Water pressure switch wiring for a pump
A water pressure switch is a switch in the pump supply: it starts the motor when pressure drops and stops it at the set value. The rule is short. A small single-phase pump can run directly through the switch contacts, while a three-phase pump must go through a contactor. The reason is simple: pressure switch contacts are rated 16 A (that is the figure in the PM5/PT5 and Condor MDR 2 documents), and a motor load is often too heavy for them.
A switch that "will not turn off" is often a wiring mistake rather than a defective unit. Setting the cut-in and cut-out pressure is covered in our separate article on adjusting a water pressure switch. This one is about wiring only.
Before you connect anything
- Contact rating: compare the current and AC3 power on the cover or datasheet with the motor nameplate. For example, the Condor MDR 2 allows 2.2 kW at 230 V 1~ (EN 60947-4-1) and 1.1 kW at 120 V.
- Ingress protection: PM/PT and MDR 2 are IP 44. That means splash protection, not jets or condensation. A damp well or pit needs an extra enclosure.
- Breaker in the panel: a dedicated breaker on the pump line. The switch goes after it.
- Power off: open the breaker and test for dead, instead of assuming the pump is idle.
Single-phase pump: direct wiring
For a household borehole or surface pump up to about 2 kW the simplest scheme is a pressure switch that breaks the supply between the panel and the pump. Most water-supply stations work this way.
| Switch terminal | Connect to |
|---|---|
| Line, terminal 1 | Live L from the breaker |
| Line, terminal 2 | Neutral N (second live on some models) |
| Motor, terminal 1 | Live L to the pump |
| Motor, terminal 2 | Neutral N to the pump |
| Earth screw on the housing | PE from the panel and PE of the pump |
The number of poles that open depends on the model and marking differs, so open the cover and read the diagram on it. If it disagrees with the table above, the manufacturer diagram wins.
- Open the breaker and confirm the line is dead.
- Pass the cable through the gland and strip it so the bare copper sits fully inside the terminal.
- Supply to Line, pump cable to Motor, earth to the PE screw.
- Tighten the screws and give each wire a light tug (M4 terminal screws like to loosen).
- Close the cover, switch on and check the cycle: the pump starts on falling pressure and stops at the upper setpoint.
Three-phase pump: pressure switch plus contactor
Do not run a three-phase motor through pressure switch contacts. Make the switch a control element: it closes the contactor coil circuit and the power contacts of the contactor start the pump. The switch then carries only the small coil current and lasts much longer.
| Part | Connection |
|---|---|
| Power circuit | Motor protection breaker, then contactor power contacts (1-2, 3-4, 5-6), then pump U, V, W |
| Control circuit | L1, pressure switch contact, selector or push button, overload relay contact (NC), coil terminal A1 |
| Return | Coil terminal A2 to neutral N (for a 230 V coil) |
| Earth | PE from the panel to the motor frame and to the switch earth screw |
A 230 V coil is easy to feed between L1 and neutral. A 400 V coil goes between two phases, and then the switch breaks one of them. Set the overload protection to the nameplate current.
One detail we keep running into: with no manual override in the control circuit, the pump has to be started with a jumper during repairs. Add an Auto-0-Manual selector. It costs next to nothing.
Earthing and safety
- Earth the switch, the pump frame and the pipe if it is metal.
- Do not tie PE and N together at the switch. The earth screw is for the protective conductor only.
- Fit a 30 mA RCD on the pump line if the well or pit is damp.
- Never exceed the contact rating. If in doubt, add a contactor.
- Do not wire a switch with a damaged diaphragm or housing: water inside the enclosure is the usual outcome.
When a pressure switch is not enough: VFD or soft starter
If the pump starts with a jerk, pressure swings, or you need dry-run protection, the pressure switch alone no longer does the job. The next step is a variable frequency drive with a pressure sensor, or a soft starter.
| Task | Switch + contactor | Soft starter | VFD |
|---|---|---|---|
| Simple on/off | Yes | Yes | Yes |
| Soft start, less water hammer | No | Yes | Yes |
| Steady line pressure | No | No | Yes, with a pressure sensor |
| Single-phase supply | Yes | Model dependent | Yes, dedicated single-phase models |
Single-phase options are covered in VFD for a single-phase pump, and soft starting in soft starter for a pump. Catalog: variable frequency drives and soft starters. If you replace the switch with a 4-20 mA sensor, read pressure sensor for a pump station. For generator operation see this guide.
Frequently Asked Questions
Can I wire a three-phase pump straight to a pressure switch?
If pump power and inrush current fit within the contact rating it is possible on paper, but we do not do it with three-phase motors. Pressure switch contacts are mostly rated 16 A (PM5/PT5 per the manufacturer manual). A contactor with overload protection gives the switch a much longer life.
Which terminal gets the phase and which the neutral?
Supply goes to the Line terminals, the cable to the pump goes to the Motor terminals, and the separate earth screw is for PE. On a single-phase pump the switch normally breaks the live conductor. Check the marking on the cover of your model.
How many kW can a pressure switch handle?
Read the datasheet of your model. As an example, the Condor MDR 2 lists Ie 16 A and 2.2 kW motor power (AC3) on a 230 V single-phase supply. The PM5 manual gives 250 V and 16 A. If the pump is bigger, add a contactor.
Why do pressure switch contacts burn or weld?
Usually the current is too high for the contacts, or the gap between cut-in and cut-out pressure is too small, so the switch chatters and the contacts cannot clear the arc. A sagging supply voltage prolongs the starting current and makes it worse.
When should I use a VFD instead of a pressure switch?
When you want soft starting, steady pressure without swings, or dry-run protection in a single device. Single-phase input drives exist for single-phase supply, and three-phase drives for normal three-phase pumps. A soft starter gives soft starting only, without pressure regulation.
Summary
Single-phase pump up to 2 kW: wire it through the switch. Three-phase: use a contactor. Check earthing, IP 44 and contact rating before the first start. If the switch chatters, pressure swings, or you want dry-run protection, move to a variable frequency drive or a soft starter. Not sure what fits your pump? Send us the power and supply type and we will suggest a scheme.
