Domestic Water Pump Sizing

Total dynamic head is just a sum of four things the pump has to overcome. The first is the static lift — the raw vertical distance from the water level in the suction tank to the critical point on the discharge side, usually the highest or furthest outlet. That distance has to be climbed regardless of flow. The second and third are the friction losses in the suction and discharge pipework, which grow with the square of the flow and depend on the pipe size, length, and fittings.

The fourth term is the one beginners forget: the pump must still leave usable pressure at the outlet. A tap on the top floor needs perhaps 1 to 2 bar to work properly, and that pressure is head the pump has to supply on top of the lift and losses. Converting it is simple — multiply bar by 10.197 to get metres — but leaving it out is a common reason a freshly commissioned system underperforms at the extremities.

Add the four together and you have the head the pump curve must reach at the design flow. The design flow itself is the peak demand trimmed by a diversity factor, because in a real building not every fixture runs at once. Pin those two numbers down — head and flow — and you have defined the duty point that everything else follows from.

Once the duty point is fixed, the power follows a short chain. Hydraulic power — the useful power actually delivered to the water — is ρgQH, the density times gravity times flow times head. It is the irreducible minimum; no pump can do the job for less. For water this works out conveniently, since a litre per second lifted one metre is just under ten watts.

Real machines are not perfect, so two efficiencies stand between hydraulic power and the switchboard. Dividing by the pump efficiency accounts for the losses inside the casing and impeller, giving the shaft power the motor must actually turn. Dividing again by the motor efficiency gives the electrical input power — what the cable and breaker have to carry. A small water pump at 65% pump and 90% motor efficiency draws roughly 1.7 times its hydraulic power from the supply.

The last step is practical rather than physical. You add a margin, here 15%, to cover an off-design duty point, ageing, and supply dips, then round up to the nearest standard motor frame. The standard kW series — 4, 5.5, 7.5, 11 and so on — is what manufacturers actually build, so the recommendation is always a motor you can specify and buy, not a theoretical figure.

A calculation like this is built for speed — enough to price a pump on a tender and size the cable and starter — but it leans on assumptions that final design has to firm up. The biggest is the loss figures. They should come from an actual head loss calculation on the real pipe routes, not a placeholder, because friction is where an optimistic estimate quietly undersizes the pump. If you have a Darcy or Hazen–Williams result for the suction and discharge runs, use it.

Efficiency is the second soft spot. The 65% default is a reasonable middle for small and medium pumps, but the true value depends on the specific machine and where the duty point sits on its curve. A pump run far from its best efficiency point can be markedly less efficient, which pushes the motor up a frame. Once a candidate pump is chosen, replace the default with the figure from its curve and re-check.

Finally, remember what this does not do. It does not draw a system curve and intersect it with the pump curve to confirm the real operating point, it does not stage multiple pumps or account for variable-speed control, and it does not check NPSH on the suction side — the separate guard against cavitation. Treat the output as a solid first cut for the proposal, then close those gaps when the design moves from screening to detailed.