Submersible pump for well or borehole: which diameter and depth suit which pump
Submersible pump for a well or borehole: diameter, depth and correct selection
When choosing a submersible pump for a well or borehole, the first two questions you need to ask are simple: what is the diameter of my well or borehole and how deep is the water level? The answers to these two questions determine almost everything else – the type of pump, its power, the diameter of the body, the length of the inlet hose and also whether the entire system will work reliably for years, or the customer will return after three months with a broken impeller.
From practice I know that most problems in pump selection stem precisely from the fact that people either do not know the exact dimensions of their water source, or they confuse the depth of the well with the depth of the water level. These two things are fundamentally different. In this article we will systematically go through the entire issue: from types of water sources, through dimensions and types of pumps, to specific recommendations for common situations in practice.
Dug wells versus drilled boreholes: fundamental differences
Before we get to the pumps themselves, it is important to understand what type of water source we are dealing with. In practice, we encounter two basic types: dug wells and drilled boreholes. Each of them places different demands on the pump.
Dug well
A dug well usually has a diameter from 80 cm to 150 cm, occasionally even more in old stone or brick masonry wells. The depth of dug wells in our conditions is mostly between 5 and 15 meters, in some areas even 20 meters, but this is rather an exception. The water in a dug well comes from so-called shallow groundwater – the first water-bearing horizon. This water is more vulnerable to contamination from the surface (fertilizers, pesticides, runoff), but the volume of water in a dug well is usually larger, which allows for more powerful pumping without the risk of exhausting the source.
For a dug well, it holds that the pump diameter is not a critical limitation – almost any submersible pump on the market will fit into a well with a diameter of 100 cm. Here, the main factors are power, submersion depth and the required flow rate.
Drilled borehole
A drilled borehole is a technically more complex source. It is drilled to a greater depth – usually from 20 to 100 meters, in some geological conditions even deeper. The water comes from a deeper water-bearing horizon and is usually cleaner and microbiologically safer. The borehole is equipped with a PVC or steel casing, the internal diameter of which is a key parameter. The most common boreholes have a casing with an internal diameter of 110 mm (4"), 125 mm (5") or 150 mm (6").
It is precisely here that the first critical point of selection arises: the pump must fit into the casing. Pump manufacturers specify the outer diameter of the pump body, and it must be smaller than the internal diameter of the casing – with at least a 10 mm margin on each side, ideally more, to ensure sufficient water flow around the pump body (motor cooling).
Casing diameter of the borehole and pump diameter: what you need to know
This is the most common source of errors when ordering a pump online. The customer looks at the performance, price, flow rate – but forgets about the pump body diameter. And then it turns out that a pump with a diameter of 101 mm simply cannot be inserted into a borehole with a 110 mm casing, because at least 4–5 mm of space is needed on each side for installation and water flow.
Standard casing and pump diameters
| Borehole casing (internal diameter) | Maximum pump diameter | Typical designation | Note |
|---|---|---|---|
| 110 mm | max. 98–100 mm | 4" pump | Most common borehole in family homes |
| 125 mm | max. 113–115 mm | 4" or 5" pump | Less common, larger water reserves |
| 150 mm | max. 138–140 mm | 5" or 6" pump | Larger homes, farms, commercial use |
| 200 mm and more | up to 185 mm | 6" and larger pumps | Industry, irrigation, public boreholes |
For dug wells with a diameter over 60 cm, these limitations are practically irrelevant – we focus mainly on performance and submersion depth. A pump such as Submersible Pump SKM 100 for Wells and Boreholes is a typical example of a compact submersible pump suitable precisely for dug wells and smaller boreholes, where reliability and easy installation are important.
Water level depth vs. well depth: why it is different
This distinction is absolutely crucial and is surprisingly often forgotten in practice.
Well or borehole depth is the total depth from the ground surface to the bottom – a technical dimension you get from the documentation from the driller or hydrogeologist. A borehole may, for example, have a depth of 60 meters.
Water level depth (static level) is the distance from the ground surface to the water level in a resting state – that is, when we are not pumping from the borehole. For example, 15 meters from the surface.
Dynamic level is the water level depth during active pumping – the water level drops because we are pumping faster than the borehole is refilling. For example, during pumping, the level drops to 25 meters.
For pump selection, the dynamic level plus the height to which we need to pump the water (e.g., into a tank or to the highest outlet in the house) is decisive. The sum of these distances (in meters) plus pipe losses forms the so-called total head that the pump must overcome. This topic is discussed in more detail in the article Discharge and pump performance: how to calculate what you need for a garden or well.
Types of submersible pumps by construction
Not every submersible pump is built for the same purpose. For wells and boreholes, multi-stage centrifugal pumps are primarily used, which can overcome a large head at a relatively small body diameter. There are, however, other constructions as well.
Multi-stage submersible pumps (for boreholes)
This is the gold standard for drilled boreholes. The pump has a cylindrical shape and can have 3, 5, 7, or even more pumping stages (sections), with each stage adding to the head. A typical 4" multi-stage pump can pump water up to 60, 80, or even 120 meters in height while maintaining a body diameter of around 95–98 mm. The motor is usually wet-rotor (cooled by flowing water) and located below the pump section.
These pumps are designed for continuous operation and long hours of continuous running. Their Achilles' heel is water quality – if the water contains sand or other abrasive particles, they wear out faster.
Peripheral and centrifugal submersible pumps (for wells)
Simple single-stage pumps with a higher flow rate but lower head – usually up to 30–40 meters – are commonly used for dug wells. They are cheaper, easier to maintain, and suitable for shallower sources with a high water level. These are precisely the pumps you find in a typical garden or household water supply.
Drain and sewage pumps
These do not belong in wells. They are designed for pumping larger volumes of water containing fine impurities (construction pits, basements, floods). They do not have sufficient discharge, their motor is not dimensioned for long-term operation, and they do not meet hygiene requirements for potable water. The difference between clean water pumps and sewage pumps is discussed in detail in the article Clean water pump vs. sewage drain pump: what is the difference and when to use which.
How to choose a pump according to depth: specific practical scenarios
We now move to the most practical part. Based on experience from real customer orders, I have compiled typical scenarios and recommendations for each of them.
Scenario 1: Dug well, water depth up to 10 m, diameter 100 cm
This is the most common case for older family homes and cottages in the countryside. The well is dug, masonry or concrete ring, the dynamic water level is around 6–8 meters from the surface. We need water to the house (about 10 meters above the level) and also to the garden.
Recommendation: A single-stage submersible pump with a lift of 20–30 m, flow rate of 2–4 m³/h, power of 370–750 W. The pump diameter is not a limitation. It is necessary to ensure a check valve on the discharge nozzle and a floating switch (protection against dry running). The total pumping head in this case will be about 18–20 m (8 m depth + 10 m lift + losses), which a standard pump can handle without problems.
Scenario 2: Dug well, water depth 12–18 m
Deeper dug wells are less common, but in some areas (e.g. on slopes, in dry areas) quite common. Here we already need a pump with a higher lift. A single-stage pump with a maximum lift of 30 m may be at the limit – especially if water has to be pumped up significantly higher in the house.
Recommendation: A two-stage or multi-stage pump with a lift of 40–60 m, power of 750–1100 W. Be careful, higher power does not necessarily mean higher flow – always check the performance/flow curve (QH curve) in the pump's technical documentation.
Scenario 3: Drilled well, casing 110 mm (4"), dynamic level 20–40 m
This is today the most common scenario for new family homes outside urban areas. The well is drilled to 40–60 meters, the static water level is around 15–25 meters, and it drops to 30–40 meters during pumping. The house has two bathrooms, a garden of 500–800 m², and requires potable water supply.
Recommendation: A 4" multi-stage submersible pump with an outer diameter of max. 96–98 mm, lift of 60–80 m, flow rate of 2.5–4 m³/h, power of 750–1500 W. The pump is ideally installed 2–3 meters above the bottom of the well (protection against sediment intake) and at least 1–2 meters below the dynamic level (protection against dry running). A check valve must be installed directly above the pump on the discharge pipe.
Scenario 4: Drilled well, casing 110 mm, dynamic level 50–80 m
Deep wells are typical for poor geological conditions or for areas with low groundwater levels. A dynamic level of 60–80 m places extreme demands on the pump – and most cheap submersible pumps from catalogs simply cannot cope with such a lift at a real flow rate.
Recommendation: A 4" multi-stage pump with 7–10 stages, a lift of 100–130 m (at a nominal flow rate of 2 m³/h), power of 1.5–2.2 kW. Here it is important to look not at the maximum lift (at zero flow), but at the lift at the required flow rate – for example 2 m³/h. This is exactly what the QH curve is for. In addition, it is necessary to dimension the cable supply – with long cables (50+ meters), voltage losses increase and the pump may have problems with starting or motor overheating.
Scenario 5: Garden pumping from a pond or tank, not from a well
In this case, a submersible pump from a borehole is not the right choice. For garden pumping from an open source, a surface pump or a self-priming pump is more suitable – or a gasoline pump if electricity is not available. For example, Gasoline water pump BZP-20 is self-priming and suitable precisely for garden applications from surface water sources – irrigation, pool filling, water transfer. For even larger garden pumping volumes, there is Gasoline pump BZP-30 with a power of 4850 W and a lift of 30 m.
Technical parameters you must compare when choosing
When choosing a submersible pump, it is not enough to look only at the price and power in kW. Here is a list of parameters you should focus on and what each of them means in practice:
- Outer diameter of the pump body – must be at least 10 mm smaller than the inner diameter of the well casing
- Maximum lift (Hmax) – the height in meters to which the pump can push water at zero flow; in practice, we work at 60–70 % of this value
- Maximum flow rate (Qmax) – liters per minute or m³/hour at zero lift; the actual flow rate at your pumping head will be lower
- QH curve – the graphical dependence of flow and lift; the most important parameter for real dimensioning
- Power (W or kW) – electricity consumption; also affects the dimensioning of the electrical installation and fuses
- Maximum submersion depth – some pumps are not designed for submersion deeper than 20–30 m (due to pressure on mechanical seals)
- Maximum temperature of the pumped water – irrelevant for wells and boreholes (groundwater is 8–12 °C), but important for special applications
- Maximum granulometry of solid particles – how large in mm the solid particles the pump can handle; important for sandy wells (typically max. 0.2–0.5 mm)
- Length and cross-section of the power cable – extending the cable must be done with the correct cross-section, otherwise voltage drop and motor failure may occur
- Motor protection – IP rating (minimum IP68 for submersible pumps), protection against dry running, thermal protection
Installation and safety components without which the pump will not last long
Even a properly selected pump is the basis, but its lifespan and reliability depend significantly on what we install around it. From practice, I know that customers who save on accessories often return the pump for warranty much sooner – in most cases, the failure was caused by incorrect installation, not a product defect.
Check valve
The check valve must be installed directly on the pump's discharge nozzle (or on the first meter of pipe above it). It prevents backflow of water into the pump and well when turned off, thus protecting the piping from water hammer and the pump from sudden reverse rotation. Without a check valve, it can happen that after each shutdown, water flows back into the well and when restarting, the pump "battles" with an empty pipe.
Dry running protection
If the pump runs without water, the motor overheats (loses cooling) and is destroyed in a few minutes. Protection can be achieved with electrodes (level sensors), a float switch, or an external current relay (monitors motor current – current drop = pump is pumping air). This protection is absolutely necessary for low-yield wells.
Pressure expansion tank
If the pump supplies the household water supply network, a pressure tank (hydrophore) reduces the number of pump starts per hour. Without a tank, the pump would start with every tap opening – up to 50–100 times a day in a typical house. This dramatically shortens the motor and starter capacitor lifespan. The tank size is dimensioned according to the pump flow – roughly 20–50 liters for a typical family home.
Supply pipe and its diameter
The discharge pipe from the pump to the house (or to the hydrostatic tank) must have a sufficient diameter – too narrow a pipe increases friction losses and reduces the actual performance. For pumps up to 3 m³/h, a DN 25 (1") pipe is sufficient, for more powerful pumps DN 32 (1¼") or DN 40 (1½") is suitable. The pipe in the well must be made of a material suitable for potable water (PE, stainless steel) and must withstand pressure.
A detailed step-by-step installation procedure can be found in the article Installation of a submersible pump in a well or borehole: step-by-step guide.
Choosing a pump and well yield: what should not be overlooked
One of the most serious mistakes is choosing a pump that is too powerful for a well with low yield. The yield of a well (or borehole) is the amount of water the source can provide per hour without the water level dropping below the pump. A typical family well has a yield of 0.5–2 m³/hour.
If we install a pump with a flow rate of 4 m³/h into a well with a yield of 0.8 m³/h, the pump will exhaust the water supply within 15–20 minutes of operation, start pumping air, and without protection against dry running, it will be damaged. Therefore, it is important to know the hydrological assessment of the well (yield) – this document should be provided by the driller to every customer.
If the well yield is insufficient for the required flow rate, the solution is an accumulation tank. The pump from the well fills the tank (e.g., 500–1000 liters) at a low but continuous flow rate, and the household is supplied from the tank by another pump unit with a higher flow rate. This solution is more expensive to implement, but reliably solves the problem of low yield.
Protecting the pump and extending its lifespan
A submersible pump should last 8–15 years with proper installation and maintenance. Practical experience shows that early failures usually have the same causes:
- Dry running – the most common cause of motor damage; solution: water level sensor
- Too many starts per hour – without a sufficient expansion tank, the motor cycles dozens of times per hour; solution: a pressure tank with sufficient capacity
- Sand and sediments – erosion of impellers in sandy wells; solution: filtration, use of a pump tolerant to fine sand
- Incorrect voltage and current – voltage drop with long cables or weak fuses; solution: correct cable cross-section, protective relay
- Corrosion from aggressive water – hard or iron-rich water; solution: stainless steel pump or stainless steel components
A detailed guide on winter protection and long-term maintenance of submersible pumps can be found in the article Maintenance and winterization of pumps: how to extend the lifespan of gasoline and submersible pumps. If you are troubleshooting – the pump is not pumping, has lost pressure, or is making unusual noises – see the article Common pump problems: why it doesn't pump, loses pressure, or overheats.
When a submersible pump is not enough: when to reach for a gasoline pump
A submersible pump requires a permanent electrical connection and a fixed installation. There are situations where this is not possible or suitable:
- A garden cabin without electricity
- Emergency pumping during power outages (floods, emergency situations)
- Construction – water supply before electrical installation is completed
- Agricultural work far from electricity sources
In these cases, a gasoline pump is the right choice. For example, Gasoline pump H-BZP-30 with a power of 9690 W and a lift of up to 95 m can pump water from depth and to a height comparable to many submersible pumps – without the need for an electrical grid. For smaller garden and portable use, there is Gasoline pump BZP-10 – lightweight, self-priming, suitable for rough terrain. A comparison of submersible and gasoline pumps in terms of use can be found in the article How to choose a water pump: gasoline, submersible or drainage?
Frequently asked questions (FAQ)
How do I determine the diameter of my well if I don't have documentation?
The most reliable way is to contact the company that drilled the well – they should have the documentation. If this is not possible, you can estimate the diameter with simple measurements: lower a rigid rod with a crossbar into the well and measure the diameter at the point where the crossbar rests on the casing. Alternatively, call a hydrogeologist or service company – most offer a free inspection of the well and issue a report. Never rely solely on an estimate – a diameter of 110 mm vs. 125 mm is a difference that determines whether the pump will fit or not.
Can I install a more powerful pump than the yield of my well?
Technically yes, but without protection against dry running, it is risky. A more powerful pump will quickly drain the well, start drawing air, and the motor will be damaged. If you need a higher instantaneous flow rate than the well yield provides, invest in an accumulation tank – the pump from the well fills the tank slowly, and you draw water from the tank using a more powerful unit. This solution is safe and very common in practice.
What is the minimum distance of the pump from the bottom of the well?
The recommended minimum distance of the pump from the bottom of the well is 1 to 2 meters. The reason is simple: fine sediment, sand, and silt settle at the bottom of the well, and if the pump draws water too close to the bottom, these solid particles damage the impellers and clog the filters. At the same time, the pump must be submerged at least 0.5–1 meter below the dynamic water level to prevent it from running dry.
What does IP68 protection mean for a submersible pump?
IP68 protection means that the device is permanently protected against water ingress when submerged to a depth specified by the manufacturer (usually 20–50 m). For submersible pumps in wells and boreholes, IP68 is the minimum required standard. Pumps with lower protection (e.g., IP67) are waterproof but not intended for long-term submersion under pressure, which is insufficient for wells and boreholes.
Do I need a pressure tank (hydraulic accumulator) with a submersible pump?
There is no legal obligation, but from a practical standpoint, a pressure tank is strongly recommended for any pump supplying a household. Without a tank, the pump turns on every time a tap is opened – in a house with two bathrooms, this can happen 80–120 times a day. Motors and starting capacitors are not designed for such frequent operation and wear out prematurely. A pressure tank with a volume of 24–50 liters will solve the problem at a reasonable price.
Can I extend the power cable of the pump with a cheap extension cord?
Absolutely not. Extending the power cable of a submersible pump must be done using a cable with sufficient cross-section – for greater distances (20+ meters) and power of 750 W or more, a minimum cross-section of 2.5 mm² is required. A thin extension cord causes voltage drop, the motor receives less power, overheats, and the winding burns out. In addition, the connection must be waterproof (submersible – suitable connector or sealed joint) – an improvised insulated joint in water is a direct risk to your life.
Conclusion: the correct choice is the basis for long life
A submersible pump is a long-term investment – a correctly selected and installed pump can last for a decade or longer without major problems. An incorrectly selected or poorly installed pump will cause problems already in the first season. The key parameters are always three: the diameter of the water source (determines the maximum pump diameter), the dynamic water level (determines the required head), and the yield of the source (determines the maximum flow rate you can pump without the risk of depletion). In addition, every installation should include a check valve, dry-run protection, and a pressure tank.
If you are unsure about the selection for a specific situation – do not choose blindly based on price. Read the technical documentation of the well, determine the yield and static/dynamic water level, and then compare the QH curves of the pumps that are in question. All other questions about selection, performance, and installation can also be found in the article Common questions about water pumps: selection, performance, depth, consumption, and spare parts.
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