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How To Select the Right Capacitor For Your Pool Pump Motor

WRITTEN BY: Inyo Pools

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This guide discusses the two kinds of capacitors that could be used on pool pump motors: start capacitors and run capacitors. It also addresses how to know a capacitor is failing and how to replace it.

Click Here to View All Motor Parts (Capacitors, Bearings, Switches & More)

Step by Step

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Step 1

CAPACITOR TYPES - A pool pump can have two types of capacitors: a Start capacitor and A Run Capacitor. The Start capacitor is switched into the the motor's winding circuits on start up to help the motor turn over and come up to speed. When the motor is close to running speed, the Start capaciotor is switched out of the winding circuits. The Run capacitor is incorporated in the motor circuit to help increase running efficiency. It remains in the circuit as long as the pump motor is running. The start capacitor will generatlly be rated at 50-400 MFD and 125 or 250 VAC. The run capacitor will generally be rated at 15-50 MFD and 370 VAC. The capacitor in the picture is a Start Capacitor rated at 124-149 MFD 125 VAC.

Step 2

MOTORS WITH CAPACITORS - Not all pool pump motors use capacitors. There are four common types of residential pool pump motors: Split Phase, Capacitor Start, Permanent Split Capacitor and Capacitor Start/ Capacitor Run.
The Split Phase motor is used extensively in spa and jetted tubs, and above ground pools. This design has a start winding and a start switch, but no capacitors.

The Capacitor Start motor is used commonly is in ground pool pumps.The starting torque is higher (150-175% of full load) and starting current lower than the split phase equivalent. The operation is similar to a split phase in that there is a start switch to take the start winding and capacitor out of the circuit once the motor reaches 2/3 to 3/4 of full speed.

The Permanent Split Capacitor motor does not have a start switch and is often referred to as a “switchless” motor. It does use a Run Capacitor that remains in the circuit at all times. The PSC design has significantly less starting torque than the capacitor start design.
The Capacitor Start/ Capacitor Run motor design is used to increase efficiency in run mode. Both Start and Run Capacitors are used. In run mode, PSC and Cap. Start/Cap. Run motors are the same.

Step 3

START CAPACITOR LOCATION - START capacitors are usually hard or glassy looking plastic enclosed can that are typically, though not always, found under the non-drive end canopy of the motor.

Click Here to Find Your Start Capacitor

Step 4

RUN CAPACITOR LOCATION - RUN capacitors are usually the METAL (aluminum or steel) enclosed can capacitors that are most of the time, but not always, found under the "hump" on top of the motor - also referred to as the capacitor housing or "doghouse". This picture shows the location of the run capacitor under a doghouse.

Click Here to Find Your Run Capacitor

Step 5

START CAPACITOR FAILURE - Usually the effect of a failed START capacitor is a "sluggish startup" or "humming with no startup. When a START capacitor is starting to go bad, you might see some leakage of electrolytic oil, and when it fails the vent opens and expells the electrolytic oils. A failed start capacitor will have a physical hole in the vent after it has failed.

Step 6

RUN CAPACITOR FAILURE - The effect of a failed RUN capacitor is usually "hi amps" and sometimes "humming with no start" and often times motor won't run in the hi amp condition without tripping thermal protector. When a RUN capacitor is starting or has gone bad, the end of the capacitor that has the terminals and sometimes the opposite end without terminals bows outward.
The capacitance reading would be significantly lower than rated if its failed. There isn't a "vent" typically on RUN capacitors -- they expand/bow outward with a fail safe internal structure so that the bowing out of the terminals breaks the contacts internally in the capacitor in a fail safe mode.

Step 7

REPLACING START CAPACITOR - See our guide on "How To Replace a Pool Pump Capacitor". When you replace a capacitor, never put in a lower rated capacitor. If you can't find your size capacitor, it is acceptable to use the one rated one step higher. If you pump motor is continually burning up capacitors, try using the next one up in size. Capacitors are susceptible topower variations or increases. If you can't control your system power, you may need a capacitor with a little higher rating. Use this link to purchase the Start Capacitor you need.

Step 8

REPLACING RUN CAPACITOR - To replace a Run Capacitor, remove the Capacitor Cover (bump) by removing the two screws. Then follow the precautions above to disconnect and replace the capacitor. As stated above, when you replace a capacitor, never put in a lower rated capacitor. If you can't find your size capacitor, it is acceptable to use the one rated one step higher. If you pump motor is continually burning up capacitors, try using the next one up in size. Capacitors are susceptible topower variations or increases. If you can't control your system power, you may need a capacitor with a little higher rating. Use this link to purchase the Run Capacitor you need

Click Here to View All Motor Parts (Capacitors, Bearings, Switches & More)

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SB Posted: 10/1/2016

I have a 2-speed AO Smith spa pump motor (Century 7-196012-03) which sometimes fails to start. Since it's the circulating pump a start failure causes an overheat condition. The other day when I turned it on it hummed only, but when I rapped it with my knuckles it started running. The shaft seems to turn smoothly by hand, and when running it doesn't sound bad, but a little louder than the single-speed pump (same motor). The run capacitor in both motors look fine. Does this sound like a capacitor problem, motor problem, sticky pump, something else?? Many thanks, SB (And nice site, I appreciate the tech info!)
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Dave B. Posted: 9/29/2016

My pump motor would not turn on at all. It would just hum for 10 seconds or so. I replaced the capacitor and now it comes on fine, but only on high. Any thoughts would be sincerely appreciated!
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InyoPools Product Specialist Dennis R. Posted: 9/2/2016

capacitor - Here is the link to a #628318-308 capacitor.
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Anonymous Posted: 8/31/2016

need a replacement capacitor for a genteq 27L1358 30uf 370vac 50/60 hz manufacturer # 628318-308. any suggestions on what to buy
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Christer Posted: 7/31/2016

Excellent resource and troubleshooting. Thank you!
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InyoPools Product Specialist Dennis R. Posted: 2/9/2016

User1 - This is a characteristic of low voltage. You may have your pump motor set up for 220V with a supply voltage of 110V. See Step #4 of our guide on "How To Determine Why a Motor Won't Start or Shuts Off" for other suggestions.
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User1 Posted: 2/6/2016

What could be my problem if the pump keeps turning on and off right after the power is turned on. The pump goes on and off about 5 times and then the GFI breaker goes off. Any thoughts anyone?
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Jean Posted: 1/10/2016

THANK YOU! Finally someone who explains repairs simply and concise. I had been trying to find out the difference in Start and Run Capacitors and you spelled it out. Again, thank you from a lady trying to do home repairs - and lots of them!
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Anonymous Posted: 7/13/2015

Excellent tutorial.
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ken Posted: 4/29/2015

Good definitions and procedures.
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