ends of the pump curve.

The 181 metres is the head at zero flow, with the delivery valve shut. It is the maximum pressure the pump can build while moving no water at all. The 50 metres is what is left at the top of the flow range, 10.8 m3/h. Your real duty point sits somewhere between the two, and every cubic metre an hour you ask for costs you head.

So a pump advertised as "181 m" will not deliver a useful flow 181 metres up. That single misunderstanding is behind a large share of borehole pumps that get installed and then disappoint. Size against the figure at your actual flow, and ask for the curve if you are near the limit.

What else do you have to buy with the pump?

This is where borehole orders most often arrive incomplete. A 4 inch submersible set is three separate purchases, not one.

1. The wet end. That is what a 4SD listing is: the impeller stack, the shaft and the brass outlet. No motor.
2. The motor. A 4 inch submersible motor matched to the kW the maker publishes for that pump model. A 4SD8/17 is listed at 2.2 kW, so it takes a 2.2 kW motor.
3. The control box, single phase only. It holds the run capacitor and the thermal cutout. DAFU offer a single phase motor with the capacitor built in only on special request, so the standard motor needs the box.

On top of those you still need riser pipe, flat cable sized for the run, a waterproof splice kit, a safety rope and clamps. None of that is in the pump box.

Part	Item	Price
Wet end	DAFU 4SD8/17, 2.2 kW	$72.00
Motor	DAFU 4SDM2200S, 2.2 kW, 220 V single phase	$110.00
Control box	Control box for 2.2 kW motor, 220 V	$15.00
Complete set	Pipe, cable and fittings not included	$197.00

Single phase or three phase?

DAFU build their 4 inch motors in both: single phase at 220 to 240 V 50 Hz and three phase at 380 to 415 V 50 Hz, both rewindable, both 2850 rpm, insulation class F, protection IP68, with a maximum water temperature of 35 C and a minimum well diameter of 4 inches.

The practical fork comes at 2.2 kW. Our single phase motors stop there, at 4SDM2200S. Above it the range is three phase only: 4 kW, 5.5 kW and 7.5 kW, starting with the 4SD4000T at $168. So if your head and flow land above 3 HP, you are also deciding to run a three phase supply to the well, and that is worth settling before you order rather than after the pipe is in the hole.

Where you have the choice, three phase is the easier life: no control box, no capacitor, a gentler start and less trouble with the long cable run down a deep borehole.

When is a deep well pump the wrong choice?

Often enough that it is worth saying plainly. A 4SD set is a specialist tool for a narrow, deep hole.

• The water is shallow. A cistern, a spring box or a well under about seven metres is surface pump work. A jet pump like the KJS750 at $77 sits in the daylight where you can service it.
• You want pressure, not lift. If a roof tank already holds the water and the top floor taps are weak, the answer is a booster such as the DAM400X at $153.
• The water is dirty. A 4SD is a clean water pump, and grit chews its stages. For a flooded basement or a drainage pit use a unit like the DFS-S750 at $74.
• The well is under 4 inches. It will not go down, and forcing it damages the cable.
• The well cannot keep up. Oversizing is the expensive mistake here. A pump that outruns the well drags the level down to its own intake and the motor loses the flow it needs for cooling. Franklin Electric's guidance is that a 4 inch high thrust motor needs about a quarter of a foot per second of water moving past it, and that a pump set below the well screen needs a flow sleeve, since water entering above it never passes the motor. Hang the pump below the pumping level with margin, above the screen, never on the bottom.

Not sure which family your situation falls into? Our guide on vortex, jet, centrifugal and submersible pumps sorts it by where the water sits.

Frequently asked questions

Does 4SD8/29 mean the pump lifts water 29 metres?

No. The 29 is the number of impeller stages inside the pump, not a depth. DAFU publish a head range of 181 to 50 metres for that model, depending on how much water you draw. A 4SD3/13 has thirteen stages and is published at 98 to 24 metres.

Does the pump come with the motor?

No. A 4SD listing is the wet end on its own. You buy the 4 inch motor separately, matched to the kW that the maker publishes for that pump model, and on single phase you add a control box as well.

Do I need a control box with a three phase motor?

No. The control box exists to hold the starting and running capacitors for a single phase motor. A three phase motor starts on its own supply and needs only proper overload protection in the panel.

How deep should the pump hang in the well?

Below the pumping water level with a safe margin so it never breaks suction when the level drops, above the well screen so water flows past the motor and cools it, and clear of the bottom so it is not sitting in sediment. If the only place to hang it is below the screen, fit a flow sleeve.

What size cable do I need down the borehole?

It depends on the motor rating and the length of the run, and it is the item people undersize most. Too small a cross section over a long run drops voltage at the motor and shortens its life. Send us the motor kW and the depth and we will tell you the size.

Sources: head, flow and power figures are DAFU's own published 4SD data, dafupump.com. Motor cooling guidance from Franklin Electric.