Figure 1: Float switch
This article covers float switch wiring. In short, a float switch is a mechanical switch that floats on top of a liquid surface. As the liquid level changes, a mechanical switch inside the float switch is either connected or disconnected. Therefore, it is capable of sending a signal to: trigger a motor to fill the tank, a valve to open to empty the tank, high water level alarm, or both depending on the media level within the tank. To learn exactly how a float switch works, read our float switch technical article. Properly installing and wiring of the float switch ensures it’ll work as intended. This article discusses mainly water tank float switches, but the same installation and wiring concepts apply for different media.
A float switch consists of the floating switch and the electrical wire. The electrical wire needs to be fixed in a position that isn’t going to change the depth of the float switch, as seen in Figure 2. It can either be fixed to a bracket on top of the water tank, or along the side/pipe running down inside the tank. Figure 3 also shows installation with a counterweight. It is important that there are no obstructions below or above the float switch that it could get stuck on as it moves with the water level. The depth, or length of the cable, between the fixture point of the float switch and bracket, determines the total extension of the float and the consequent distances between the pump stopping and starting level.
Figure 2: Properly installing a float switch
Figure 3: States of a float switch
Figure 3 shows an example installation of a float switch with a counterweight and how it can be in two different states (high and low). Depending on your wiring, at the top and bottom of the water movement different functions can occur. A float switch is typically wired to have one operation. This one operation can be to fill a tank, empty a tank, or a high-water alarm. Looking at Figure 3 and the two points below, the same installation can have different functions depending on wiring.
Float switches are either single function or double function (changeover contact). The function means if they are normally open (NO) or normally closed (NC). Single function float switches are either NC or NO and then they have a common and ground wire. Double function float switches have an NO, NC, and common wire. For a single function float switch, the ground wire needs to always be properly grounded. For a double function float switch, the wire not used needs to be properly isolated.
Important Note: Different float switch brands and types classify NO and NC differently. Please read your brand-specific wiring diagram to know what wiring state to use.
Figure 4 shows different options for wiring single and double function float switches by MAC. Figures 1 & 2 are for single function and Figures 3 & 4 are for double function. Wiring for filling applications can use diagrams 2 & 4 and for emptying applications can use diagrams 1 & 3.
Figure 4: Brand MAC float switch wiring diagrams