Float Switch Wiring Diagram: Pump Level Control Connections

A float switch is one of the simplest level-sensing devices in industrial and domestic use: a buoyant float attached to a switch mechanism that opens or closes contacts as liquid rises or falls. Simple does not mean trivial. Wiring a float switch incorrectly can fill a tank to overflow, run a pump dry until the motor burns out, or fail to drain a flooded sump. The key decisions are: is the switch normally open or normally closed, and what logic does the application need -- fill or empty?

NO vs NC Float Switches

Float switches come in two basic contact configurations:

Normally Open (NO)

The contacts are open when the float is in its resting (low liquid) position. As liquid rises and lifts the float, the contacts close. The float switch closes the circuit when the level is HIGH.

Use NO configuration for:

• Empty/drain control -- close the contacts (start the drain pump) when the tank is full
• High-level alarm -- trigger an alarm when liquid reaches a threshold

Normally Closed (NC)

The contacts are closed when the float is in its resting (low liquid) position. As liquid rises and the float lifts, the contacts open. The float switch opens the circuit when the level is HIGH.

Use NC configuration for:

• Fill control -- open the contacts (stop filling) when the tank is full; contacts close (resume filling) when level drops

Some float switches are configurable -- the same device can be wired NO or NC depending on which terminals are used. Vertical float switches (pendant type) typically have a single NO or NC designation that cannot be changed. Ball float switches on a pivot arm can often be inverted to switch the effective contact logic.

Single Float Switch: Direct Pump Wiring

For small sump pumps, bilge pumps, or fountain pumps below about 2A, the float switch can interrupt the pump supply directly without an intermediate contactor or relay.

Wiring (Single-phase, pump filling a tank):

Goal: Pump runs when level is LOW, stops when level is HIGH. Use: NC float switch (contacts closed at low level, open at high level).

1. Live (L) from supply to float switch input terminal.
2. Float switch output terminal to pump live input.
3. Neutral (N) from supply direct to pump neutral terminal.
4. Earth from supply to pump earth terminal.

When the tank is low, float drops, NC contacts are closed, pump runs. When tank fills and float rises, NC contacts open, pump stops. Simple and effective for small loads.

Safety Note

Float switches in wet locations must be IP-rated for submersion or splash (IP67 or IP68 for fully submersed applications). Check the switch manufacturer's IP rating before installing in a tank or sump. Never handle float switch wiring or connections while the system is energized and the tank is wet. Working near water and mains electricity is a genuine electrocution risk -- isolate the supply before any maintenance.

Float Switch with Contactor for Larger Pumps

Most industrial pumps draw more current than a float switch can handle directly. A standard float switch contact rating is 10A or 16A at 250VAC -- a 5.5kW pump at 230V draws approximately 24A full-load, well above that limit.

In this case, the float switch operates the coil of a contactor, and the contactor's main contacts switch the pump supply.

Wiring (Single-phase pump, empty/drain logic):

Goal: Pump drains the tank when level is HIGH, stops when level drops LOW. Use: NO float switch (contacts open at low level, close at high level).

Control circuit:

1. L (control supply hot) → float switch input (NO contact)
2. Float switch output → contactor coil A1
3. Contactor coil A2 → N (control supply neutral)

Power circuit:

1. L (mains) → contactor L1
2. Contactor T1 → pump live
3. N (mains) → pump neutral (bypasses contactor)
4. Earth → pump earth terminal

When the tank level is low, the float is down, NO contacts are open, contactor coil is de-energized, pump is off. When level rises and float lifts, NO contacts close, contactor A1 is energized, main contacts close, pump runs and drains the tank.

For three-phase pumps, use a three-phase contactor (L1/L2/L3 to T1/T2/T3) and add a thermal overload relay between T1/T2/T3 and the motor terminals. The float switch still only controls the contactor coil -- no change to the control circuit logic.

Two Float Switch Level Control (Fill Application)

Using a single float switch for fill control creates a problem: the pump or valve cycles on and off rapidly around the set point as waves or turbulence bounce the float. Two float switches create a differential band (hysteresis) -- the pump starts at a low level set point and stops at a high level set point.

Component placement:

• Float switch 1 (LOW): Set at the minimum acceptable level. When liquid drops below this point, float 1 drops to its low position.
• Float switch 2 (HIGH): Set at the maximum acceptable level. When liquid rises above this point, float 2 rises.

Wiring logic (fill pump, latching control with two floats):

Both float switches are NC configuration. Use a latching relay or a self-latching contactor circuit:

1. L (control hot) → Float 2 NC contact input (high-level switch)
2. Float 2 output → Float 1 NC contact input (low-level switch)
3. Float 1 output → Contactor coil A1
4. Contactor NO auxiliary (13/14) bridges across Float 1 (seal-in)
5. Coil A2 → N (control neutral)

Behavior:

• Tank empty: Float 1 is down (NC closed). Float 2 is down (NC closed). Contactor energizes through Float 1.
• Contactor seals in through auxiliary 13/14.
• Tank fills. Float 1 rises -- but contactor is sealed in through its own auxiliary, so it stays on.
• Float 2 rises and NC contact opens. Coil circuit breaks. Contactor drops out. Pump stops.
• Tank empties. Float 2 drops and NC contact closes again. Now Float 1 is also down (NC closed). Coil energizes through Float 1 again. Cycle repeats.

This gives you a low-start, high-stop band. Adjusting the float positions (cable length on pendant floats) sets the operating range.

Reed Switch Float Switches

Magnetic reed switch floats (common in HVAC condensate pans and small process tanks) work on a different principle. A magnet in the float body moves past a sealed reed switch capsule fixed to the outside of a stem or tube. The contacts open or close as the magnet passes.

Reed switches typically handle 0.5A to 2A -- they are control-level devices only. Wire them to a relay or PLC input, never directly to a pump motor. Their small contact gap makes them susceptible to arcing damage if overloaded.

Visualizing Float Switch Logic in CircuitDiagramMaker

Float switch circuits are not complex, but the NO/NC logic in combination with fill vs empty intent confuses people regularly. Drawing the circuit in CircuitDiagramMaker -- with the float switch symbol in its resting (low liquid) position and annotating the contact state at that position -- makes the logic explicit before any wiring is done. You can also annotate the float positions (high level, low level) and the expected pump state at each condition. This kind of annotated diagram is useful documentation to leave with a maintenance team.

Create Your Own Float Switch Wiring Diagram

• Show the float switch symbol with a label indicating NO or NC and the low-level (resting) contact state
• For direct wiring: live through float switch to pump live; neutral direct to pump
• For contactor control: float switch in the control coil circuit (A1/A2); power circuit through contactor main contacts to motor
• For two-float differential control: show Float 2 (high) NC in series with Float 1 (low) NC, and contactor seal-in across Float 1
• Annotate the liquid level position each float occupies and the pump state (ON/OFF) at each condition

Create your own float switch wiring diagram -- free

Key Takeaways

• A normally open (NO) float switch closes when the float rises (high level); use NO for empty/drain logic.
• A normally closed (NC) float switch opens when the float rises (high level); use NC for fill logic.
• Check the float switch contact current rating -- most are 10-16A AC. For larger pumps, use the float switch to control a contactor coil, not the motor directly.
• Two float switches with a seal-in (latching) contactor circuit create a differential band, preventing rapid on-off cycling around a single set point.
• IP rating is critical in wet applications -- use IP67 or IP68 for submersed float switches.
• Reed switch floats handle milliamps to 2A; always route them through a relay or PLC input for pump control.
• Always isolate the electrical supply before working near float switches in a tank or sump environment.