Two pumps sit side by side on a supplier listing. One says 20 metres maximum head, the other says 18 metres. The 20 metre pump looks like the better buy, and for a great many commercial installations it is the one that will fail to do the job. The problem is not the pump. It is that a water pump specification sheet is written in a language most buyers were never taught, and the two figures that get printed largest are the two that tell you least about what the pump will actually deliver on your site.
This guide walks through a commercial pump datasheet line by line: what each figure means, which ones decide whether the pump works, and which ones are marketing headlines with a technical definition hiding behind them. It is written for facilities managers, contractors and procurement staff specifying pumps for plant rooms, pumping stations, construction sites and commercial drainage, not for domestic buyers.
The Two Headline Figures on Every Pump Listing
Almost every pump is advertised on two numbers: a maximum head and a maximum flow rate. On a European datasheet these appear as Hmax and Qmax. Both are real, measured, honest figures. Both are also endpoints, and an endpoint is not an operating condition.
What Hmax Actually Means
Hmax is the total head the pump develops at zero flow. It is the shut-off head: close the discharge valve completely, and this is the pressure the pump can hold. At Hmax the pump is delivering nothing at all.
The most useful way to think about Hmax is not as a height the pump can lift to, but as the maximum resistance the pump was designed to overcome. That resistance is the vertical lift plus every friction loss in the pipework, not just the lift. A pump quoted at 20 metres facing 15 metres of static lift has far less margin than it appears to, because the pipe run, bends, valves and strainer all add head the pump must also overcome.
There is no reliable rule of thumb for converting Hmax into a usable working head. The relationship depends entirely on the shape of that specific pump’s curve. Anyone offering you a fixed percentage to derate by is guessing. You have to read the curve.
What Qmax Actually Means
Qmax is the flow the pump delivers at zero total head. Free discharge, outlet open, at the pump’s own level, with no lift and no restriction. It is the other endpoint of the same curve.
UK commercial datasheets quote flow in litres per minute or cubic metres per hour, sometimes both. The conversions worth committing to memory are 1 m³/h = 16.67 l/min and 1 l/s = 3.6 m³/h. If you are reading a US datasheet, note that a US gallon is 3.785 litres while an imperial gallon is 4.546 litres. Those differ by 20 per cent, and gallons per minute on an American sheet are always US gallons.
Why Maximum Head and Maximum Flow Can Never Happen Together
This is the single most useful thing to understand about a pump specification, and it is the reason the two headline figures mislead so consistently.
Hmax and Qmax are the two axis intercepts of one continuous curve. Head and flow are inversely related along that curve: add resistance and flow falls, remove resistance and head falls. Hmax occurs at zero flow. Qmax occurs at zero head. A pump advertised as “20 m head, 400 l/min” delivers 20 metres at zero litres per minute, and 400 litres per minute at zero metres. Every real operating point sits somewhere on the curve between those two extremes, and neither headline figure is achievable in any installation you will ever build.
The pump curve (red) runs between the two headline figures. The system curve (grey) is what your installation demands. Where they cross is the only condition the pump will ever run at.
The Duty Point Is the Number That Actually Matters
The duty point is where the pump curve crosses the system curve. The pump curve is what the pump produces. The system curve is what your installation demands, and it is the sum of the static head (the vertical lift from liquid level to discharge point), the friction head (losses in pipe, bends, valves, non-return valves and strainers) and any residual pressure needed at the outlet.
You do not get to choose the duty point independently. The pump will find it. Your only levers are which pump you buy and how you install the pipework. This is why sizing from Hmax is so damaging: it selects a pump whose duty point sits far to the left of where it should, in a region where recirculation, vibration and high radial loads chew through seals and bearings.
The best efficiency point (BEP) is the flow at which that impeller is most efficient. Good practice is to select a pump whose duty point sits within roughly 10 per cent of BEP flow, and to treat about 70 to 120 per cent of BEP flow as the acceptable operating window. Outside that band, life expectancy falls away regardless of how well the pump is built. Our guide to understanding pump head and flow rate covers how to calculate total dynamic head for your own system, and how to read a pump performance curve takes the curve itself apart in detail.
Reading the Motor Data
P1 and P2 Are Not the Same Number
A good datasheet gives two power figures. P2 is the shaft power, the mechanical power delivered at the motor shaft. This is the figure manufacturers headline as the pump’s rated power. P1 is the input power, the total electrical power drawn from the supply. P1 is what your meter sees, and what your cable and protective device have to carry.
The gap between them is the motor’s inefficiency, and it is not small. A typical small submersible quoted at P2 of 0.55 kW can draw a P1 of 0.85 kW. A buyer sizing the electrical supply from the headline 0.55 kW figure is 35 per cent short. On a bank of pumps, or on a generator-backed site, that is the difference between a working installation and nuisance tripping.
Watch the horsepower conversion too. European datasheets generally use metric horsepower (0.7355 kW), while US sheets use imperial horsepower (0.7457 kW). The difference is small on its own, but stacked on a P1 and P2 mix-up it produces a materially wrong figure. Work in kilowatts where you can.
The 50 Hz and 60 Hz Trap
This one catches out experienced buyers. A great many pump datasheets circulating online are American or Japanese documents rated at 60 Hz. The UK supply is 50 Hz, and an induction motor on 50 Hz runs about 17 per cent slower.
The affinity laws make that speed drop bite hard. Flow is proportional to speed, so it falls about 17 per cent. Head is proportional to the square of speed, so it falls about 31 per cent. Power is proportional to the cube. A pump advertised at 30 metres of head on a 60 Hz sheet delivers somewhere around 21 metres on a UK supply. It runs, it sounds perfectly healthy, and it quietly misses the duty. Always check the frequency stated on the sheet before you compare two pumps.
Voltage, Phase and Full Load Current
UK single phase is 230 V, three phase is 400 V line to line, both at 50 Hz. Three phase is normal for commercial and industrial pumps above roughly 2.2 to 3 kW, though the exact crossover varies by manufacturer. Datasheets write these as 1 x 230 V ~ and 3 x 400 V ~.
Full load current is the current drawn at rated load, and it is what sizes your cable, breaker and overload, not the rated kW. Bear in mind that direct on line starting draws several times full load current for a few seconds. On a site with a constrained supply, or one running on a generator sized from running current alone, that starting surge is what trips the installation. At 50 Hz a two pole motor runs at roughly 2,800 to 2,900 rpm under load and a four pole motor at roughly 1,400 to 1,450 rpm.
Protection, Insulation and Duty Cycle
What an IP Rating Does and Does Not Promise
The IP code has two digits. The first is protection against solids and dust, the second against water.
| Code | First digit (solids) | Second digit (water) |
|---|---|---|
| 4 | Objects over 1 mm, most wires and screws | Splashing water from any direction |
| 5 | Dust protected, ingress not fully excluded | Low pressure jets from any direction |
| 6 | Dust tight, no ingress | Powerful jets |
| 7 | Not applicable | Temporary immersion, 1 m for 30 minutes |
| 8 | Not applicable | Continuous immersion, depth and duration as stated by the manufacturer |
That last row is the one that costs people money. IP68 does not mean unlimited submersion. The second digit 8 is defined against conditions the manufacturer specifies, which is exactly why a good datasheet lists “IP 68” and “maximum immersion depth: 20 m” as two separate lines. The IP code on its own tells you nothing useful for installation planning. Exceed the stated depth and the cable gland and shaft seal see a pressure they were never rated for, water reaches the windings, and you have a rewind rather than a repair.
IP55 is common on surface mounted pumps. It is dust protected and resists low pressure jets, but it is not suitable for submersion or for standing water. Fitting an IP55 surface pump in a plant room or chamber that can flood is a frequent and expensive misapplication.
Insulation Class
Insulation class is the maximum temperature the winding insulation can sustain long term. Class B is 130 °C, Class F is 155 °C and Class H is 180 °C, each with a permitted temperature rise above a reference ambient of 40 °C.
A detail worth knowing when comparing two apparently identical pumps: a motor built to Class F insulation but permitted only a Class B temperature rise, often written “F/B”, has a deliberate thermal margin designed in. That margin substantially extends winding life, and it is a genuine quality signal rather than a marketing one.
Duty Cycle and Starts Per Hour
Duty types are classified S1 to S10. Only the first few appear on pump datasheets in practice.
| Duty | Meaning | Suitable for |
|---|---|---|
| S1 | Continuous duty. Constant load, long enough to reach thermal equilibrium. | Any permanent installation |
| S2 | Short time duty. Runs, then must cool fully back to ambient. | Occasional or emergency use |
| S3 | Intermittent periodic, usually with a percentage such as S3 40%. Never reaches equilibrium, relies on rest periods. | Cyclic duties within the stated percentage |
Many low cost drainage and dirty water pumps are S3 rated. Run one continuously and the windings exceed their insulation class temperature and degrade. It will work perfectly for weeks and then fail without warning. For any permanent installation, specify S1.
Check maximum starts per hour as well, often quoted as something like 20/h. In a sump or pumping station the controls and the chamber volume set the cycling rate, and exceeding the stated starts per hour overheats the motor whatever its S rating says. Sizing the chamber and setting the float switch levels correctly is what keeps that number in range.
The Limits That Are Easiest to Miss
Maximum immersion depth is set by the cable gland and seal pressure rating, not by the IP code. If the datasheet does not state one, and plenty of imported sheets do not, ask before you install. There is often a minimum submergence requirement too, because submersible motors are usually cooled by the liquid around them. Run one partly exposed and it overheats even at correct hydraulic load.
Maximum liquid temperature on standard water pumps is typically 35 to 40 °C. Commercial laundry effluent, kitchen waste, boiler blowdown and some process water routinely exceed that. Above the limit the elastomers degrade and, on a submersible, the motor loses its coolant. Failure is thermal and comparatively quick.
Maximum solids size is a spherical measurement. It says nothing about fibrous material, and UK commercial wastewater is full of wet wipes, sanitary products and rag, none of which are spheres. A vortex impeller, a single channel impeller and a grinder behave completely differently at the same quoted particle size, so read the impeller type alongside the number. Specify for the worst thing the pump will ever see, not the typical thing. Our guide on how to choose a sewage pump goes into impeller selection in more depth.
Connections and Materials
BSP, DN and NPT
European datasheets quote threaded connections in BSP, sometimes written “GAS”. The nominal size refers to the approximate bore of the pipe, not the measured thread diameter: a half inch BSP male thread measures 20.955 mm across the threads, not 12.7 mm.
| Nominal size | Thread OD (mm) | Threads per inch |
|---|---|---|
| 1/2″ | 20.955 | 14 |
| 3/4″ | 26.441 | 14 |
| 1″ | 33.249 | 11 |
| 1 1/2″ | 47.803 | 11 |
| 2″ | 59.614 | 11 |
BSPP (marked G) is parallel and seals on a bonded seal or O ring. BSPT (marked R) is tapered and seals on the thread itself. Wrapping PTFE tape round a parallel thread and expecting it to seal is a common site error. NPT threads on US datasheets are not interchangeable with BSP, having a different thread angle and pitch. They will start, they will feel tight, and they will leak or gall. On flanged connections you will see DN sizes instead, where DN50 is roughly 2 inches and DN80 roughly 3 inches, and the flange drilling (PN10 or PN16) must match as well.
Materials and Seals
“Stainless steel” without a grade is not a specification. 304 pits and fails in chloride environments, which includes coastal sites, pool plant and de-icing salt runoff. 316 adds molybdenum and holds up. Ask which one you are getting.
The casing is rarely the wear point in any case. The impeller is what erodes in abrasive water, and the seal faces are what fail. Silicon carbide against silicon carbide is the durable pairing for gritty duty; carbon and ceramic is the cheaper pairing and wears far faster. On a permanent installation, look for a double mechanical seal in an oil filled chamber, which gives you a second barrier and, where a probe is fitted, warning before water reaches the windings. That is the difference between a seal change and a rewind.
The Standards Behind the Numbers
ISO 9906 and What a Published Curve Guarantees
BS EN ISO 9906 governs hydraulic performance acceptance testing for rotodynamic pumps, and it defines acceptance grades with real tolerances. Published tolerance bands for the commonly used grades run approximately as follows, though you should confirm the exact figures and grade with your supplier:
| Grade | Flow tolerance | Head tolerance | Typically used for |
|---|---|---|---|
| 1B | ±5% | ±3% | Tightly specified process duties |
| 2B | ±8% | ±5% | General industrial and chemical |
| 3B | ±9% | ±7% | Slurry and general duty |
A pump accepted to Grade 3B may legitimately deliver 7 per cent less head than the published curve and still pass. On a 20 metre duty that is 1.4 metres. If your system was sized with no margin, a fully compliant pump can still fail to do the job. Worth asking which grade a curve was verified to, and whether the individual unit was tested at all rather than the type.
IE Efficiency Classes and MEI
Motor efficiency classes IE1 to IE4 are defined under IEC 60034-30-1. Under current GB ecodesign rules, three phase motors from 0.75 kW upward generally have to meet at least IE3. There is an exemption that matters a great deal here: motors designed to run wholly immersed in water are outside the scope. A submersible pump datasheet showing no IE class is not a compliance failure, and its absence tells you nothing about quality. A surface mounted three phase motor above 0.75 kW that is not IE3, on the other hand, is a genuine flag.
Separately, clean water rotodynamic pumps fall under retained ecodesign rules requiring a Minimum Efficiency Index of at least 0.40 since January 2015, alongside a legal obligation to publish the MEI value and performance curves. If a supplier cannot produce an MEI figure and a curve for an in-scope pump, that is a compliance red flag rather than a documentation gap. Note that MEI is quoted for the full impeller diameter, so a trimmed impeller version of the same pump will be less efficient than the headline figure suggests.
WIMES, If You Are Buying for a Water Company Framework
WIMES stands for Water Industry Mechanical and Electrical Specifications. It is a set of standardised specifications and data sheets maintained by the Pump Centre in collaboration with the UK water companies, supported by life cycle cost models. Documents are purchased individually.
The important distinction is that WIMES is a procurement and contractual specification, not legislation. You comply because a water company contract or a MEICA designer requires it, not because a statute does. Broadly, the 1.xx series covers mechanical equipment and the 3.xx series covers electrical, with WIMES 1.02 covering submersible pump units and WIMES 3.03 covering LV motors. If you are tendering into a water company framework, expect the specification to be named in the contract documents and check which issue applies before you quote.
A Line by Line Walkthrough
Pulling it together, here is what a typical commercial pump datasheet gives you and what each line should prompt you to do.
| What the sheet says | What it actually means | What to do about it |
|---|---|---|
| Hmax / max head (m) | Head at zero flow. Shut-off condition. | Ignore for sizing. Use the curve. |
| Qmax / max flow (l/min, m³/h) | Flow at zero head. Free discharge. | Ignore for sizing. Use the curve. |
| Performance curve | Every real operating point. | Plot your system curve on it and read the intersection. |
| P2 (kW) | Shaft power. The headline rating. | Use for pump comparison only. |
| P1 (kW) | Electrical input power. | Size supply, cable and running costs from this. |
| Voltage / phase / frequency | Supply required. | Confirm 50 Hz. Check phase availability on site. |
| Full load current (A) | Current at rated load. | Size cable, breaker and overload. Allow for starting surge. |
| IP rating | Ingress protection class. | Meaningless alone. Read alongside immersion depth. |
| Max immersion depth (m) | The real submersion limit. | If absent, ask before installing. |
| Insulation class | Winding temperature capability. | F or H preferred. F/B indicates built-in margin. |
| Duty (S1, S3 40%) | How continuously it may run. | S1 for anything permanent. |
| Max starts per hour | Cycling limit. | Check against chamber volume and control settings. |
| Max liquid temperature | Thermal limit of seals and cooling. | Check against peak temperature, not average. |
| Max solids size (mm) | Largest sphere that passes. | Read with impeller type. Consider rag, not just size. |
| Discharge (BSP, DN) | Connection size only. | Do not size pipework from it. Check BSP against NPT. |
| Materials | Corrosion and abrasion resistance. | Ask for the stainless grade. Check impeller and seal faces. |
| NPSH required (m) | Suction condition the pump needs. | On suction lift, calculate NPSH available with a margin. |
| Weight (kg) | Handling and lifting implication. | Over ~25 kg, design the lifting arrangement. |
On NPSH specifically, one nuance is worth carrying with you: NPSH required is defined at the point where head has already dropped 3 per cent because cavitation has begun. It is a measured threshold, not a safe operating point, which is why a margin of roughly 0.5 to 1 metre is standard practice. NPSH required also rises with flow, so a pump that is comfortable at duty can cavitate when the system runs faster than designed. Our guides on pump suction lift and NPSH and on pump cavitation cover this properly.
Recommended Pumps from AES Rewinds
Specifying a pump and want the full datasheet before you commit? At AES Rewinds we stock a comprehensive range of pumps for industrial and commercial applications, and we can supply complete performance curves and technical data for anything in the range.
Browse our full range:
- Industrial water pumps for commercial transfer, boosting and process duties
- Drainage pumps and dirty water pumps for site and building drainage
- Pump controls and panels for duty cycling, level control and protection
If you have a duty point and a system curve, our team can match a pump to it. If you only have a lift height and a rough flow, we can help you work the rest out. Contact us for advice on specifying a pump for your application.
Frequently Asked Questions
What is Hmax in pumps?
Hmax is the maximum head a pump can develop, measured at zero flow. It represents the total resistance the pump was designed to overcome, combining vertical lift and pipework friction. At Hmax the pump delivers no water at all, so it should never be used as a sizing figure. See the section above on what Hmax actually means for the full explanation.
What is Qmax on a pump?
Qmax is the maximum flow rate a pump can deliver, measured at zero head. That means free discharge with the outlet open at the pump’s own level and no restriction in the pipework. Like Hmax it is an endpoint of the performance curve rather than a realistic operating condition.
How do you read a pump data sheet?
Start with the performance curve rather than the headline figures. Work out your system’s total dynamic head, plot it against the curve and find the duty point. Then check the practical limits in this order: P1 for electrical sizing, frequency, IP rating with maximum immersion depth, duty cycle, maximum liquid temperature, solids size with impeller type, and connection size.
What are the typical specifications for a commercial water pump?
A complete commercial datasheet gives flow and head ranges with a performance curve, P1 and P2 power, voltage, phase, frequency and full load current, IP rating, insulation class, duty type, maximum immersion depth, maximum liquid temperature, maximum solids size, discharge connection, materials of construction, seal specification, NPSH required and weight. If several of these are missing, ask for them rather than assuming.
What is the ISO standard for pump performance?
BS EN ISO 9906 covers hydraulic performance acceptance tests for rotodynamic pumps, defining acceptance grades with permitted tolerances on flow, head, power and efficiency. Related standards you will see referenced are IEC 60034 for rotating electrical machines, which governs duty types and insulation classes, and IEC 60529 for the IP code.
Does IP68 mean a pump can be submerged permanently?
No. The second digit 8 means continuous immersion under conditions the manufacturer specifies, not a universal depth or duration. The figure that actually governs installation is the stated maximum immersion depth, which is a separate line on the datasheet. IP68 also does not automatically confer resistance to high pressure jets, so it is not a licence to pressure wash the unit.
Why does my pump deliver less than the datasheet says?
The three usual causes are sizing from Hmax rather than the duty point, ignoring friction losses in the pipework, and comparing against a 60 Hz datasheet on a 50 Hz UK supply, which costs roughly 31 per cent of head. Undersized discharge pipework and a partially blocked strainer produce the same symptom by adding friction the original calculation never allowed for.
Key Takeaways
- Hmax and Qmax are the two ends of one curve. Hmax happens at zero flow, Qmax at zero head, and neither can occur in a working installation.
- The duty point, where the pump curve crosses your system curve, is the only condition the pump will actually run at. Size from it, and aim to sit near the best efficiency point.
- Size electrics from P1 and full load current, never from the headline P2 rating, which can understate the load by a third.
- Check the datasheet frequency. A 60 Hz sheet read on a 50 Hz supply overstates head by around 31 per cent.
- IP68 is meaningless without the stated maximum immersion depth beside it, and S3 duty pumps will not survive continuous running.
- Ask for the stainless grade, the impeller type and the seal face material. Those three decide how long the pump lasts in your water.
Related Articles
- Understanding Pump Head and Flow Rate, how to calculate the total dynamic head your system actually demands.
- How to Read a Pump Performance Curve, taking the curve apart line by line including the best efficiency point.
- How to Calculate Pump Sizing, a worked method for matching a pump to an industrial application.
- A to Z of Pump Terminology, a glossary for every term you will meet on a datasheet.
- Pump Suction Lift and NPSH Explained, essential reading before any surface mounted suction lift installation.