control wiringHow do I install and wire my float switch? Where can I find a float switch circuit diagram? Where can I find a float switch wiring diagram? You asked, and today, we answer.


Wiring a float switch isn’t necessarily hard, but it can be a little confusing if you don’t have a visual aid or two. Remember that what you’re wiring is a means of turning things on and off. Thinking carefully about when you want something off, and when it should turn on, will help you as you visualize the wiring and apply the schematic to real world control.


We’re going to look at a progression of straightforward pump control arrangements using float switches. We’ll look at single and double switch arrangements and how to wire them, and then look at equivalent circuits using Kari series float switches.


These instructions and diagrams will serve to teach you the basics of float switch control wiring. They certainly don’t apply in all scenarios, especially when additional control equipment is needed to handle large motors. However, with a little bit of fundamentals, you’ll be wiring like an old pro in no time.


Single Float Switch Wiring


Control Schematic 2

wiring diagram for a single 2-wire normally open float switch

Control Schematic 1

wiring diagram for a single 2-wire normally closed float switch
Let’s start with the most basic float switch: a two-wire, single-pole, single-throw float switch. The rising action of the float can either close (i.e., turn on) a “Normally Open” circuit, or it can open (turn off) a “Normally Closed” circuit. Installation scenarios might include a Normally Open float switch turning on a pump to empty a tank (Control Schematic 2), or a Normally Closed float switch turning off a pump that fills a tank (Control Schematic 1). In both schematics, terminal 1 in the control circuitry represents the landing point for the float switch’s (+) wire, and terminal 2 for the (-) wire.      

So there we have it. A two-wire float switch that can easily be used for turning a pump on or off. Mount or suspend your switch at the desired level, get your wires into a water-tight junction box (or out of the liquid containment area and then into a junction box), check the connections back to your control and power equipment, and you’re done.


It’s a very simple solution, but it’s also problematic because level fluctuations will cause the float to flutter, which will turn the pump motor on and off in quick succession. And now your simple solution has burned up a pump motor. So what can we do to protect the pump motor?


Wiring For Two Float Switches


We can add a second switch to create hysteresis. Hyste-what?? Yeah, we’ll get there. Hang on.


What we need is a way to allow for a level switch to turn on and off without cycling the pump motor at the same time. We could add a time delay, but that doesn’t help monitor and respond to the conditions in the tank; it only overrides the switch. However, if we add a second switch that is identical to the first, and wire a seal-in relay around one of them, we’ll get the control we’re looking for.


Control Schematic 3

wiring diagram for two normally closed 2-wire float switches
Let’s start by looking at Control Schematic 3 with two Normally Closed switches. This circuit can be used to control a pump that fills a tank. The first switch (L) is set at the minimum liquid level desired in the tank. The second switch (H) goes at the maximum desired level.      

When the liquid is below both switches, they are both closed; the pump runs, filling the tank.  As the liquid fills past the first switch, it opens. However, seal-in relay A has been activated and closed, bypassing the now-open switch L (effectively “sealing it in”), so the pump continues to run until the high-level switch H opens. When the high-level switch opens, the motor relay P opens, stopping the motor, and seal-in relay A opens.


So no more liquid is coming into the tank from this pump. Let’s say a valve downstream of the tank is opened, allowing liquid to drain out of the tank. As the liquid level falls, high-level switch H closes. But since both low-level switch L and seal-in relay A are open, the pump motor does not start.


In fact, the liquid level in the tank must fall below low-level switch L before the motor will start. At that point, both the low-level and high-level switches will be closed, completing the circuit, and activating motor relay P to start the pump. At the same time, seal-in relay A will be activated, closing the by-pass around low-level switch L. So when low-level switch L opens as the pump fills the tank, the seal-in relay keeps the circuit closed, and the pump keeps pumping.


This cyclical action is called hysteresis. Once the liquid level falls below the low-level switch, the pump will run until both switches are open. The liquid level can fluctuate up and down, the low-level switch can open and close, and the pump will continue to run smoothly. Similarly, once the high-level switch opens, the pump will not run until both switches have closed. Regardless of level fluctuations, no more pump motor flutter.


Great! We’ve got level control, reasonable pump-motor life, everything we could want, right? Let’s wire it up. We need to wire both float switches back to our control circuitry, plus we have to add the contacts and seal-in relay A. The low-level switch wires to terminals 1 and 2, the high-level switch to terminals 3 and 4, and the contacts for seal-in relay A to terminals 5 and 6.


So that’s at least four, if not six, wires that need to be hooked up to the control circuitry. (Wiring for the seal-in relay and contacts will depend on your control equipment.) That’s not so bad: two float switches, an additional relay, and four to six wires. But what if I tell you that you can do it with just two wires? Not two additional wires, just two wires.


2-Wire Pump Control With The Kari Float Switch


That’s right. With a KARI series 2L float switch, you get the same hysteresis control using one switch and two wires instead of two switched and four or six wires. “What is this Magic,” you ask? Simple: each KARI series float switch has multiple microswitches and control circuitry built into the float.


As the single KARI series float rises with the liquid level in the tank, it tilts to one side. The microswitches inside the float activate at factory-set angles as the float tilts, and the preprogrammed control circuitry responds accordingly.


So what do you need to wire this up? We can go back to control schematic 1: just two wires between the switch and the motor control circuit, (+) wire to terminal 1 and (-) to terminal 2. No seal-in relays, no extra switches, nothing else. Two wires, and you’re done.


Bonus: 3-Wire Pump Control With The Kari Float Switch


Control Schematic 4

wiring diagram for one normally open 3-wire float switch with isolated alarm
Since that was so easy, let’s take a look at what you can do with a three-wire KARI series float switch: add an alarm signal! Instead of four wires for simple, two-level hysteresis, a KARI series 3H float switch gives you two-level hysteresis and an alarm using just three wires.      

Take a look at the Control Schematic 4. On the bottom line you have the wiring terminals for the switches providing hysteresis (wires 1 & 2). The next line up is for a high-high-level alarm (i.e., a higher level than the high-level hysteresis switch).  As with the seal-in relay above, the wiring necessary for the alarm contact will vary based on your control equipment. All that is left is installing the switch per the manufacturer’s instructions for your desired levels.


Motor Starting And Motor Control


We’ve spent quite a bit of time talking about how float switches can be used to turn pumps on and off, so it’s worth taking a moment to talk specifically about motor starting and motor control. For small motors – DC motors, motors up to 1 HP – the relay-driven contactors shown in the diagrams above are probably sufficient for starting the motor. No harm will come to these motors (or the loads they are driving) from starting and stopping via a contactor acting as an on-off switch.


For larger motors, inrush current (up to six or eight times full load current) becomes an important factor in the starting and maintenance of the motor, rendering contactors insufficient as stand-alone motor starters. Such motors need integrated controllers and overload protection in order to start safely and still be protected while running at full load. Fortunately, most motors of this size will either be controlled via a motor control center (MCC) or a dedicated control panel, both of which are fully capable of integrating control circuits and instruments like those shown above.


In all reality, most of the pumps and motors you would control with a float switch are probably large enough to require these integrated controls. While the setup is more complicated than the wiring schematics provided above, the wiring is often simplified for the end user because the system provider has done most of the work.


However, understanding the basics of float switch control wiring will help you work confidently no matter how powerful or complex the system. Everything from float switch installation to troubleshooting will become easier. And, of course, we’re always available to help out if you feel the need.



Shop APG Float Switches



top photo credit: PEO ACWA via flickr cc cropped