If your pool pump hums but won’t start, or your garage door motor groans halfway up and gives up, there’s a good chance a small cylinder bolted to the side of the motor has quietly died. That cylinder is a run capacitor, and it’s one of the cheapest, most common failure points on any single-phase motor you’ll find around a New Zealand home. The good news: once you know what it does and how to read the numbers printed on the side, swapping one out is a straightforward DIY job.
This guide walks through what a run capacitor actually does, how it differs from a start capacitor, where you’ll find them in the house, the warning signs that yours has failed, and how to pick the correct replacement so you only have to order once.
What a run capacitor actually does inside a motor
Single-phase AC motors have a problem: a single sine wave of mains power doesn’t naturally produce a rotating magnetic field, so the motor doesn’t know which way to spin. Manufacturers solve this by adding a second winding (the auxiliary or start winding) and wiring a capacitor in series with it. The capacitor shifts the current in that winding out of phase with the main winding, and that phase shift is what creates the rotating field that gets the rotor turning and keeps it turning smoothly.
A run capacitor stays in the circuit the whole time the motor is running. It’s not just there for the kick-off; it’s continuously improving the motor’s torque, efficiency, and power factor. That’s why a tired or failing run capacitor often shows up as a motor that runs hot, draws more current than it should, or struggles under load even though it technically starts.
Physically, a run capacitor for the appliances we’re talking about is almost always a CBB60 cylinder, oil-filled, rated for continuous duty, with a 450 VAC working voltage. The number that varies between applications is the capacitance, measured in microfarads (µF), and that value is set by the motor manufacturer. Get the µF wrong and the motor will run rough, run hot, or not run at all.
Run capacitor vs start capacitor — what’s different and how to tell them apart
People often lump these together, but they do different jobs.
A start capacitor is a short-burst component. It provides a big phase shift to get a heavy load moving (think a compressor with high inertia), and a centrifugal switch or relay disconnects it within a second or two of the motor reaching speed. Start caps are usually black plastic, much higher capacitance (often 100-400 µF), and rated for intermittent duty only. Leave one in circuit continuously and it cooks itself.
A run capacitor is built for permanent duty. It stays energised every second the motor runs, so it has to dissipate heat for years without drying out. Run caps are typically lower capacitance (the 4-80 µF range covers almost everything domestic), housed in an aluminium can (the classic CBB60 cylinder), and oil-filled to manage heat.
Quick ways to tell which one you’re pulling out:
- Shape and material: aluminium cylinder = almost certainly a run cap. Black plastic tub = almost certainly a start cap.
- µF value: under 100 µF = run. Over 100 µF = start.
- Label: it’ll say “Motor Run” or “Motor Start” somewhere on the printing.
- Circuit position: if there’s a relay or centrifugal switch in series with the cap, it’s a start cap. If it’s just permanently wired in, it’s a run cap.
Most of the appliances homeowners deal with use a single run capacitor and no start cap at all. The pool pump, the bathroom extractor fan, the evaporative cooler — all run capacitor only.
What appliances use them
Run capacitors are everywhere once you start looking:
- Pool and spa pumps — the most common reason Kiwis end up shopping for one. A failed run cap is usually why a previously fine pool pump suddenly hums and won’t spin.
- Air conditioner and heat pump outdoor units — both fan and compressor often have their own caps; some use a dual run cap (one can, two values).
- Washing machine motors — particularly older top-loaders with traditional induction motors.
- Garage door openers — chain-drive and belt-drive units almost universally use one on the motor.
- Ceiling fans, pedestal fans, extractor fans — any AC fan motor that isn’t a tiny shaded-pole design.
- Evaporative (swamp) coolers — both the pump and the blower motor typically have their own caps.
- Workshop gear — bench grinders, small compressors, drill presses, lathes.
Different motors call for different µF values. A small extractor fan might want 4 or 5 µF, a typical single-speed pool pump often lands around 30-40 µF, and a larger pump or compressor motor might want 50, 60, or 80 µF.
Signs yours has failed
Capacitors don’t usually fail gradually in a way you’d notice day to day. They tend to give a few warning signs and then drop dead. Watch for:
- The motor hums but won’t spin. Classic symptom. Power is reaching the motor, but without the phase shift from the cap there’s no rotating field, so it just sits there buzzing. If you give the shaft a manual nudge and it then runs, that’s almost a guaranteed cap failure.
- Slow start, then runs fine. Motor takes noticeably longer than it used to before it’s up to speed.
- Won’t start at all. Dead silent, or just a brief hum then a thermal cutout trip.
- Bulging or domed top. The top of a healthy CBB60 is flat. If it’s bulged upward, the cap has internally vented and is finished.
- Oily residue or split casing. The dielectric oil has leaked. Replace immediately.
- Burnt or fishy smell from the motor area. Often the cap itself.
- Motor runs hot or trips on overload after a few minutes. A weak cap (lost capacitance) makes the motor work harder.
A multimeter with a capacitance setting will give you a definitive answer: measure the cap (after discharging it) and compare to the printed µF value. Anything more than around 6% off the rated value is suspect; anything 10%+ off is dead.
How to identify the right replacement
Three numbers on the side of the old cap tell you what to order:
- Capacitance (µF or MFD). This must match. A 30 µF motor wants a 30 µF cap. Don’t “round up” or “round down” to what’s in stock — buy the right value. Common sizes are listed in the shop from 4 µF right through to 80 µF.
- Voltage rating (VAC). The CBB60 caps we stock are all 450 VAC, which covers every 230 V mains appliance you’ll find in NZ. You can always go higher in voltage rating (a 450 V cap will happily replace a 370 V or 400 V one), but never lower.
- Terminal style. This is the bit that catches people out. There are three common styles:
– L (Lug) — 6.3 mm quick-connect spade lugs on top. The most common style on pool pumps and HVAC units.
– FL (Flying Lead) — pre-attached wire pigtails with ring crimps. Common on fans, garage doors, and anywhere the old cap was hard-wired.
– CL (Core Lead) — central M8 stud for mounting plus flying leads. Less common but turns up in some older or industrial gear.
Match the terminal style to what came out and you skip the rewiring step entirely. For example, a typical 30 µF pool pump cap with spade terminals is a CBB60 L 30 µF 450V — same can, same lugs, same numbers, in and out in ten minutes.
If you’re not sure what to order, the Find Your Capacitor wizard walks you through it in three steps: tell it the machine, the µF, and confirm the voltage, and it’ll point you at the right SKU.
Wrapping up
A run capacitor is a small, cheap, predictable part with one job: keep your single-phase motor spinning smoothly. When it dies, the motor usually can’t start, can’t run, or can’t run efficiently — but the motor itself is almost always fine. Replace the cap and you’ve got the appliance back for the cost of a coffee or two, instead of replacing the whole unit.
The three things to nail down before ordering: the µF value (must match exactly), the voltage (450 VAC covers everything domestic), and the terminal style (L, FL, or CL). Get those right and it’s a same-day fix.
If you already know your numbers, browse the shop and order direct from the NZ-bonded warehouse. If you’re not sure what you need, start with the Find Your Capacitor wizard on the home page — it’ll save you a return trip to the toolbox.