There are two nuts under that cover and they do different things. Get them backwards and you will spend an hour going in circles, or worse, set a cut-out above what your tank is rated for.
This is the procedure for how to adjust an air compressor pressure switch, written the way we would talk somebody through it on the phone. Read the safety section. It is short and it is the part people skip.
Before you touch anything
Four things, in this order, every time.
- Kill the power and lock it out. There is line voltage on the contacts inside that cover, and on a three phase machine it is 230 or 460 volts sitting an inch from your knuckles. Breaker off, locked, tagged.
- Bleed the tank down to zero. Open the drain and let it go all the way to zero on the gauge. You will need to run the compressor up and down to test your setting anyway, so start from empty.
- Find your tank's maximum allowable working pressure. It is stamped on the ASME nameplate on the tank. This is a hard ceiling, not a suggestion.
- Find your safety relief valve setting. It is stamped on the valve. Your cut-out has to be below it, with margin. If you set cut-out at or above the relief setting, the relief valve becomes part of normal operation, which it is not designed to be and which will destroy it.
If you cannot read the tank nameplate because it has been painted over or rusted off, stop. An unidentifiable tank is a separate problem, and our guide to tank rust and when to replace a tank covers what to do about it.
The two numbers you are setting
Cut-in is the low pressure at which the switch closes and starts the motor. Cut-out is the high pressure at which it opens and stops the motor. The gap between them is the differential.
Common factory settings you will run into:
| Machine type | Typical cut-in | Typical cut-out | Differential |
|---|---|---|---|
| Small single stage, portable | 90 PSI | 120 PSI | 30 |
| Shop single stage, 60 to 80 gallon | 95 to 105 PSI | 125 to 135 PSI | 30 |
| Two stage industrial | 140 PSI | 175 PSI | 35 |
| Two stage set low for tool work | 110 PSI | 140 PSI | 30 |
One rule that matters more than any of the numbers above: keep the differential at 13 PSI or more. Below that, the contacts chatter as pressure approaches cut-out, the motor starts and stops rapidly with no time to cool, and you will burn up a motor or a set of contacts in a remarkably short time. When somebody tells us their compressor is short cycling right after they adjusted the switch, a differential set too tight is usually what happened.
Identify your switch
Pop the cover with the power off. You are looking for how many adjustment nuts or screws are under there.
Two adjusters. Square D Pumptrol 9013, most Furnas and Hubbell units, Condor MDR-11. These are adjustable differential switches and they are what this article is about. You will see one large nut on a heavy spring and one small nut on a lighter spring.
One adjuster. Fixed differential switch. The single nut moves cut-in and cut-out together and the gap between them is set at the factory and cannot be changed. You can shift the whole band up or down. You cannot widen or narrow it.
If yours has one adjuster and you need a different differential, you need a different switch, not a different technique. Our pressure switch range covers both types.
The procedure
Step 1: Set the range with the large nut
The large nut is the range nut, also called the main calibration nut. It moves cut-in and cut-out together, shifting the whole band up or down without changing the spread.
Turn it clockwise to raise both settings and counter-clockwise to lower them. Move it a quarter to a half turn at a time. These springs are more sensitive than they look, and a full turn will overshoot.
Step 2: Set the differential with the small nut
The small nut is the differential nut. On most switches it changes cut-out only, widening or narrowing the gap between the two points while cut-in stays where you put it.
Clockwise widens the gap by raising cut-out. Counter-clockwise narrows it. Same rule on movement: a quarter turn at a time.
Step 3: Close up and test
Put the cover back on before you energize anything. Restore power, let the compressor run up, and watch the gauge.
Note two numbers: the pressure at which it stopped, and then bleed air until it starts again and note that pressure. Those are your actual cut-out and cut-in, and they will not match what you calculated in your head on the first try.
Step 4: Repeat
Power off, bleed down, adjust, close up, test. It takes three or four cycles to land on a setting. Anybody who tells you they nail it in one pass is either lucky or not checking.
Write the final numbers on a label inside the cover with the date. The next person to open that cover, possibly you in four years, will be glad.
Things that go wrong
It starts and stops rapidly near cut-out. Differential is too tight. Widen it back out past 13 PSI. If it still chatters after that, you may have a tired switch or a gauge and switch reading differently, and the switch is cheap enough to just replace.
It never reaches the new cut-out. You may have asked for more pressure than the pump can make. A worn single stage pump that used to hit 135 may top out at 110 now, and turning the switch up does not fix a pump. Our guide on a compressor that will not build pressure works through that.
It runs, reaches cut-out, then will not restart later. Check the unloader. The little valve on the side of the switch is supposed to bleed the head pressure off when the switch opens so the motor has an unloaded start. A stuck unloader leaves pressure on top of the piston and the motor cannot break away, which usually presents as a hum and then a tripped breaker.
The relief valve pops. Stop and lower the cut-out immediately. Cut-out is at or above the relief setting, the relief valve has now been cycled, and it may not seat correctly again. Replacing a safety relief valve that has been popped is cheap insurance.
The numbers drift after a few weeks. Old switches lose calibration as the spring fatigues and the contacts pit. If you are chasing it repeatedly, replace the switch rather than keeping it in tune.
When not to adjust it at all
Two situations where the switch is not your answer.
If tools are starving, raising cut-out is treating a symptom. Tools starve because of CFM, pressure drop through undersized piping, or a restrictive coupler at the drop, and raising system pressure just costs energy while masking it. Roughly every 2 PSI of extra system pressure adds about 1 percent to your compressor's energy use, forever.
And if the compressor is short cycling, look at storage and leaks before you touch the differential. A tank that is too small for the load, or a system leaking hard, makes a compressor cycle constantly no matter how the switch is set.
Frequently Asked Questions
Which nut raises the cut-out pressure?
Both can, and that is the confusing part. The large range nut raises cut-in and cut-out together, moving the whole band up. The small differential nut raises cut-out only, widening the gap. If you want more pressure overall, use the large nut. If you want the compressor to run longer between starts, widen the differential with the small nut.
How much differential should an air compressor have?
At least 13 PSI, and most machines are set around 30 to 35. Below 13 PSI the contacts chatter near cut-out and the motor cycles rapidly with no cooling time, which destroys motors and contacts quickly. Wider differential means fewer starts per hour and longer motor life, at the cost of a wider pressure swing at your tools.
Can I set my cut-out higher than the tank rating?
No. The maximum allowable working pressure stamped on the ASME nameplate is a hard limit, and your cut-out must sit below both that and the safety relief valve setting with margin. Exceeding it puts you outside the tank's certification and makes the relief valve part of normal operation, which is not what it is for.
Why does my compressor start at a different pressure than the switch says?
Printed markings on a switch body are nominal, not calibrated, and gauges drift too. Trust what you observe on the gauge during an actual run and stop cycle, not the numbers printed on the switch. If your gauge itself is suspect, put a known good one on a test port before you chase the switch.
Should I just replace the switch instead of adjusting it?
Often, yes. A pressure switch is an inexpensive part with a finite life, and if the contacts are pitted, the unloader is sticking or the setting drifts after you adjust it, you are maintaining something that wants replacing. Match the amp rating, the voltage, the port configuration and whether you need an on/off lever and an unloader.
Bottom line
Power off, tank at zero, know your tank rating and your relief setting. Large nut moves the whole band, small nut changes the spread, quarter turn at a time, test between every adjustment, and never go below 13 PSI of differential. If you find yourself adjusting the same switch repeatedly, the switch is telling you something.
For how the part works and when it has failed rather than drifted, see our air compressor pressure switch guide. If the real complaint is rapid cycling, start with air compressor short cycling instead.
