Float Switch Connection Diagram: Wiring NO and NC Float Switches for Pump and Tank Level Control

Float Switch Connection Diagram: Wiring NO and NC Float Switches for Pump and Tank Level Control — interactive diagram. Open it in the editor to customise components and wiring.

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A float switch connection diagram shows how to wire a buoyancy-actuated level switch to a pump or alarm circuit, using normally-open or normally-closed contacts to control liquid level automatically.

A float switch is a mechanical switch whose contacts open or close in response to the buoyant force of a liquid acting on a float element. As the liquid level rises, the float rises with it; as it falls, the float descends. This movement operates the switch contacts at predetermined high and low levels. Float switches are the simplest and most reliable form of liquid level control and are used in sump pumps, water tanks, sewage pits, industrial process vessels, and bilge pumps.

Contact configurations determine the pump control logic. A normally-open (NO) float switch is open when the float is in its rest (low-level) position and closes when the float rises to the actuation point. Wiring a NO float switch in series with a pump motor circuit means the pump starts when the level rises and the float lifts — used to empty a sump or bilge (pumping down). A normally-closed (NC) float switch is closed in the rest position and opens when the level rises — used to fill a tank (the pump runs while the float is low and the circuit is closed; it stops when the level rises and the float opens the contacts).

Dual-float installations use two float switches to provide separate start and stop levels, eliminating pump short-cycling. The lower float switch (set to the start level) and the upper float switch (set to the stop level) control a latching relay or motor starter contactor. When the lower float activates, the contactor energises and latches; when the upper float activates, the contactor releases. This arrangement provides a hysteresis band between start and stop levels proportional to the vertical distance between the two floats.

For three-phase pumps, the float switch circuit controls the coil of a motor starter contactor, not the motor winding directly. The float switch contacts carry only the contactor coil current (typically 50–500 mA), while the contactor contacts carry the full motor current. This separation is essential — connecting float switch contacts directly to three-phase motor terminals will destroy the switch.

Cable entry into the float switch body must be sealed to the IP rating required for the installation environment.

How to wire float switch connection diagram

Determine the liquid level control requirement Decide whether the application is pump-down (sump emptying — pump runs when level is high) or pump-up (tank filling — pump runs when level is low). This determines whether a NO or NC float switch is needed. Document the start level and stop level heights.
Select the float switch current rating Verify the float switch contact rating is adequate for the circuit it directly controls. For single-phase domestic pumps, use a float switch rated for the full motor current. For motor starters and contactors, the float switch only needs to handle the contactor coil current, allowing a lower-rated float switch.
Mount the float switch at the correct level For a single-float system, mount the switch at the level where the pump should start or stop. The float's travel angle must be clear of the tank wall, inlet pipes, and other obstructions. For a dual-float system, mount the lower switch at the start level and the upper at the stop level.
Route the float switch cable to the control panel Run the cable from the float switch to the pump control panel or distribution board. Protect the cable from mechanical damage with conduit or cable tray. Ensure the cable entry into the enclosure is sealed against moisture and the entry into the float switch body meets the required IP rating.
Wire the float switch into the pump control circuit For a NO float switch controlling a contactor coil: connect one float switch conductor to the live supply (via a fuse or MCB) and the other to the contactor coil terminal A1. Connect coil terminal A2 to neutral. The contactor main contacts then carry the pump motor supply. Confirm the pump motor terminals are connected to the correct contactor output terminals.
Test the installation at low water level before filling With the tank or sump empty, confirm the float switch is in its rest position and the pump is in the expected state (on or off depending on the wiring logic). Slowly fill the vessel and verify the pump starts or stops at the correct level. Check for hysteresis with a dual-float system.

Specifications

Typical float switch contact rating (single-phase domestic)	240 V AC, 10–15 A
IP rating for continuous submersion	IP68
IP rating for splash and intermittent immersion	IP67
Control circuit voltage (typical)	240 V AC or 24 V AC/DC
Float switch operating temperature (typical)	-10 °C to +70 °C (check datasheet for specific material)
Dual-float hysteresis band (typical installation)	100–300 mm vertical separation between start and stop floats

Safety warnings

All electrical installation work in wet, damp, or hazardous locations must comply with applicable wiring regulations (IEC 60364, NEC/NFPA 70, BS 7671, AS/NZS 3000) and must be carried out by a licensed electrician. Swimming pools, sewage pits, and similar locations have additional zone restrictions on equipment placement.
Always isolate, lock out, and verify dead the pump motor circuit and the control supply before working on any float switch wiring or contactor connections.
Float switches in sewage, waste water, or chemically aggressive liquids must be rated for the specific liquid. Standard water float switches may be degraded by fuels, solvents, or strong acids and alkalis.
Do not connect float switch contacts directly to three-phase motor winding terminals. The switch must control the motor starter coil. Direct connection will exceed the float switch contact rating and result in switch failure or arcing.
Submersible pump cable and float switch cable must be rated for continuous submersion (typically rated to IEC 60245-1 or equivalent). Standard PVC cable will absorb moisture and fail in submersion applications.

Tools needed

Digital multimeter with continuity and AC voltage functions
Insulated screwdrivers
Cable strippers and crimping tool
Conduit bender and fittings (where conduit is used)
Approved voltage indicator
Pump test vessel or commissioning access to the tank/sump

Common mistakes

Wiring a NO float switch where an NC switch is required (or vice versa), causing the pump to run continuously or never start.
Mounting the float switch so the float cannot swing freely — obstructions cause the switch to stick in one position and the pump to run dry or overflow.
Connecting the float switch directly to a three-phase motor, bypassing the motor starter contactor.
Using a single float switch without hysteresis, causing the pump to short-cycle — the pump starts and stops repeatedly at the same level, overheating the motor winding.
Failing to seal the float switch cable gland to the required IP rating, allowing moisture ingress into the wiring enclosure.

Troubleshooting

Pump runs continuously and does not stop when tank is full: Cause: Float switch contacts stuck closed (float obstructed), or NC switch used where NO is required, keeping the circuit energised at high level. Fix: Isolate the pump circuit. Manually operate the float switch and verify with a multimeter that the contacts open and close as expected. Check float travel is unobstructed. Confirm the NO/NC configuration matches the circuit wiring diagram.
Pump does not start even when the sump is above the start level: Cause: Float switch contacts not closing (float obstructed or switch failed), blown control fuse, or loose connection at the contactor coil terminals. Fix: Isolate the circuit. Test the float switch contacts with a multimeter. Replace the control fuse and check contactor coil terminal tightness. Manually lift the float and confirm the switch closes electrically.
Pump short-cycles — starts and stops rapidly: Cause: Single float switch used with no hysteresis. The pump empties the sump to just below the switch actuation point, the switch opens, the pump stops, the level rises slightly, the switch closes, and the cycle repeats. Fix: Install a second float switch at a lower level to create a hysteresis band. Wire through a latching relay or motor starter auxiliary contact so the pump runs until the upper stop level is reached.

Frequently asked questions

What is the difference between a NO and NC float switch?

A normally-open (NO) float switch has open contacts when the float is at its lowest position (gravity resting) and closes when the float rises. An NC switch has closed contacts at rest and opens when the float rises. The correct choice depends on whether the pump should run when the level is high (sump emptying: NO) or low (tank filling: NC).

Can I connect a float switch directly to a three-phase motor?

No. Float switch contacts are rated for small currents, typically 5–15 A at 240 V AC for single-phase residential types. A three-phase motor draws significantly more current at starting. The float switch must control the coil of a motor starter contactor or relay, which then switches the motor circuit.

How do I prevent pump short-cycling with a float switch?

Use two float switches — a lower switch at the pump start level and an upper switch at the pump stop level — wired through a latching relay or motor starter with a hold-in auxiliary contact. This creates a hysteresis band. The pump runs between the start and stop levels rather than cycling on and off at the same level.

What IP rating should a float switch have for a sewage pit?

A sewage or sump application requires a float switch rated at least IP68, meaning it is dust-tight and suitable for continuous immersion. The cable gland and entry point must also be sealed to the same rating. IP67 (immersion up to 1 m for 30 minutes) is the minimum for intermittent submersion applications.

How do I wire a float switch to trigger a high-level alarm instead of controlling a pump?

Connect the float switch contacts in series with the alarm sounder or indicator lamp and a suitable supply voltage. Use a NO float switch so the alarm activates only when the level rises to the actuation point. Add a relay if the alarm circuit voltage or current differs from the float switch rating.