What diameter of a borehole or well do I need for a 3" submersible pump?
What well or borehole diameter do I need for a 3" submersible pump?
This question is among the most common ones we address with customers before they choose a home water well with a submersible pump. And it's not a trivial question – it has happened to us several times that a customer ordered a pump, brought it home, and only during installation into the borehole realized that it simply wouldn't fit. Result: returning the product, a new order, lost time, and in the worst case, another drilling company visit. This can easily be avoided if you know what to look for and how to correctly read the pump and borehole parameters.
In this article, we will explain in detail what the designation "3 inches" means for submersible pumps, what the actual outer diameter of the pump is, what the minimum requirements for the borehole or well diameter are, where common mistakes are made, and how to verify everything before purchasing or ordering. We will also look at the differences between boreholes and shaft wells and provide specific practical examples.
What does "3 inches" mean for a submersible pump – and why it's not enough to know just that
Submersible pumps are traditionally marked in inches according to the so-called hydraulic diameter, i.e., the diameter of the hydraulic part of the pump (the housing in which the impeller is located). This is not the outer diameter of the entire pump body. And it is precisely here that the most common mistake occurs.
A pump marked as 3" (three inches) has a hydraulic diameter of approximately 76 mm, which is 3 × 25.4 mm. However, the outer diameter of the entire pump body, including the motor, terminal box, and cable gland, can be as much as 93–98 mm, i.e., almost 10 cm. And this is the dimension that must fit into the borehole or well.
For safe installation, it is necessary that there is a clearance of at least 10–20 mm on each side between the outer casing of the pump and the inner wall of the borehole. This gap is not only for installation – it is essential for the water to circulate and cool the pump motor. If the pump is "snug" in the borehole without a gap, the motor overheats and its lifespan is reduced.
So when we talk about a "3 inch" pump, we are talking about a category, not an exact outer dimension. Always verify the specific outer diameter of the pump in the manufacturer's technical specifications, or in the product details in the online shop. Most 3" pumps have an outer diameter of 96–98 mm, some more compact models even 93 mm.
Minimum borehole diameter for a 3" pump
For 3" class pumps, the general recommendation is that the borehole must have an inner diameter of at least 110 mm. In practice, we most commonly encounter boreholes DN 110 and DN 125 (according to the outer diameter of the casing: 110 mm or 125 mm casing = inner clear diameter of approx. 100–115 mm).
It is important to distinguish:
- Outer diameter of the casing – this is the diameter of the steel or plastic pipe into which the borehole is drilled. This diameter is provided by the driller and is written in the borehole documentation.
- Inner clear diameter of the casing – this is the actual free space inside the borehole. The wall thickness of the casing is usually 5–8 mm, so you subtract twice the wall thickness from the outer diameter.
Concrete example: a casing DN 125 (outer diameter 125 mm, wall thickness 6 mm) has an inner clear diameter of 113 mm. A 3" pump with an outer diameter of 96 mm fits into it with a gap of about 8.5 mm on each side – this is an acceptable minimum, but for a smoother installation process and better cooling, a borehole with an inner clear diameter of at least 120 mm is better.
If you have an old borehole and do not know its exact parameters, it is advisable to contact the company that drilled the borehole or to request the borehole passport. If you don't have the passport, you can measure the diameter improvisationally – lower an object of known diameter (e.g., a PVC pipe) into the borehole and check into which diameter it fits without resistance.
What to do with a DN 100 or smaller borehole?
There are quite a few old boreholes in Slovakia, especially from the 80s and 90s, drilled to a diameter of DN 100 (outer diameter of the casing 100 mm, inner clear diameter only 88–92 mm). A 3" class pump simply won't fit into such a borehole.
In such a case, you have several options:
- Drilling a new, wider borehole – the cleanest solution, but also the most expensive. A new borehole today costs from 50 to 120 € per meter of depth, depending on the geological conditions.
- Use of a 2" pump – there are submersible pumps with an outer diameter of around 70–72 mm, designed specifically for narrow boreholes. However, they have lower performance.
- Surface suction water pump – if the water level is at most 7–8 m below ground level, you can consider a surface water pump, which does not require lowering the pump into the borehole. More on this can be found in the article Surface vs. Submersible Water Pump: Which is Better for My Water Source?
- Re-casing or enlarging the borehole – the driller can in some cases enlarge the borehole by milling, but it is not always geologically possible.
Wells (shaft, dug) and 3" pump – different rules
With dug or shaft wells, the situation is significantly simpler. Shaft wells typically have an internal diameter of 800 mm, 1000 mm or 1200 mm – which is much larger than the pump. The diameter is not an issue here.
With wells, other factors need to be considered:
- Well depth and water level – the pump must be submerged at least 0.5–1 m below the minimum water level (during pumping). You need to know how deep the water level is and how the well reacts to prolonged pumping.
- Presence of silt and impurities – dug wells are more prone to sedimentation. The pump should not rest on the bottom but be suspended at least 0.5–1 m above the bottom to avoid sucking up sediments.
- Electrical installation – you must bring electricity to the wet environment of the well with sufficient protection (IP rating, circuit breakers). This is a professional task for an electrician.
- Water quality – dug wells are more vulnerable to contamination. If you plan to use the water for drinking, read the article "Drinking water from a well through a water treatment plant: what must the system and pump meet?"
Practical examples from practice – what happens most often
Example 1: New borehole, unknown casing diameter
The customer had a borehole drilled and the company delivered a "passport" only in paper form, where it was written "borehole 125". The customer interpreted this as an internal diameter of 125 mm. In reality, it was the outer diameter of the casing, and the actual internal diameter was only 113 mm. The 3" pump with an outer diameter of 96 mm fit, but with minimal clearance. The installation took an unreasonable amount of time and we had to carefully guide the pump using guide lines.
Lesson: Always ask the driller for the actual internal clear diameter of the casing, not just the outer dimension.
Example 2: Old well with casing DN 100
The customer had a borehole from 1994, casing DN 100, internal diameter only 88 mm. He ordered a 3" pump based on a neighbor's recommendation. The pump did not fit into the borehole at all. The solution was to order a special 2" pump with power adjusted for a family house, which was slightly cheaper, but with lower performance. The customer now has to plan for a longer time to fill the garden.
Lesson: Always verify the actual clear diameter of the borehole before ordering. If you have an older borehole, assume DN 100 unless you have documentation.
Example 3: Shaft well, pump stuck on the cable
The customer had a dug well with a diameter of 1 000 mm and successfully installed a 3" submersible pump. The problem occurred during removal for service – the cable wrapped around the discharge pipe several times and the pump got stuck in the well. He had to call a technician.
Lesson: When lowering the pump into a shaft well, always secure the cable and hose together using tie wraps every 0.5–1 m. The pump must be suspended on a safety rope, not just on the discharge hose or pipe.
Example 4: Borehole DN 125, but with sediment at the bottom
The customer had a borehole that had not been used for several years. After reactivation, he wanted to use it for garden irrigation. When we started the pump, it began to suck up mud and fine particles. After several hours, the pump jammed. The borehole had to be cleaned – using a compressor or specialized technology – before the pump could be installed.
Lesson: A long-term unused borehole should always be cleaned before installing a pump. This service costs hundreds of euros, but it protects the pump, which is worth several times more.
What 3" pumps you can find on atria.sk and what you need to know before ordering
On atria.sk, there are several domestic water supply systems with submersible pumps in the 3" class. All are designed for boreholes or wells with sufficient diameter and differ mainly in performance, maximum lift height and purpose (drinking water vs. technical water).
Domestic water supply system with submersible pump 3 SQIBO - 0.55 / 50l for drinking water into the well is suitable for less productive sources and smaller households. The SQIBO 0.55 kW pump has an outer diameter in the 3" class, i.e. approx. 96 mm – before installation, always check the clear diameter of your borehole or well.
For households with higher consumption or a deeper borehole, the Domestic water supply system with submersible pump 3 SQIBO - 0.75 / 50l for water into the well and borehole with a 0.75 kW motor and higher pumping capacity – can handle deeper boreholes and higher water consumption.
If you are looking for a solution with intelligent pressure control, take a look at Domestic water supply system with submersible pump IBO 3 SDM24 / 50l - RTS for water into the well – this set has RTS pressure regulation, a practical feature for comfortable household water supply without noticeable pressure fluctuations.
For those interested in drinking water and irrigation from a well or borehole, there is Domestic water supply system with submersible pump 3Ti-20 / 50l - RTS for drinking water into the well and borehole. The 3Ti-20 pump is a proven model with a compact design suitable for most standard boreholes with casing DN 125 or larger.
If you are looking for a more economical option primarily for garden irrigation from a well, take a look at Domestic water supply system with submersible pump SCR-0.50 / 50l into the well, for irrigation – performance-wise suitable for smaller boreholes and shaft wells, cost-effective.
If you are unsure which model is most suitable for your water source and needs, also read the articles "How to choose a domestic water supply system: well, borehole or surface source?" and "What pump power do I need for a domestic water supply system?" – both will help you make an informed decision.
Technical parameters of the borehole and their correct reading – summary table
| Drilling designation | Outer Ø of casing | Wall thickness | Clear inner Ø | Suitable for 3"? |
|---|---|---|---|---|
| DN 100 | 100 mm | 5–6 mm | 88–90 mm | NO – pump will not fit |
| DN 110 | 110 mm | 5–6 mm | 98–100 mm | LIMITED – only for pumps with Ø ≤93 mm |
| DN 125 | 125 mm | 6 mm | 113 mm | YES – standard for most 3" pumps |
| DN 150 | 150 mm | 6–8 mm | 134–138 mm | YES – comfortable clearance, ideal |
| Borehole ≥800 mm | 800–1200 mm | — | ≥750 mm | YES – no diameter restrictions |
How to measure the diameter of a borehole – if you don’t have documentation
Not every customer has access to the borehole passport or can contact the company that drilled the well. If you are in this situation, you can help yourself:
- Take a rigid PVC pipe with an outer diameter of, for example, 100 mm and carefully lower it into the borehole. If it goes down without resistance, the clear diameter of the borehole is greater than 100 mm. Repeat the process with pipes of 110 mm, 120 mm, etc., until you find a pipe that fits tightly or slides down.
- If you have access to the borehole, you can also use a simple calibration string with a knot or a ball of precise diameter.
- In case of doubt, call a specialist – a hydrogeologist or a company dealing with boreholes. Measuring the diameter of a borehole is a quick and inexpensive task that will save you a lot of problems.
Depth of the borehole and pump length – what not to forget
Along with the diameter, you also need to consider the depth. Submersible 3" pumps are usually 40–70 cm long (depending on the number of stages). If your borehole is shallow and the water level is at maximum, the pump must be fully submerged – and there must still be enough space above it for the discharge elbow and to secure the cable.
The minimum required water depth above the pump’s suction inlet is usually 0.5 m. If the water level drops to this limit while the pump is still running, it will suck in air and may be damaged or burn out. Therefore, it is important to know the yield of the borehole – how many liters per minute or per hour the borehole can supply. If the yield is low and the pump power is too high, the pump will quickly drain the well.
This topic is discussed in more detail in the article "What pump power do I need for a home water supply?" – we recommend reading it alongside this article, as both together form a complete basis for the correct selection.
Material of the casing and its impact on installation
Borehole casings can be made of different materials – steel (galvanized or stainless) and PVC/HDPE plastic. Each material has different properties that can affect the installation of the pump:
- Steel casing – strong, but can corrode and corrosion can settle on the inner walls, reducing the effective clear diameter. For old steel boreholes, always account for the fact that the actual clear diameter may be 5–10 mm smaller than stated in the documentation.
- Plastic casing (PVC/HDPE) – a modern standard, smooth inner surface, no corrosion problems. The diameter does not change over time. Pump installation is smoother.
Proper installation – guidelines that will extend the life of the pump
Even if you have the correct borehole diameter and the right pump, a poor installation can quickly damage the pump. Here are the basic rules:
- Always start the pump on a stainless steel safety rope with sufficient load capacity (min. 50 kg). Never only on a hose or pipe.
- Secure the cable, rope and discharge hose together with tie wraps every 50–100 cm to prevent tangling.
- Suspend the pump at least 0.5–1 m above the bottom of the borehole. In silted boreholes, even higher.
- The pump must be placed at least 0.5–1 m below the minimum water level during pumping. If you are unsure of the water level, use a float switch or a dry-run switch.
- Before starting the pump, check that the discharge pipe is properly connected and that the cable is not damaged.
For more details on the correct procedure, see the article Assembling a home water supply with a submersible pump step by step.
Most frequently asked questions (FAQ)
3" pump – what is its actual outer diameter?
The designation 3" refers to the hydraulic (functional) diameter of the pump, not the outer diameter of the body. Most submersible pumps of class 3" have an outer diameter of the body in the range 93–98 mm. The exact dimension is always found in the technical sheet of the specific model. Always verify it before ordering, as differences between manufacturers do exist.
Can I use a 3" pump in a borehole with a casing DN 110?
It depends on the inner clear diameter of the casing. A DN 110 casing with a wall thickness of 5–6 mm has a clear diameter of only 98–100 mm. If the pump has an outer diameter of 96 mm, the clearance is only 1–2 mm on each side – which is too little for reliable installation and motor cooling. In such a borehole, we recommend using a compact 2" pump or considering another water source.
Do I need to consider the diameter if I have a dug well?
No – dug wells usually have a diameter of 800–1,200 mm, which is more than sufficient for any 3" submersible pump. In the case of dug wells, you need to consider other parameters: the depth and stability of the water level, yield, water purity, and the method of securing the pump inside the well.
How deep must the pump be submerged?
The pump must be submerged at least 0.5 m below the minimum water level that occurs during long-term pumping (not just the current still water level). At the same time, it must be at least 0.5–1 m above the bottom of the borehole to avoid sucking in mud and sediments. In practice, this means you need to know the dynamic water level in the borehole – i.e., the water level during pumping, not at rest.
What happens if the pump is too tight in the borehole?
If there is not enough clearance (at least 10 mm on each side) between the pump housing and the borehole wall, water cannot properly circulate around the motor and cool it. The motor overheats, which shortens the life of the winding insulation and causes premature wear. In extreme cases, the pump can weld to the casing wall upon contact. In addition, tight fitting makes installation and removal of the pump more difficult.
Can I check the diameter of my borehole myself?
Yes. The simplest way is to lower solid cylindrical objects (e.g., PVC pipes) with gradually increasing outer diameters. The pipe that still freely fits into the borehole, but the next larger one does not – the smaller one gives you the clear diameter of the borehole. Alternatively, contact the company that drilled the borehole or a hydrogeologist who can measure the diameter professionally.
Conclusion: correct dimensions save time, money and nerves
Selecting a 3" submersible pump is not only about performance or price. Before choosing a specific model, you must know three basic things: the clear inner diameter of your borehole or well, the outer diameter of the specific pump you are considering, and the dynamic water level in the borehole during pumping. If these three parameters fit together, you are already halfway to success.
In most cases, the combination of: a borehole with a casing DN 125 (clear diameter 113 mm) + a 3" pump with an outer diameter of 96 mm = a reliable installation with sufficient clearance for cooling. If you have an older borehole and do not know its parameters, always verify them before ordering – practical experience shows that most installation problems stem precisely from insufficient verification of dimensions before purchase.
If you have answered the question about the diameter and are interested in the next steps, read the article Pressure tank in a home water supply: what it is used for and how to set it up? or Maintenance and servicing of a home water supply: what and how often to check? – both will help you make full use of your water supply and extend its lifespan.
Do you have a question on this topic?
Having trouble deciding or dealing with a specific situation in your household? Write to us – we are happy to help.