Float Switch Symbol Guide: Schematics, Wiring & Regional Standards

The standard float switch symbol on an electrical schematic is a circle intersected by a diagonal line, often featuring a small solid dot or secondary half-circle to represent the buoyant float and internal pivot mechanism. In IEC standards, it appears as a standard limit switch modified with a specific float actuator tag, while NEMA uses the distinct circle-and-line motif. Understanding these symbols is critical before wiring any pump, valve, or alarm circuit, as misinterpreting the normally open (NO) or normally closed (NC) state will cause your system to run dry or overflow.

The Complete Float Switch Symbol Reference Table

Before pulling wire or terminating connections, verify the exact switch state required by your circuit. The table below covers the primary schematic representations you will encounter on residential, commercial, and industrial prints.

Symbol Representation	Standard	Switch State	Typical Application
Circle with diagonal line up-right, solid dot at top-right	NEMA	Normally Open (NO)	Sump pump (turns ON when water rises and lifts float)
Circle with diagonal line up-right, solid dot at bottom-left	NEMA	Normally Closed (NC)	Tank fill valve or high-level alarm (turns OFF/alarms when water rises)
Standard limit switch line with 'F' or half-circle actuator tag	IEC 60617	Normally Open (NO)	Boiler low-water cutoff (closes to start feed pump when water drops)
Standard limit switch line with 'F' tag, cross-bar indicator	IEC 60617	Normally Closed (NC)	High-level shutoff for chemical dosing tanks
Two switch lines ganged together with a single float actuator	NEMA / IEC	DPDT / Changeover	Lead/lag pump alternation or simultaneous alarm + pump start

Regional Standards & Variants (NEMA vs. IEC vs. Old UK)

Electrical drawings do not use a universal language. The symbol your engineer or electrician uses depends heavily on the regional standard governing the project. Assuming a NEMA symbol applies to an IEC drawing is a fast track to wiring a pump backward.

NEMA (US & NEC Context)

In North America, the National Electrical Code (NEC) context relies heavily on NEMA Z55.1 and legacy JIC standards. The float switch is drawn as a distinct, standalone component—a circle with a diagonal line. The position of the solid dot relative to the diagonal line dictates the NO/NC state. This visual metaphor represents the physical mercury tilt switch or microswitch inside the float housing.

IEC 60617 (EU, UK, & Global)

The International Electrotechnical Commission (IEC) takes a modular approach. Instead of a unique shape for every sensor, IEC uses a base limit switch symbol (a standard NO/NC contact line) and adds an actuator modifier. For a float switch, you will see a small half-circle or a letter 'F' attached to the actuator line. According to standard IEC diagram practices, you must read the base contact state first, then apply the float logic. This reduces the total number of symbols an engineer needs to memorize but requires careful reading of the actuator tags.

Old UK (BS 1646)

On legacy British drawings (pre-harmonization with IEC), you may encounter BS 1646 symbols. These often bypass graphical metaphors entirely, using a simple rectangular box with the letters "FS" (Float Switch) or "LS" (Level Switch) printed inside, alongside terminal numbers (e.g., C, NO, NC). If you are retrofitting a pump station in an older UK facility, always trace the terminal numbers rather than relying on the box shape.

Rows People Get Wrong & Faded Marking Interpretation

Even experienced journeyman electricians and hobbyists trip up on specific aspects of float switch logic. Here is where the errors happen and how to fix them.

The "Normally" Misconception

The most common mistake is misunderstanding the word "Normally" in NO/NC. In electrical theory, "normal" refers to the unactuated, de-energized, shelf state of the switch. For a hanging float switch in an empty sump pit, gravity pulls the float down. Therefore, the "normal" state is hanging down.

A Normally Open (NO) float switch is open when the pit is empty, and closes when the water lifts the float. People frequently get this wrong by assuming "normal" means "normal operating water level," leading them to wire an NC switch when the schematic calls for an NO switch. Always picture the switch hanging in the air on the bench to determine its normal state.

Safe Interpretation of Faded or Missing Markings

Pump environments are harsh. Humidity, sulfur gases from sewage, and UV exposure can fade the physical NO/NC labels on a float switch housing within a year. If you are troubleshooting a circuit and the physical switch markings are illegible, do not guess. Use your digital multimeter (DMM) to verify the state.

The Bench Test Procedure:

1. Disconnect the float switch wires from the circuit (wire-nut the supply side for safety).
2. Set your DMM to the continuity or resistance (Ohms) setting.
3. Place one probe on the common (black) wire and the other on the switched wire (usually blue for NO, brown for NC on standard 3-wire pigtails).
4. Hold the float pointing straight down (empty tank state). Note the reading.
5. Flip the float 180 degrees so it points straight up (full tank state). Note the reading.
6. Interpretation: If the DMM reads < 1 ohm (or beeps) when pointing down, it is a Normally Closed (NC) switch. If it reads OL (open loop) when pointing down and < 1 ohm when pointing up, it is a Normally Open (NO) switch.

Frequently Asked Questions

What does a normally closed float switch symbol look like?

On a NEMA schematic, a normally closed float switch symbol looks like a circle with a diagonal line pointing up and to the right, with a solid dot positioned at the bottom-left of the circle. On an IEC schematic, it looks like a standard limit switch contact with a cross-bar (indicating NC) and a half-circle or 'F' tag attached to the actuator arm. In practice, an NC switch passes current when the tank is empty and breaks the circuit when the water rises to lift the float.

How do I wire a schematic float switch symbol to a 120V sump pump?

For a standard residential pedestal or submersible sump pump, the schematic will typically call for a Normally Open (NO) float switch. The switch is wired in series with the hot (black) leg of the 120V circuit. The black wire from the breaker panel connects to the black pigtail on the float switch. The blue pigtail (NO switched leg) from the float switch connects to the black wire on the pump motor. The white (neutral) and bare/green (ground) wires bypass the switch and connect directly to the pump and outlet.

NEC-style guidance note: Under modern NEC Article 210.8, 120V sump pumps in basements or crawlspaces generally require GFCI protection. Ensure your breaker is a GFCI/AFCI combination type as required by your local AHJ, and be aware that a tripping GFCI on a sump pump can cause flooding; some local jurisdictions allow specific exceptions for dedicated sump circuits, so always consult your local inspector.

Why does my P&ID use a different float switch symbol than the electrical drawing?

This is a classic point of confusion on commercial jobs. A Piping and Instrumentation Diagram (P&ID) uses ISA (International Society of Automation) standard 5.1 symbols, which focus on the instrument's function and location rather than its electrical contacts. On a P&ID, a float switch is represented by a circle with the letters "LS" (Level Switch) or "LSH" (Level Switch High) inside, often with a line indicating if it's mounted in the field or on a panel.

The electrical schematic, however, uses NEMA or IEC symbols to show exactly how the electrical contacts (NO/NC) wire into the relay logic or PLC input. You need the P&ID to know where the switch is physically located and what process variable it monitors, but you must use the electrical schematic to know which terminal gets wired to the pump contactor coil.