Upper tank level control supplied by lower tank

Among the variations of automatic level systems for the control of water reservoirs, the level control of the upper tank supplied by the lower tank is the most common, this system is widely used to fill water tanks in places where artesian wells, cisterns or in rural properties where there is a need to take water from dams to reservoirs on the property.

This is speaking of residential systems without counting the countless industrial uses, whether in processes, machines and equipment for controlling fluid systems.

In this system, the level sensor used is a float that acts as a switch and can be an open or closed contact for the command system, for this system three sensors are used, two of which are used to control the level of the main reservoir (upper tank) and one for minimum level control of the lower tank, this sensor for the lower tank is a protection so that the pump is not activated when there is no liquid in the liquid source tank.

For the upper tank level control system supplied by the lower tank the following materials are used:

  • 3 level sensors;
  • 1 contactor (compatible with the power of the pump to be connected);
  • 1 Electric water pump.

The electrical diagram below represents the connection of the cables for this circuit.

Upper tank level control supplied by the lower tank.


In this system, two sensors control the level of the main tank and one sensor controls the minimum level of the lower tank, this sensor in the lower tank works as a protection against lack of water, preventing the pump from wearing out or burning due to dry starting. The sensors in the upper reservoir work in the open contact position while in the lower reservoir the sensor works in the closed contact position.

The basic condition for the system to work and the pump to pour the liquid into the upper reservoir is that there is liquid in the lower reservoir above the minimum level stipulated by the protection sensor when the liquid above the minimum this sensor is close and allows the upper reservoir to control starting and stopping the pump.

In the upper tank when the liquid level starts to drop and is below the maximum level sensor, the contact closes, but the pump is not yet activated because the second sensor of the lower tank (minimum level sensor) is not yet closed. The moment the liquid is below the minimum level sensor of the upper tank, the pump is started and starts to draw the liquid from the lower tank to the upper one.

In the contactor, a normally open contact is in parallel with the upper tank level sensors providing a seal contact, this way when the liquid starts to rise and opens the low-level sensor of the upper tank again, the pump still does not turn off providing that the top tank fills, only turning off the pump when the top-level sensor of the top tank is opened, in series with these sensors is the bottom tank sensor that is able to shut down the entire system whenever it is open or when there is no liquid in the bottom tank to be pumped up.

Level sensor.

Security for your pump.

With the use of the sensor in the lower tank, a safety element for the pump is prevented from being activated when there is no liquid in the tank, the famous dry failure.

Considering that of the entire upper tank level control system supplied by the lower tank, the pump is the most expensive element the addition of a simple level sensor is a great case to think about

This described system can be used both in systems for single-phase and three-phase pumps, the control is the same for any pump use, requiring only the correct dimensioning of the contactor for the different pump powers.

About the author

Henrique Mattede is a Professor of Electrotechnics and a technology enthusiast. Creates videos and articles about electrical on the internet to bring free knowledge to