The correct symbol for float switch representation on a schematic depends entirely on whether you are reading an electrical wiring diagram (NEMA/IEC) or a Piping and Instrumentation Diagram (ISA 5.1). In electrical schematics, it is drawn as a limit switch actuated by a liquid level lever; in P&IDs, it is a bubble tagged "LS". Below is the direct reference data you need to read, draw, and wire these components without guessing.

Master Reference Table: Float Switch Schematic Symbols

Use this table to translate between the three dominant drafting standards. NEMA and IEC are used for panel wiring and electrical schematics, while ISA is used for process flow and P&IDs.

Switch Type NEMA Symbol Description IEC 60617 Description ISA 5.1 P&ID Tag Practical Meaning (At Rest / Gravity)
Normally Open (NO) Limit switch symbol with wavy liquid line attached to lever; contacts open. Standard NO contact (angled line) with mechanical float link. LSL (Level Switch Low) or just LS with NO note. Open circuit when hanging down. Closes when liquid rises and tilts float up.
Normally Closed (NC) Limit switch symbol with wavy liquid line; contacts closed (line passes through). Standard NC contact (angled line with cross) with mechanical float link. LSH (Level Switch High) or LS with NC note. Closed circuit when hanging down. Opens when liquid rises and tilts float up.
Changeover (SPDT) Limit switch with wavy line; common terminal splitting to NO and NC. Common terminal branching to one NO and one NC contact via float link. LST (Level Switch Transfer) or dual-tagged LSH/LSL. Three wires. Common to NC is closed at rest; Common to NO is open at rest.

For a deeper dive into general electrical schematic conventions, refer to the All About Circuits wiring diagram reference. For process engineering contexts, the Engineering Toolbox ISA symbol guide provides the full P&ID bubble nomenclature.

Regional Standards: NEC, IEC, and Legacy UK Practices

While the schematic symbols dictate how the switch is drawn, regional codes dictate how the physical switch is wired and identified in the field.

North America (NEC / NEMA)

The National Electrical Code (NFPA 70) does not mandate specific internal wire colors for manufactured float switches, but industry standard practice (used by Liberty Pumps, Rule, and Zoeller) for a 3-wire SPDT switch is:

  • Black: Common (Line or Load connection)
  • Brown: Normally Open (Pump-Up / High-Level alarm)
  • Blue: Normally Closed (Pump-Down / Low-Level cutoff)

NEC-style guidance: When wiring a 120V piggyback float switch in a damp location (like a sump pit), the receptacle must be GFCI protected per NEC Article 210.8(A)(5). Your local AHJ has final authority on whether a hardwired pump requires a local disconnect.

Europe & Global (IEC 60446 / IEC 60617)

IEC 60446 strictly defines brown as Line (L) and blue as Neutral (N) for mains power. However, for control drop cables on MAC3-style float switches, the Black/Brown/Blue convention mirrors the US standard. If you are wiring a 230V pump, the Brown wire from the float switch is often spliced into the Brown (Line) mains feed, which causes massive confusion for apprentices. Always sleeve the float switch's brown wire with black tape to denote it as a switched leg, not a continuous mains line.

Legacy UK (Pre-Harmonization)

If you are troubleshooting an older UK installation (pre-2004), you may encounter red (Line), black (Neutral), and green/yellow (Earth). In these older panels, float switch drop cables were often pulled using standard 3-core lighting cable (Red/Yellow/Blue). In those cases, the Yellow wire was typically used as the switched return. Always verify with a meter; never trust legacy colors blindly.

The "Rows People Get Wrong" Notes

When reading the reference table above, misinterpreting the "Normal" state is the most common cause of flooded basements and burnt-out pump motors.

Mistake 1: Confusing "Normal" with "Empty Tank"
In electrical schematics, "Normal" refers to the state of the switch when it is hanging straight down under the influence of gravity (unpowered, at rest). It does not mean the tank is empty. A Normally Open (NO) switch is open when hanging down. As the water rises, the float's buoyancy overcomes gravity, tilting the switch and closing the contacts.
Mistake 2: ISA P&ID Bubbles vs. Electrical Contacts
Do not confuse an ISA "LSH" (Level Switch High) bubble with an electrical "NC" contact. An LSH tag means the switch trips when the level gets too high. To achieve this, the electrical schematic will usually show a Normally Closed contact wired in series with a pump motor contactor coil. When the water rises, the NC contact opens, dropping out the contactor and stopping the pump.

Safe Interpretation When Markings Are Faded or Missing

On jobsites, you will frequently encounter float switches where the printed NO/NC diagram on the casing has been eaten away by UV exposure or chemical vapors. Never guess the wiring based on wire color alone, as previous technicians may have spliced in non-standard cables.

Bench-Test Protocol:

  1. De-energize the circuit. Lock out the breaker and verify dead with a tested non-contact voltage tester and a multimeter.
  2. Set your digital multimeter to Continuity mode (the diode/sound symbol).
  3. Identify the Common wire (usually Black, but verify). Place one probe on the Black wire.
  4. Let the float hang straight down (gravity state). Place the second probe on the Brown wire, then the Blue wire. The wire that beeps is your NC (Pump-Down). The wire that reads "OL" (Open Loop) is your NO (Pump-Up).
  5. Flip the float completely upside down. The continuity states must reverse. If they do not, the internal microswitch or mercury tilt bulb is mechanically stuck; discard the unit.

Decision Path: Picking the Right Switch for Your Application

Stop guessing which switch type to buy. Use this decision tree to select the exact configuration and part number for your specific pumping or control scenario.

Application Scenario Required Action Electrical State Needed Concrete Pick / Part Number
Residential Sump Pump (Emptying a pit) Turn pump ON when water rises, OFF when empty. NO (Closes on rise). Piggyback plug preferred for easy swap. Liberty Pumps 10SN (NO Piggyback, ~$28)
Tank Fill Valve (Stopping a pump from overfilling) Break the circuit to stop the pump when tank is full. NC (Opens on rise). Wired in series with contactor coil. MAC3 MAC/3-10 NC (Bare wire SPDT, use Blue/Black, ~$22)
High-Level Alarm (Sounding a buzzer if pit overflows) Trigger alarm circuit only when water hits the rim. NO (Closes on extreme rise). Independent tether. Hydrolevel Raptor 10 (NO, adjustable tether, ~$35)

The Default Recommendation: If you are wiring a standard 120V residential sump pump and need a reliable, code-compliant setup without hardwiring splices in a damp pit, your concrete pick is the Liberty Pumps 10SN piggyback NO float switch. Plug the switch into the GFCI receptacle, plug the pump into the switch's piggyback end, and set the tether length for a 4-inch drawdown. For 240V well pumps or 3-phase industrial lift stations, abandon piggyback plugs entirely; use a 3-wire SPDT float switch (like the MAC3) wired into a NEMA 4X control enclosure housing a definite-purpose contactor and motor overload relay.