Pool Pump Capacitor: Types, Specs, Testing, and Replacement Selection
A pool pump’s capacitor fails after just 3-8 years and often causes issues like humming without starting, making it a frequent culprit. Replacing one costs between $15-$40, while ignoring it can lead to motor damage costing $200-$600+. This guide covers the function of different capacitor types, how to locate yours, testing procedures, and what to order when replacement is necessary. Begin with our comprehensive pool pump troubleshooting resource if you’re unsure whether a faulty capacitor is the problem.
Video guide
Video: “How To Test A Pool Pump Motor Capacitor” by Inyo Pools
What a Pool Pump Capacitor Does
There are two distinct types of pool pump capacitors. They do different jobs at different times.
Start capacitor: This component supplies 150-175% of full-load torque, critical for getting a pool pump motor spinning from rest. The centrifugal switch disconnects it after about 2-3 seconds when the motor hits two-thirds of its speed. When a faulty start capacitor causes your pool pump to hum but fails to start properly, replace or check this part first.
Run capacitor: The run capacitor stays in the circuit the entire time the pump runs. It improves running efficiency, reduces heat buildup in the motor windings, and shifts the current phase in the run winding. A failed run capacitor causes high amp draw, overheating, and sometimes a pump that starts briefly then trips its thermal protector.
Why capacitors fail: Age and heat are the primary causes. Start capacitors also fail when the centrifugal switch sticks. Instead of cutting the start cap out of the circuit after 2-3 seconds, a stuck switch keeps it energized continuously. We see this pattern most often on older motors where the centrifugal switch has developed carbon buildup. A start capacitor designed for 2-second bursts fails almost immediately when left in the circuit full-time.
Start capacitor vs. run capacitor: key differences
| Spec | Start Capacitor | Run Capacitor |
|---|---|---|
| MFD range | 50-400 MFD | 15-50 MFD |
| Voltage | 125 VAC or 250 VAC | 370 VAC (upgrade: 440 VAC) |
| Physical can | Hard plastic, cylindrical | Metal (aluminum or steel) |
| Location | Non-drive end canopy (under cover) | “Doghouse” hump on top of motor |
| Circuit role | Switches out at full speed | Stays in during operation |
| Failure sign | Hole opens in vent, electrolytic fluid visible | Ends bow outward |
| Symptom | Humming, won’t start | High amp draw, overheating |
Inspect start capacitors rated between 50 and 400 microfarads (MFD) for a voltage of either 125 or 250 volts alternating current (VAC). Run capacitors, in contrast, usually measure around 15 to 50 MFD at 370 VAC. Ensure you don’t swap them; each has distinct functions and can’t replace the other effectively.
440 VAC run capacitors: Standard units rated at 370 VAC offer a safety buffer against voltage spikes and operate cooler under normal conditions, making them a practical choice for those who want reliability. For a 240VAC pump, the minimum safe capacitor rating is calculated as 340V, meaning 370V is the bare minimum you should consider. Going with the 440V version ensures better surge protection and can extend the lifespan of your equipment.
Dual capacitor for two-speed motors: Two-speed pumps sometimes use a single can with two capacitance values, for example, 30+10 MFD. This dual capacitor has three terminal blocks (one common, one for high speed, one for low speed). Match both values when ordering.
Pentair Centurion housing note: The Pentair Centurion motor housing accepts a maximum capacitor diameter of 1-3/4 inches. The common 50mm (approximately 2-inch) replacement capacitors don’t physically fit. Confirm the diameter before ordering.
Which Capacitor Does Your Motor Have? (Motor Type Codes)
Before ordering any capacitor, identify which type your motor uses. A.O. Smith motor type codes appear on the motor data plate (the sticker on the motor housing, often on the side or back):
Worth checking.
Worth checking.
Worth doing.
| Code | Motor Type | Capacitor(s) Needed |
|---|---|---|
| CX | Switchless (PSC) | Run capacitor only |
| CS | Capacitor Start | Start capacitor only |
| SP | Split Phase | No capacitor |
| CP | Capacitor Start/Run | Both start and run |
How to read the data plate: Look for a flat label on the motor housing. The type code (CX, CS, SP, or CP) appears with HP, Amps, Volts, and RPM. Take a photo of the entire label before ordering anything.
Capacitor Start (CS) is the most common motor type for in-ground residential pool pumps. If you have a standard Hayward, Pentair, or Jandy single-speed pump, it’s almost certainly a CS motor requiring a start capacitor.
Variable speed pumps: Variable speed pumps use permanent magnet motors. They have no start capacitor and no centrifugal switch. If your pump is a Hayward TriStar VS, Pentair IntelliFlow, or similar variable-speed model, this guide doesn’t apply.
Above-ground pump note: Above-ground pool pumps from Intex, Bestway, and similar brands often use Split Phase (SP) motors with no capacitor at all.
How to Find the Capacitor on Your Pump
Run capacitor location: Look for the “doghouse”, a raised rectangular hump on top of the motor housing, usually held down by 2-4 small screws. Remove the screws, lift the cover, and the run capacitor (a metal can) is inside.
Start capacitor location: The start capacitor sits at the non-drive end of the motor, the electrical end where the power conduit enters. There’s a round or rectangular cover held by 2 screws. Remove it and the start capacitor (hard plastic can) is inside.
Motors with both: CP-type motors have both capacitors. Check both covers. The start capacitor is always at the electrical end. The run capacitor is always under the doghouse hump.
Polaris PB4-60 booster pumps: These use a start capacitor only, no run cap. The housing design is different from standard pool pump motors.
How to Test a Pool Pump Capacitor (Multimeter Method)
Tools needed: Digital multimeter with a capacitance setting (marked µF or mF)
Step 1: Turn off pump at circuit breaker. Not just the timer switch, cut power at the breaker. Lock it out if possible.
Step 2: Discharge the capacitor. Lay the metal shaft of an insulated screwdriver across both terminals at the same time. You may see a small spark. Repeat twice to ensure full discharge.
Step 3: Remove the capacitor leads. Note which lead goes to which terminal (take a photo). Disconnect the leads from the terminals.
Step 4: Set multimeter to capacitance (µF setting).
Step 5: Touch probes to capacitor terminals. Polarity doesn’t matter for most capacitor tests.
Step 6: Read the display. Compare the reading to the MFD value labeled on the capacitor body.
Interpreting results:
- Reading within ±10% of labeled value = capacitor is good
- Reading near zero = capacitor has failed
- Reading far below the 10% range = capacitor is failing
If your 7.5 MFD capacitor registers below 6.75 or above 8.25 MFD, you’ll want to replace it. A reading of just 2.1 MFD indicates significant degradation and failure.
Same again.
Alternative ohms test (if multimeter has no capacitance setting):
Verify the multimeter is set on its minimum ohms (resistance) scale. Connect probes to the capacitor’s terminals. With a healthy capacitor, you should see the meter rapidly rise then gradually return close to zero as the cap charges via the multimeter’s internal supply. Should a faulty capacitor be present: the displayed value will remain at zero uninterrupted or climb sharply and stay fixed.
See the TroubleFreePool capacitor reference for additional testing documentation.
Capacitor specs: what to order
Four rules determine the correct replacement capacitor:
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Bottom line.
Rule 1. Match MFD exactly (or go slightly higher, never lower). A 30 MFD replacement can be swapped for a 32-33 MFD, but never a 25 MFD. Lower MFD causes inadequate starting torque and early failure.
Rule 2. Match or exceed voltage rating. A 370 VAC run cap can be replaced with a 440 VAC unit. Never install a lower-voltage capacitor.
Rule 3. Match physical diameter. The capacitor must fit inside the motor housing. Pentair Centurion requires 1-3/4” maximum. Measure the failed capacitor before ordering.
Rule 4. Use the motor model number, not the pump housing number. The pump housing number (e.g. Hayward SP2607X10A) doesn’t identify the motor inside. Read the motor data plate for the motor catalog or part number. Always use the motor model number to identify the correct capacitor.
Never install a run capacitor larger than the original, this causes motor overheating and destruction.
Motor-to-capacitor cross-reference examples:
| Motor Model | Capacitor Required |
|---|---|
| Century B128 (1.0 HP) | 25 MFD run cap |
| Century B130 (2.0 HP) | 30 MFD run cap |
| AO Smith C48L2N134B1 (1.5 HP) | 161-193 MFD start cap (BC-161) |
| Hayward C48K2N143B1 | 161-193 MFD, 110V start cap |
| Polaris PB4-60 booster pump | 124-149 MFD start cap (BC-124) |
For a more complete cross-reference, the INYOPools capacitor selection guide covers dozens of motor models.
Where to buy: Local electric motor repair shops carry the widest selection for immediate pickup. Start capacitors are also commonly stocked at Ace Hardware. For online ordering with motor-specific search: INYOPools and PoolSupplyWorld.
Ready to swap it out? See our step-by-step guide on how to replace the capacitor for the full disassembly and reinstall procedure.
FAQ
How do I know if my pool pump capacitor is bad without a multimeter?
Visual inspection often reveals failure. On a start capacitor, look for a hole or crack in the vent at the top of the plastic can, failed start caps release their electrolytic fluid through the vent. On a run capacitor, look at the ends of the metal can. Bowed or bulging ends mean the capacitor has failed internally. Oil residue or brown staining around the cap is also a failure indicator.
Can I run my pool pump without a capacitor?
No, on any motor that requires one. A Capacitor Start (CS) motor relies on the start capacitor to generate enough torque to spin up from a standstill. Without it, the motor hums and heats up but never starts. Continuing to attempt starts without a functioning capacitor overheats the motor windings. A PSC (CX) motor similarly can’t run efficiently without its run capacitor.
How often do pool pump capacitors fail?
Run capacitors typically last 5-10 years under normal conditions. Start capacitors last 3-8 years. Lifespan shortens if the centrifugal switch sticks, a stuck centrifugal switch keeps the start capacitor energized continuously and destroys it within seconds of each start attempt. If you’re replacing start capacitors more than once every 2-3 years, test the centrifugal switch. For ongoing pool equipment maintenance schedules, capacitor inspection should be included in annual motor service.
Is a run capacitor the same as a start capacitor?
No. They differ in MFD range (15-50 MFD for run vs. 50-400 MFD for start), voltage rating (370 VAC for run vs. 125-250 VAC for start), physical construction (metal can for run vs. hard plastic for start), and circuit role (run stays in circuit during operation. Start is removed at full speed). They’re not interchangeable. Installing a start capacitor in the run cap position, or vice versa, damages the motor. For pool equipment electrical problems involving other equipment, the same motor-type principles apply across pool heaters and filter motors. See our guide to pool pump problems for the full symptom-to-diagnosis map.