Pool Pump Capacitor: Types, Specs, Testing, and Replacement Selection
A pool pump capacitor is a small electrical component that can fail in as little as 3-8 years, and when it does, it’s the most common reason a pump hums but won’t start. Replacing one costs $15-$40. Ignoring it and burning out the motor costs $200-$600+. We put together this guide to cover what each capacitor type does, how to find yours, how to test it, and exactly what to order when you need a replacement. Start with our full pool pump troubleshooting guide if you’re not yet sure the capacitor is the cause.
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, and they do different jobs at different times.
Start capacitor: A pool pump start capacitor provides 150-175% of full-load torque to spin the motor from a standstill. It is only in the circuit for 2-3 seconds, the centrifugal switch removes it once the motor reaches about two-thirds of full speed. Once the motor is running, the start capacitor is out of play until the next startup. When a pool pump hums and won’t start, the start capacitor is the first thing to check.
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 |
Start capacitors are rated 50-400 MFD at 125 or 250 VAC. Run capacitors are rated 15-50 MFD at 370 VAC. They are not interchangeable.
440 VAC run capacitors: Standard run capacitors are rated 370 VAC. Upgrading to a 440 VAC-rated run capacitor provides a safety margin for voltage surges and runs cooler under normal conditions. For a 240VAC pump, the minimum safe capacitor rating is 240 × 1.414 = 340V, so 370V is the absolute floor. The 440V version handles surges better and typically lasts longer.
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 do not 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):
| 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 does not 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 does not 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
Example: A 7.5 MFD capacitor should read between 6.75 and 8.25 MFD. A reading of 2.1 MFD on a 7.5 MFD capacitor means replace it.
Alternative ohms test (if multimeter has no capacitance setting):
Set the multimeter to its lowest ohms (resistance) setting. Touch probes to the capacitor terminals. A good capacitor causes the meter reading to jump high and then slowly drift back toward zero as the capacitor charges from the meter’s internal battery. A failed capacitor: the reading stays at zero the entire time, or jumps high and stays there permanently.
See the TroubleFreePool capacitor reference for additional testing documentation.
Capacitor specs: what to order
Four rules determine the correct replacement capacitor:
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) does not 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 cannot 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 significantly 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 are 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.