Short definition
Cut-in pressure is the lower threshold at which the pressure switch energizes a well pump or booster pump; cut-out pressure is the upper threshold at which it de-energizes. The two values define the operating band of the pump cycle. Most WA private wells run 30/50 psi (cut-in 30, cut-out 50) or 40/60 psi.
What it is
A private well or booster-pump system can’t run continuously — it would short-cycle on every fixture demand and burn out the motor. Instead, a pressure switch on the discharge side energizes the pump when system pressure drops to the cut-in value, and de-energizes it when pressure rises to the cut-out value. Between cycles, a pressure tank (hydro-pneumatic tank or bladder accumulator) holds water at pressure to handle short demands without starting the pump.
The standard differential between cut-in and cut-out is 20 psi. A 30/50 setting gives a 20 psi differential; a 40/60 setting gives the same differential at higher pressure. This 20 psi spacing matches the design assumption of most residential pressure tanks and pump motors — narrower differentials cause short-cycling, wider differentials cause uncomfortable pressure swings at fixtures.
The pressure switch has two adjustment nuts inside its cover:
- Long-spring nut. Adjusts the cut-in pressure while preserving the differential. Turn clockwise to raise cut-in (and cut-out by the same amount).
- Short-spring nut. Adjusts the differential — the spread between cut-in and cut-out. Used carefully and rarely; if you change the differential, you also need to re-pre-charge the pressure tank to roughly 2 psi below the new cut-in.
Why it matters to a homeowner
If your well pump is short-cycling (turning on and off every few seconds during demand), or fixture pressure swings noticeably during a shower, the pressure switch and the tank pre-charge are the first things to check. If you want firmer fixture pressure (the 30/50 default sometimes feels weak at upper-floor fixtures), you can raise to 40/60 — but you must also raise the tank’s air pre-charge to roughly 38 psi (2 psi below the new cut-in) for the system to work correctly.
Adjusting the pressure switch requires turning off power at the well-pump breaker first. The contacts inside the cover are line voltage. Most homeowners can do the adjustment, but if you’re not comfortable working in an energized control box, it’s a $100–$200 service call.
Common failure modes
- Pitted contacts. After years of arcing, the switch contacts pit and weld; pump won’t start, or won’t stop.
- Diaphragm failure inside the pressure switch — erratic on/off cycling at apparently random pressures.
- Mis-set differential without re-pre-charging the tank — short-cycling.
- Frozen switch in an uninsulated WA well house during a cold snap — moisture inside the switch case freezes and locks the diaphragm.
Washington note
WA private wells (Olympic Peninsula, Mason County, Whatcom, Kitsap, rural Jefferson and Snohomish, east-of-Cascades agricultural properties) overwhelmingly use Square D Pumptrol-style mechanical pressure switches at 30/50 or 40/60 psi. The 30/50 setting is the residential default; 40/60 is common where fixture pressure feels weak. East-of-Cascades cold snaps drive the “frozen switch” failure mode every few winters — a heated well-house or insulated control enclosure prevents it.