A practical wiring guide for cable float switches: reading NO/NC contacts, wiring direct to a small pump vs through a contactor, fill vs drain logic, and a
A cable float switch is the simplest way to automate a pump, but it is wired wrong more often than any other part of the level-control loop. The mistake is almost always the same: treating the float as a generic on/off part instead of respecting its contact rating and its fail direction. This guide walks through the wiring that actually survives commissioning, for both the direct-pump case and the contactor case.
Most cable floats carry a changeover (SPDT) contact with three terminals: common (C), normally-open (NO) and normally-closed (NC). With the float hanging free in its rest position, C–NC are closed and C–NO are open; when the liquid lifts the float past its trip angle the contacts change over. Colour codes vary by factory, so do not trust the wire colour alone — check the datasheet and prove the contact with a multimeter before energising anything.
Before touching terminals, decide what the float is for, because that decides whether you use NO or NC:
The safety rule is that a failed or jammed float should leave the pump in the safe state — for a sump that means pumping on (fail to run, so the pit does not overflow), which is another reason the empty-pit circuit uses NC.
There are two ways to land the load:
| Approach | When to use it | Notes |
|---|---|---|
| Float switches the pump directly | Small single-phase pump, load below the contact rating | Float carries full motor current; add motor protection upstream |
| Float switches a contactor coil | Three-phase motors, larger pumps, AC-3 duty | Contacts only see the coil current; the contactor handles the motor |
| Float into a PLC / level controller | Multi-level or multi-pump control | Float is a dry signal input; logic and hysteresis live in software |
For anything above a fraction of a kilowatt, the contactor route is cheaper in the long run: the float contacts stay cold, and the contactor takes the arcing and the motor inrush. Wire the float in series with the contactor coil (through the control circuit fuse), and let the contactor's power contacts switch the motor.
The trip level is set by where you tie the cable off, not by the switch. Tie the cable shorter for a higher trip point, longer for a lower one, and leave enough slack that the float swings freely without the cable pulling it sideways. Order a standard cable length (2, 3, 5, 10 or 15 m) rather than cutting to an odd length, and remember a longer cable lowers the effective trip point.
ELEHPD supplies cable float switches with SPDT contacts rated for direct pump control, in cable lengths from 2 to 15 m, alongside the magnetic contactors and level controllers that pair with them in the same panel.
On a typical SPDT cable float the brown/red is common (C), blue is normally-open (NO) and black or yellow is normally-closed (NC). The order is not universal, so always confirm against the datasheet and prove it with a continuity meter before connecting a load.
Only for a small single-phase pump whose running and inrush current sit inside the contact rating (for example 10(8) A at 250 V AC). Above that, or on any three-phase motor, wire the float as a signal into a magnetic contactor coil so the contacts only switch the coil, never the motor.
That decides NO vs NC. To fill a tank, use NO so the contact closes when the float falls and starts the pump. To empty a sump or pit, use NC so the contact opens when the float rises and stops the pump. Getting it backwards fills the pit or runs the pump dry.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.