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  1. #1
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    Calculating Capacitor Size


    howdy all,

    i often come across ac units with faulty start capacitors for compressors and often not able to read what the size is of original one. Is there a way of calculating the size of capacitor needed?
    Also what happens if you run a capacitor that is to big for the compressor? like a 80 instead of a 60 as an example.
    sorry, prob a basic question...



  2. #2
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    Re: Calculating Capacitor Size

    "...A motor that requires a 7.5 mfd capacitor will not work efficiently with a 4.0 mfd capacitor. Much the same way, a motor will not run properly with a weak capacitor. This is not to imply bigger is better, because a capacitor that is too large can cause energy consumption to rise. In both instances, be it too large or too small, the life of the motor will be shortened due to overheated motor windings. Motor manufacturers spend many hours testing motor and capacitor combinations to arrive at the most efficient combination. There is a maximum of +10% tolerance in microfarad rating on replacement start capacitors, but exact run capacitors must be replaced. Voltage rating must always be the same or greater than original capacitor whether it is a start or run capacitor. Always consult manufacturers to verify correct capacitor size for the particular application. ..."

    As you can see from the above text replacing a capacitor needs to be considered well and should be an exact replacement. Generally speaking for compressors it should not be too difficult to contact your wholesaler and get a match for the compressor model in question.
    Hope this helps.
    Reality is an elusion created by alcohol deficiency. Quaff and enjoy. [Yorkshire, UK]

  3. #3
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    Re: Calculating Capacitor Size

    Thanks for that. Just wish they would specify the capacitor size on circuit diagrams etc. Not always possible to contact manufacturers when on call etc. And sometimes it is important to get it up and running again!

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    Re: Calculating Capacitor Size

    I assume your question is for a specific compressor.

    Can you post the model and manufacturer?
    It might make it easier to tell you the correct capacitor capacity.
    "All things are possible until they are proved impossible - and even the
    impossible may only be so, as of now."

  5. #5
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    Re: Calculating Capacitor Size

    Quote Originally Posted by HVACRSA View Post
    howdy all,

    i often come across ac units with faulty start capacitors for compressors and often not able to read what the size is of original one. Is there a way of calculating the size of capacitor needed?
    Also what happens if you run a capacitor that is to big for the compressor? like a 80 instead of a 60 as an example.
    sorry, prob a basic question...
    This is not a FIX all but a rule, a basic method for motor capacitors.

    (2650 x Full load amps ) divided by supply Voltage = MFD

    Example 2650 x 8amps = 21200 divided by 240volts = 88.33 MFD

    Plus or minus say 10% so call it 95 MFD it will not be correct but will get you out of a hole and the kit back working if that is the real fault and the capacitor has not failed due to other factors etc.

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