Time: 20–30 minutes. Skill: homeowner-comfortable with a screwdriver and able to isolate the circuit at the breaker. Cost: the price of one replacement capacitor (NZD incl GST).
If your bore pump hums and trips, won’t start, or keeps cutting out, the cause is often a failed capacitor — and here’s the good news: on most submersible bore pumps the capacitor is not down the bore with the wet end. It lives in the surface control box beside the bore head, which makes it one of the more accessible pump-electrical fixes for a NZ DIYer.
Where the capacitor actually is
This is the key thing that trips people up. A submersible bore pump motor sits at the bottom of the bore, sometimes tens of metres down. The starting/run capacitor is almost always housed in a control box (start box) mounted at the surface — on a wall, post, or pump shed. So you do not need to pull the pump to replace the capacitor. (Some surface-mounted jet or “shallow well” pumps instead carry the cap in the motor terminal box, like other pressure pumps.)
If your fault really is in the submerged motor or the drop cable, that’s a job for a bore-pump specialist with lifting gear — not a DIY task.
How the fault shows up
- The motor hums or buzzes but won’t start, then trips the overload
- The breaker trips on start-up
- Repeated short-cycling or cutting out under load
- A bulged, domed, leaking, or scorched capacitor inside the control box
Safety first
- Isolate the power at the breaker or isolator and confirm the supply to the control box is dead before opening it.
- The capacitor holds a dangerous charge even with the power off — discharge it before touching the terminals (Step 4).
- In NZ, fixed mains wiring must be done by a licensed electrician. Bore control boxes are frequently hard-wired and sit outdoors in damp conditions — if it’s hard-wired or you’re unsure, use a sparky.
Match the value — never guess from the model
Bore-pump capacitor values vary widely with motor size and depth; start capacitors can run into the tens-to-hundreds of microfarads and run capacitors into the low tens, with voltage ratings commonly 250V or 400–450V AC. These are only ballparks. Read the µF and voltage on your old capacitor or the motor/control-box nameplate, match the µF exactly, and meet or exceed the voltage rating. The wrong value will damage the submerged motor — an expensive mistake to fix.
Not sure what the markings mean? The find-your-capacitor wizard decodes them.
What you’ll need
- The replacement run capacitor — match the µF (within ±5%) and equal-or-higher voltage to the one fitted now.
- Phillips and flat screwdrivers.
- An insulated screwdriver, or a 10–20 kΩ bleed resistor with insulated leads, to discharge the old capacitor safely.
- A multimeter, if you want to confirm the diagnosis before swapping.
- Your phone — to photograph the wiring before disconnecting anything.
Steps
1. Isolate the power
At the breaker/isolator feeding the control box. Confirm it’s dead before opening the box.
2. Open the surface control box
It’s usually weatherproof — check for moisture or insect ingress while you’re in there. Photograph the wiring first.
3. Identify the capacitor(s)
— one or more cylindrical cans wired to the terminals or relay.
4. Discharge each capacitor
By bridging its terminals with a bleed resistor (around 10–20 kΩ, 5 W) or a proper capacitor discharge tool — a bare insulated-handle screwdriver across the terminals also drains it but sparks violently and can pit the terminals, so use that only as a last resort. Do this every time.
5. Record and remove the leads
Noting exactly which connector goes where (label them).
6. Read the old capacitor
Note the µF, voltage, and whether it’s a start or run cap, then source an exact µF match at equal or higher voltage.
7. Fit the new capacitor
Into the same position and push the connectors firmly onto the matching terminals.
8. Reseal the control box
Properly — the weatherproof gasket matters outdoors — and check nothing is pinched.
9. Restore power and test
The pump should start cleanly and run without tripping. If it still hums or trips, power off; the fault may be down the bore.
If the new capacitor doesn’t fix it
If a correctly-matched cap doesn’t solve it, suspect the parts you can’t easily reach: the submerged motor winding, the drop cable, a failed relay in the control box, a stuck pressure switch, or a dry/low bore. A persistent hum-and-trip after a good cap often means a motor or cable fault down the bore — call a bore-pump specialist with the gear to lift and test it.
Get the right bore-pump capacitor
Browse the full pump capacitors range or go to the well & bore selection. Not sure which one? Use the find-your-capacitor wizard to match your old cap in a minute.
Capacitors are shipped tracked, around 2 weeks via NZ Post or courier, prices in NZD incl GST, backed by a 90-day DOA guarantee.
NZ-owned. NZD incl GST. 90-day DOA guarantee.