Why does a pump start then stop a few minutes later?
A pump that stops after minutes of running is being switched off by a protector: either the thermal overload buried in the motor windings, or the electronic controller on the pipework. The timing points to which one.
| What you notice |
Most likely cause |
| Runs 2-10 minutes, goes quiet, works again 10-30 minutes later |
Thermal overload tripping, often a drifted run capacitor |
| Cuts out in seconds with a fault light on the controller |
Dry-run protection: lost prime or low water |
| Starts and stops every few seconds while a tap is running |
Waterlogged pressure tank (short-cycling) |
| Trips the switchboard breaker the instant it starts |
Shorted capacitor or winding fault: stop and call an electrician |
The give-away is the recovery. A thermal overload won’t reset until the motor has cooled, typically 10 to 30 minutes. Dry-run protection retries within minutes or waits for its reset button, usually with a fault light showing.
How does a weak bore pump capacitor make the motor overheat and cut out?
A run capacitor that has drifted below its rated microfarads is the classic cause of a bore or pressure pump that starts, runs for a few minutes, then trips out. On a single-phase 230V pump motor, the capacitor feeds the auxiliary winding and keeps the magnetic field rotating evenly. As the capacitor ages it loses capacitance: a 30µF part might measure 24µF, 20% low. The motor still starts, but the weakened field means more slip and higher current under load, so the windings heat up. After a few minutes the thermal overload inside the motor opens and the pump goes quiet; once it cools (typically 10 to 30 minutes), it runs again, until the next trip. A healthy capacitor measures within ±5% of its printed value; anything further out is replacement territory, and a two-minute multimeter capacitance test confirms it.
This is why the fault feels so random, and why it worsens gradually as the capacitance keeps falling. A pump that ran twenty minutes between trips in spring might manage five by mid-summer, when hotter weather and heavier watering load push the windings over the limit sooner.
The starts-then-stops pattern is only one symptom of a dying capacitor. Humming without starting, tripping on start-up and a bulged or leaking can are the others: see the signs your pump capacitor is failing for the full list.
How do you test a bore pump capacitor without pulling the pump?
On a surface-mounted pressure or jet pump, the capacitor sits under the terminal cover at the back of the motor. On most 4-inch submersible bore pumps you don’t need to lift anything either: three-wire submersible motors keep the capacitor in the control box on the shed wall. Only two-wire submersibles carry the capacitor and relay down the bore inside the motor. Those are a pump technician’s job.
To test it:
- Isolate the power. Unplug the pump, or switch off its dedicated breaker. If the pump is hardwired and you’re not confident working on mains equipment, have a registered electrician take it from here.
- Open the terminal cover or control box and photograph the wiring before touching anything.
- Discharge the capacitor across both terminals with a 10-20 kΩ bleed resistor or a discharge tool.
- Disconnect the capacitor leads and note the printed rating, for example 30µF ±5%, 450V.
- Measure it on a multimeter’s capacitance (µF) setting. Full walk-through in how to test a pump capacitor with a multimeter.
- Compare to the label. Outside ±5% of the printed value, say 26µF on a 30µF label, means replace it. A reading of OL or near zero means it’s dead.
If the capacitor fails the test, swapping it is a ten-minute job on a plug-in pump: the bore pump capacitor replacement guide covers it step by step.
What else makes a pressure pump cut out?
If the capacitor measures within tolerance, work through the water side before blaming the motor.
- Dry-run protection. Bore water tables drop in late summer; if the pump out-draws the bore, the controller sees loss of flow and trips. This is the cut-out doing its job, not a fault.
- Blocked foot valve or intake screen. A fouled screen starves the pump, pressure never reaches cut-off, and the dry-run logic eventually trips. If the pump runs but pressure is weak first, start with pressure pump not building pressure.
- Waterlogged pressure tank. A tank that has lost its air charge makes the pump start and stop every few seconds. Check the pre-charge with a tyre gauge: it should sit about 20 kPa below the pump’s cut-in pressure.
- Voltage drop on long cable runs. A pump shed hundreds of metres from the switchboard can run at low voltage under load, which raises current and heat: the same overload trip as a weak capacitor.
- Controller fault. Electronic pressure controllers fail too. If the capacitor tests good and the water supply is proven, the controller becomes the prime suspect.
What size capacitor does your bore or pressure pump take?
Single-phase bore and pressure pumps in New Zealand almost all use a CBB60 polypropylene run capacitor rated 450V, sized between 8µF and 70µF depending on motor power. From manufacturer documentation: a 0.37kW shallow-well jet pump takes 10µF, a 0.75kW jet pump takes 16-20µF (DAB Aquajet 102 M is 16µF, Grundfos JP5 is 20µF), a 1.8kW deep-well jet (Davey PJ90-PJ150) takes 30µF, and 4-inch submersible borehole motors range from 25µF at 0.75kW to 70µF at 2.2kW (Bianco SB4 series). Match the µF on the label of your existing capacitor; within ±5% is fine. The voltage rating must be the same or higher, so 450V can replace 400V but never the reverse. The µF value is sized to the motor windings, not the brand, so read your own label rather than copying someone else’s pump.
Documented sizes for common NZ pressure and bore pumps:
| Pump model |
Type |
Motor |
Run capacitor |
| Davey 95S / 95D |
shallow/deep-well jet |
1.2 kW |
20µF 450V CBB60 |
| Davey PJ90 / PJ115 / PJ150 |
deep-well jet |
1.8 kW |
30µF 450V CBB60 |
| Grundfos JP5 |
jet pressure |
0.78 kW |
20µF 450V CBB60 |
| Grundfos JP6 |
jet pressure |
1.06 kW |
25µF 450V CBB60 |
| DAB Aquajet 102 M |
jet pressure |
0.75 kW |
16µF 450V CBB60 |
| Bianco MULTI34 |
multistage pressure |
0.55 kW |
16µF 450V CBB60 |
| Bianco SB4-3/11-0.75 |
4-inch submersible bore |
0.75 kW |
25µF 450V CBB60 |
| Bianco SB4-4/18-1.5 |
4-inch submersible bore |
1.5 kW |
55µF 450V CBB60 |
| Bianco SB4-8/13-2.2 |
4-inch submersible bore |
2.2 kW |
70µF 450V CBB60 |
These are documented sizes, not a fitment guarantee. Manufacturers revise motors, so always match the µF and voltage printed on your own capacitor. Browse every documented Davey pump capacitor size, or answer three questions in the capacitor wizard to get to the right part. Replacement CBB60s start from NZ$29.95 for the common 30µF size, with tracked NZ delivery. Each product page shows the delivery time for that size.
A few larger single-phase motors use a separate electrolytic start capacitor (body code CD60) alongside the run capacitor: you’ll see two cans under the cover. Replace each like-for-like: a start capacitor never stands in for a run capacitor. Our motor start capacitor guide covers how to read and match one.
When should you call an electrician or pump technician?
- Plug-in surface pump: testing and replacing the capacitor is a straightforward job once the pump is unplugged and the capacitor discharged.
- Hardwired pump or controller: most permanent bore installations are hardwired, so assume yours is unless you can see a plug. If you’re not confident working on mains equipment, have a registered electrician do it.
- Two-wire submersible, or any pump that needs pulling from the bore: that’s a pump technician with lifting gear.
- The switchboard breaker trips instantly: possible shorted winding or earth fault. Stop and call an electrician; don’t keep resetting it.
- The capacitor tests fine but the overload keeps tripping: the motor may be failing; have a repair-versus-replace conversation with a pump service agent.
Frequently asked questions
Can a bad capacitor make a water pump trip the breaker?
Yes, two ways. A capacitor that has drifted low overloads the motor gradually, tripping its thermal cut-out after minutes of running. One that has failed short can trip the switchboard breaker the instant the pump starts. If so, stop resetting it and call an electrician.
Why is my jet pump not building pressure before it cuts out?
That points to the water side rather than the capacitor: lost prime, an air leak in the suction line, a blocked foot valve or a worn impeller. The pump runs but can’t reach cut-off pressure, so the dry-run protection eventually trips it. A weak capacitor shows up as overheating and thermal trips, not as low pressure.
How long does a pump thermal overload take to reset?
Typically 10 to 30 minutes: it’s a bimetal switch that closes once the windings cool, which is why the pump “fixes itself” until the next trip. Don’t treat it as a duty cycle: every overheating event ages the winding insulation.
What size capacitor does a 4-inch submersible bore pump use?
Documented Bianco SB4 borehole motors run 25µF at 0.75kW, 45µF at 1.1kW, 55µF at 1.5kW and 70µF at 2.2kW, all CBB60 type, 450V. On three-wire submersible motors the capacitor is in the control box at the surface, so confirm the size from the label there rather than from the pump itself.
Can I replace a pump capacitor myself?
If the pump plugs into a socket, it’s a simple swap once you’ve unplugged it and discharged the capacitor: match the µF on the label (within ±5% is fine) and use the same or a higher voltage. If the pump or its controller is hardwired, or you’re not confident working on mains equipment, have a registered electrician do it.