220 volt pressure switch wiring diagram
This is a free printable 220 volt pressure switch wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A 220-volt pressure switch wiring diagram shows how to control high-power equipment (air compressors, water pumps, hydraulic systems) using a pressure-sensitive switch that automatically starts and stops the equipment based on system pressure. The diagram identifies: 220V single-phase or three-phase power source, pressure switch contacts responding to system pressure (typically adjustable from 40-120 PSI), motor starter contactor providing soft-start and overload protection, and an electric motor operating the pump or compressor. When system pressure drops below the low setpoint (typically 60 PSI), the pressure switch contact closes, energizing the contactor and starting the motor. As pressure increases, when reaching the high setpoint (typically 100 PSI), the switch contact opens, de-energizing the contactor and stopping the motor. The diagram shows: pressurized system feedback path from the pump/compressor outlet to the pressure switch sensing port, pressure gauge for visual pressure indication, and pressure relief valve preventing overpressurization. Modern pressure switch designs include: electronic transducers converting pressure to 4-20mA signals for remote monitoring, wireless transmission to building management systems, and multi-stage switching logic for staged equipment operation.
How to wire 220 volt pressure switch wiring diagram
- Isolate all power and depressurise the system Switch off and lock out the circuit breaker. Open the tank drain valve to release all stored pressure before opening the pressure switch housing. A pressurised system can eject the switch housing with force.
- Identify supply, motor, and earth terminals inside the switch Most pressure switches have four terminals: L1 in, L2 in, L1 out (to motor), L2 out (to motor), plus an earth stud. Some have a single-pole switching arrangement — verify the switch schematic printed inside the housing lid.
- Connect supply to input terminals Wire the incoming 220V supply L and N to the designated input terminals. Ensure supply cable is protected by a correctly-rated circuit breaker upstream.
- Connect motor circuit from output terminals Run motor supply cable from output terminals to the motor terminals or to a contactor if motor exceeds direct switch rating. Include a thermal overload relay in the motor circuit for protection.
- Earth the switch housing and motor frame Connect the circuit earth conductor to the earth terminal inside the switch and to the motor frame earth terminal. Test earth continuity from switch housing to consumer unit earth bar — should read under 0.5 ohm.
- Set differential and cut-out pressure Pressurise system manually if possible, or energise and let the compressor run. Adjust cut-out pressure first, then set differential. Test that the switch opens at cut-out and closes at cut-in pressure as intended.
- Test for correct operation and check for arcing Run several full cycles. After 10 cycles, re-open the switch housing (power off, depressurised) and inspect contacts for pitting or discolouration that indicates the contacts are being overloaded.
Frequently asked questions
How do I set the cut-in and cut-out pressure on a pressure switch?
The main spring nut (larger) adjusts the cut-out pressure — tighten to increase, loosen to decrease. The differential nut (smaller) adjusts the gap between cut-in and cut-out. A typical compressor runs at 8 bar cut-in and 10 bar cut-out. Always adjust with system depressurised and power isolated.
Can I wire the pressure switch directly without a contactor for a small compressor?
Only if the pressure switch is rated to handle the full motor running current — check the switch nameplate. Many small compressor pressure switches are rated 15–16A and can switch the motor directly. For motors above 2.2kW, always use a contactor to protect the switch contacts from welding.
My compressor runs but does not build pressure — the pressure switch is not the problem?
Correct. A pressure switch that activates but pressure does not rise indicates a mechanical fault: failed check valve, worn pump rings, loose fittings, or a leaking relief valve. Isolate the tank from the pump and test pump output directly. Pressure switch wiring faults show as no-start, not low pressure.
Why does my compressor motor hum but not start when pressure switch calls for it?
The motor is seeing supply voltage but cannot develop starting torque. Most common causes: failed start capacitor, weak supply voltage causing insufficient starting torque, or seized pump. Test capacitor MFD first — a dead start capacitor causes exactly this symptom and is the cheapest fix.
Is the pressure switch earth connection mandatory?
Yes. The pressure switch housing is metal and is directly connected to pressurised metalwork. If a live conductor touches the housing without an earth, the entire plumbing system becomes live. Earth the switch housing to the circuit earth via the cable gland or a dedicated earth terminal inside the switch.
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