Float Switch Connection: Tank Filling and Tank Emptying Explained
Mohit VekariaShare
For tank filling, the control should request pumping at the low level and stop at the high level; tank emptying needs the opposite sequence. The float's documented contact pair and the controller's input logic determine the connection. Identify the exact float model, switching states and input type before wiring. A wire-colour example from a different float is not a reliable connection guide.
Separate the water task from the electrical connection. A pump sending water into a rooftop tank and a pump draining a sump pit may use similar-looking floats but require different start and stop behaviour.
Choose the required sequence first
| Application | At the low level | At the high level |
|---|---|---|
| Fill a receiving tank | Request pumping, if source water and other permissions are healthy | Stop filling |
| Empty a collection pit | Stop draining before the pump runs without adequate water | Request pumping |
| Protect the source tank | Block the receiving tank's pump request when the source is low | Permit operation when the configured recovery level is reached |
| High-level alarm | Normally remain inactive at a healthy level | Signal an alarm at its defined alarm level |
This table states the required function. It does not assign colours or terminal numbers. MAC3's float-switch instructions provide different diagrams for filling and emptying, and distinguish single-function and dual-function versions.
Why “normally open” needs context
Open and closed describe an electrical contact. The water level, float position and reference state must also be defined. “Use the normally open wire” is incomplete if nobody has established what the maker calls the normal position.
An electrician should use the model's contact diagram and confirm switching on isolated equipment. Some floats expose changeover contacts; others have a fixed function. The spare conductor on a changeover float must be handled as the manufacturer specifies, not left as an exposed loose end.
Check whether the float carries power or sends a signal
A float may switch an approved motor load directly, operate a contactor circuit or provide a signal to a controller. These duties are different. A headline current rating does not by itself establish suitability for a motor's starting load.
Check the switch's motor-load rating, voltage and permitted duty against the actual circuit. Where the controller expects a voltage-free contact, do not inject an external mains supply into that input. Follow both the float and controller manuals.
Set the physical movement as well as the wiring
The float must move through its intended travel without catching on a pipe, tank wall or cable. Its fixing arrangement and free cable length affect the switching levels. Set the high stop level with room for incoming water to stop safely.
MAC3's instructions explain the role of the fixture point and free cable in the level difference, and call for an overflow provision in filling duty. Apply the instructions for the float actually supplied. Keep electrical joints and connections in the locations and enclosures approved for that product.
Check the source as well as the receiving tank
A low rooftop tank can request a run while the underground sump is empty. The receiving float does not know whether source water is available. A suitable source interlock must be part of the control arrangement.
The one-float versus two-float guide explains that system choice. Here the aim is to establish the intended contact states and verify the complete connection, rather than assume that adding a second float always works with every controller.
Test four conditions before leaving automatic mode enabled
Have the installer demonstrate low-level start, high-level stop, source-empty blocking and recovery after water returns. Observe the actual float movement and pump response. Keep people clear because an automatic pump can restart during a valid demand.
If the pump stops at the wrong level, isolate it and investigate the chosen contact, controller setting and physical travel. Do not reverse random wires while the circuit is energised.
