Wiring A Compressor Pressure Switch

They are electro-mechanical devices, and through repeated cycling they do fail, so it helps to know about wiring a compressor pressure switch if you ever have to replace yours.

It will take pages and pages to try to assist in the wiring of every brand of compressor switch, so here are some specifics about the process, and some generalities too.

Safety when wiring a compressor pressure switch

If you are well grounded the power in a 120 VAC circuit can kill you. If you are working on a switch with 240 VAC, the power there will kill you if you are not very careful.

In operation a pressure switch turns the power ON and OFF to an air compressor motor. When the air compressor is plugged in there is power at the switch. When you remove the switch cover, assuming that’s the type of pressure switch your air compressor has,  liver terminals are exposed. So double check to be sure the compressor is unplugged – or the breaker to the compressor is off – before starting any work on the pressure switch.

Kill the power to the compressor completely when you are wiring a compressor pressure switch.

 Basic pressure switch wiring

You have a power supply to the air compressor. For the typical DIY type home or small workshop air compressor, that power supply is a plug on a wire that connects to a wall socket.

Plug in the compressor plug and you have power flowing to the air compressor, through (perhaps) an ON/OFF switch and to the pressure switch, and if the pressure switch is closed, power will flow through it to the start capacitor on the air compressor motor.

The pressure switch will be closed (passing electricity) if the tank air pressure is below the pressure switch low pressure set point. If the air pressure in the compressor tank is higher than the pressure switch low pressure set point, the pressure switch will be open (not passing electricity) and the power stops there.

Inside an air compressor pressure switch

In the photo above you see two views of the same pressure switch. The image on the right has the cover on, and on the left, the cover off. Note that there are two wire-ways under the floor of the pressure switch. One of these is to bring the power cord from the wall plug through the wire-way and up to the terminals on the pressure switch, and the other is to allow the wires from the switch back out of the switch and up to the motor circuit.

The wire terminals themselves are usually marked with LINE – which is the terminals for the power supply from the wall plug, and the other side is noted as LOAD or MOTOR, and the motor wires are attached to these terminals.

On the right side of the left hand image above, you can see the screw to which the ground wire is attached.

A different style of pressure switch

Of course, if you have a pressure switch like the one below, the wiring is fairly simple.

Pressure-switch-for-a-DeWalt-compressor

One wire connects to the terminal on the compressor ON/OFF switch and the other to a terminal on the motor.

Wiring a pressure switch video

Master Tool Repair was kind enough to make the following video available for folks that are rewiring their own pressure switches.

With most electrical installations, it is important to remember that attaching the ground wire is critical if you want the circuit, and user,  to be protected from shorts or faults in the system.

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Larry Henrickson

Thanks for your help. How do you get the cover off? I have an FBANG switch just like the one in your picture, but I can’t see anything holding it together. Do I need to remove the red switch button? Thanks for any help you can give.

John

If we bye pass the compressor switch it works fine . And we got a new switch and and it wired correctly

John

Motor stops turning

John

Wire a new air compressors switch to a 220 v compressors but the belt stops moving when it hits 50 psi . Any body can help

Mahmoud

Hi how are you.
I’m working in air compressor
I’m conect the pressure switch and the compressor load and un load automatically..but the problem is compressor not turn of after delay time? Why

Gerry Fitzgerald

We have a 240v air compressor that had a bad contactor switch. The old one was a Square D brand 2pole switch. The guy at the electrical parts place gave me a new Eaton 3 POLE switch. How do I connect the new one correctly?

Allen

I have a new 240V air compressor. I wired the breaker using Romex 12/3 with ground. The power cord for the compressor 4/C 12 AWG 600V has 4 wires: Black, Red, White and Green. how do I wire the pressure switch using all wires. My electrician said leave the white unconnected?

Thanks

I have two pressure switches ( one is written as base load & the other is written as peak load) and and two compressors.

Both compressors have it’s own contactors.

I have one auxiliary contactor (2 N/O +2 N/C) to switch between compressors whenever compressor needs to start.

I want to connect Coil of both contactors & both pressure switch to auxiliary contactor.

How to do the wiring for this ?

Should I use plc’s digital output to energize auxiliary contactor for switching compressors ?

Prakash

I don’t understand

AJ

Hi
My compressor (12v on board with tank) has the small round pressure switch fitted to the tank with 2 black wires coming from it which lead inside a metal cover on the end of the tank.

There is no switch on the unit but I intend to install one in the car via a relay. Is there an easy way to determine which of the two wires goes to the motor, or does it not matter which one I ground via a switch.

Max

hi, i have a pressure switch like the second one pictured how do i connect it to two compressors?

chris

Hi I am installing into the back of a Land Rover . I have a Tank installed etc . I have connected heavy gauge wire from battery to Relay (30) wire from compressor to ( 87) ….. 86 is earthed .. so 85 goes to pressure switch ( does it go to the terminals marked P or N ?)…….. I have a switch ( mounted in dash) so a wire from this goes to pressure switch ? either P or N ( need to know which ) , and what terminal do I use for wire to the relay The… Read more »

chris

Hi . The Compressor is a 12v HD for a 4×4 . just has 2 wires red + and black – . I am hard wiring in . I am using switch with fuse etc . Relay . The Pressure switch is identical to the 1st picture ( showing bare and covered . I have wired up as follows .. Permanent live -fuse-87 to relay …30 out to compressor ( live) heavy gauge cable , so just need to know about terminals 86 and 85 and what wires would go to the pressure switch as the markings are P and… Read more »

chris

Hi

I have a pressure switch . its marked N and P ( not motor / load) . What wires would go into these ? I am using 12v via relay and switch. The large terminals on the relay are 87/30 so these go to the compressor . So basically I am just using positive and negative wires , Any help would be appreciated as searched and not found anything .
Kind regards chris

Phil

Hi Can you tell me how the wiring goes please for the Red knobbed pressure switch you have shown above as i have one and there are no markings on it at all. Thanks