What does a motor start capacitor do?
A single-phase induction motor can’t produce a rotating magnetic field on its own, so it can’t decide which way to turn. A start capacitor fixes that for the first moment of start-up: it feeds a large, phase-shifted current into the motor’s start winding, giving high starting torque so the motor can break away under load: a compressor against pressure, a pump against a full column of water, a door against its springs.
Once the motor reaches roughly three-quarters of its running speed, a centrifugal switch, a start relay or a PTC device takes the capacitor out of the circuit. It is built for that intermittent duty only. If the switch or relay sticks closed and leaves it in circuit, a start capacitor overheats and fails within seconds.
A run capacitor is the opposite: a film capacitor (CBB60 or CBB65) that stays in circuit the whole time the motor runs, with a smaller value at a higher voltage. The two are never interchangeable, in either direction. Some motors (capacitor-start, capacitor-run) carry both: you’ll see a CD60 and a CBB-type can side by side. Our run vs start capacitor guide shows how to tell which layout your pump uses, and CBB60 vs CBB65 vs CD60 compares the three body codes side by side.
Which appliances use a CD60 start capacitor?
Start capacitors turn up wherever a single-phase motor has to start under load. These are the typical start-capacitor ranges we see by appliance class, all at 250V:
| Appliance |
Typical start capacitor |
Voltage |
| Compressors and air tools |
50-500µF |
250V |
| Well, pressure and bore pumps |
100-300µF |
250V |
| Pool and spa pumps (start-capacitor motors) |
150-300µF |
250V |
| Garage door openers |
150-250µF |
250V |
| Fridges and freezers |
50-150µF |
250V |
These are typical ranges, not a fitment guarantee. The value printed on your own capacitor always wins. Most NZ pool and pressure pumps are permanent-split-capacitor (PSC) motors that use only a run capacitor; a start capacitor shows up on higher-torque and capacitor-start motors. As one documented example, Davey’s 2026 parts catalogue lists a separate start capacitor for its single-phase VM multistage pumps: 100µF on the 0.75 and 1.1kW motors, 150µF on the 1.5 and 2.2kW motors, alongside the run capacitor. Davey doesn’t print a voltage for those, so read it off your own part.
How do you read a start capacitor label?
A CD60 label usually carries four things:
- Capacitance, often as a range: for example “161-193 MFD” or “161-193µF”. MFD is an older abbreviation for microfarads. The range is the manufacturer’s tolerance band: any capacitor that measures inside it is in spec.
- Voltage: for example “125VAC”, “220VAC” or “250VAC”. This is the maximum the capacitor is rated for.
- Frequency: “50/60Hz” on most parts.
- Body code and duty: “CD60”, sometimes with a duty marking showing it’s for intermittent start use.
Worked example: a label reading 161-193 MFD 125VAC describes a start capacitor with a nominal value of about 177µF, rated for 125V. That’s a common marking on US-market motors.
What size start capacitor do I need?
Match the label. If it prints a single value, match the µF on the label; within ±5% is fine. If it prints a range, choose a value inside that range where one is available. Where none of the listed values sits inside the range, pick the listed value at or above the bottom of the range, never below it, or the motor won’t have the torque to break away. For the 161-193µF example, that’s 200µF: about 4% above the top of the range. If the next listed value would be more than about 5% above the top of your range, ask us before ordering.
Voltage works the same way as on a run capacitor: the same or higher is fine, never lower. A 250V CD60 replaces start capacitors marked 110V, 125V, 165V, 220V or 250V. It does not replace one marked 330V or higher.
Don’t guess the value from the motor’s horsepower or kilowatts, and don’t copy another pump of the same brand. Start capacitor values are chosen for the specific motor winding, and an undersized one leaves the motor humming; a grossly oversized one stresses the start winding.
We list CD60 start capacitors at 250VAC in these values: 50µF, 75µF, 100µF, 150µF, 200µF, 250µF, 300µF, 350µF, 400µF, 450µF and 500µF.
What are the signs a start capacitor has failed?
- Hum, then trip. The motor hums for a few seconds without turning, then the thermal overload or breaker trips.
- It starts with a push. A fan, pump shaft or compressor pulley that spins up when given a nudge has lost its starting torque.
- Slow, laboured starts that get worse over weeks.
- Physical damage. A domed or split top, a vent that has blown open, a wet or crusty residue, or a burnt smell around the cover.
- Breaker trips the moment it starts: the capacitor may have failed short. Stop resetting it until it’s been checked.
A dead run capacitor can produce some of the same symptoms, so if your motor has both, test both while the cover is off.
Why do start capacitors keep failing?
A start capacitor that fails again within weeks or months is usually a symptom, not the fault. The common causes:
- A sticking centrifugal switch or start relay. If the capacitor isn’t switched out once the motor is up to speed, it stays in circuit and cooks. A replacement will fail the same way until the switch or relay is fixed.
- Too many starts per hour. Start capacitors are rated for a limited number of short starts. A pump that short-cycles (starting and stopping every few seconds) burns through them.
- A waterlogged pressure tank or faulty pressure switch that causes that short-cycling. Our guide to a bore pump that starts then cuts out covers the tank and pressure-switch checks.
- Low supply voltage on long cable runs, which slows the motor’s acceleration and keeps the capacitor in circuit longer.
- Heat. A capacitor mounted in a hot, unventilated pump shed or compressor housing ages faster.
How do you test a start capacitor?
You need a multimeter with a capacitance range that reaches a few hundred µF. Isolate the power, discharge the capacitor, disconnect it and photograph the wiring, then measure across the two terminals. If the label shows a range, the reading should fall inside it (allowing for the meter’s own accuracy); if it shows a single value, within ±5% of it. A reading well below the range, zero, or “OL” means replace it. The full walkthrough is in how to test a pump capacitor with a multimeter. The method is the same for start and run capacitors.
Two things trip people up. A start capacitor must be fully discharged before it will read correctly. And inexpensive handheld meters can disagree with a bench meter by more than 5% at these values, so treat a borderline reading as borderline.
Can you replace a start capacitor yourself?
On a plug-in appliance (a portable compressor, a plug-in pump), the swap is a mechanical job once the power is off and the capacitor is discharged: photograph the wiring, move the leads across one at a time, and refit the cover. A CD60 is non-polarised, so the two leads are interchangeable. On hardwired equipment, the circuit has to be isolated at the switchboard first. If you’re not confident working on mains equipment, have a registered electrician do it.
If the capacitor has a resistor soldered across its terminals, note it before you remove anything. See the FAQ below.
Where can you buy a CD60 start capacitor in NZ?
We stock 11 CD60 start capacitors from 50µF to 500µF, all rated 250VAC, ±5%, with flying-lead terminals, priced from NZ$49.95 to NZ$79.95. Tracked NZ delivery: each product page shows the delivery time for that size. Each product page also lists the common printed ranges that value covers. If your old capacitor is rated above 250V, or you can’t find a value that fits your printed range, contact us with a photo of the label before ordering.
Frequently asked questions
Can I use a run capacitor instead of a start capacitor?
No. A run capacitor (CBB60 or CBB65) has a far smaller µF value and won’t give the motor enough starting torque, and a start capacitor (CD60) isn’t built to stay in circuit continuously. Replace a start capacitor with a start capacitor and a run capacitor with a run capacitor.
Can I go higher on µF with a start capacitor?
Slightly, and only to reach a listed value. Choose a value inside the printed range where you can; otherwise the nearest listed value above the bottom of the range, no more than about 5% above the top. Never go below the range. A much larger start capacitor overstresses the start winding.
Is a 250V start capacitor OK on 230V NZ mains?
Yes, when the original is rated 250V or lower. Start capacitors on 230V single-phase motors are commonly marked 220V or 250V, and a 250V CD60 replaces either. If your old start capacitor is marked 330V or higher, a 250V part is not a substitute.
What does 161-193 MFD mean?
It means the capacitor is in spec anywhere from 161 to 193 microfarads (MFD is an old abbreviation for µF), so its nominal value is about 177µF. To replace it, choose a value inside that range, or the nearest listed value above the bottom of it: 200µF in our range, about 4% above the top.
Why is there a resistor across my start capacitor?
It’s a bleed resistor. It drains the capacitor’s charge between starts so the start relay or switch contacts don’t arc when they close again, and it makes the part safer to handle. If your old capacitor had one, fit an equivalent resistor across the new capacitor’s terminals, or ask us or an electrician before you reconnect it.