Single phase capacitor sizing

When install a motor using capacitor for starting or running methods,we must sizing the rated of capacitor suitable with motor to get correct starting torque and avoid winding from overheating and can cause a damage.

This is basically a question of motor design.There is no straightforward regular relationship between capacitance and the motor size in kW.

When replacing these capacitors, the capacitance value and voltage should be taken from the manufacturer’s plate on the motor or from the old capacitor.This must be correct within ±5% and is sometimes stipulated down to a fraction of a μF.The choice of a running capacitor is even more limited than with a starting capacitor.

How to sizing the starting capacitor?

1) A rule of thumb has been developed over the years to help simplify this process. To select the correct capacitance value, start with 30 to 50μF/kW and adjust the value as required, while measuring motor performance.

We also can use this basic formula to calculate capacitor sizing :

2) Determine the voltage rating for capacitor.

When we select the voltage rated for capacitor,we must know the value of our power supply.For safety purpose,multiply the voltage of power supply with 30%.Factors that affect the selection of the proper voltage rating of the capacitor include:
• Voltage de-rating factor
• Safety agency requirements.
• Reliability requirements
• Maximum operating temperature
• Space available

How to sizing the running capacitor?

When selecting motor run capacitors all of the required parameters above need to be identified in an organized process. Remember that, not only are the physical and basic electrical requirements important.

But the type of dielectric material and the metallization technique should be examined.The wrong choice here can have adverse effects on the overall performance of the capacitors.Please refer to motor nameplate or contact supplier or manufacture to get a accurate value of capacitor.Safety First


  1. brian k says:

    hi couldnt make out the formula for single phase [start] caps, blurry.any way to get a clear copy? thanks brian

  2. pls.
    see clear copy of single phase motor capacitor sizzing &with example

  3. Roelof van Heerden says:

    To whom it may concerin
    Thank you for valuable information. It is a long time since I last worked with these motors and I forgot to work out the size of the capacitor for staring a S/P motor. Your infog in this article helped me a lot and I hope to recover my motor which was out of order for about a year now.
    Happy new year!
    Roelof van Heerden

  4. Robert says:

    Hey, I found your post very valuable. But I got a question regarding the run capacitor. You aproximate sizing of the capacitor is in the range of 4-40 mF and at the motors I have 1.1-2.2 kW the run capacitor is sized between 35-70 uF. The difference is more than 100 times, is there a calculation error?

  5. shahid mahmood says:

    Dear sir I want to know one information we have one air-condition inside that there is a contactor voltage 110 and 50 hertz.They fix on the coil 0ne capacitor and a resistance. I want to know what is the function of this and how to calculate the capacitor and resistance

    • You charge the cap with a megger (1000v) shuve it up your ass if it fires you will instantly know the size of the micro farads whwhwhwhwahahahahaahahaaha

  6. GOOD

  7. Please send me a clear copy of this formula? Cannot make out the mathematical operations that are a part of the formula.
    Thank you.

  8. Steven says:

    Can you please send a clear copy of the formula? I can’t make out the signs in the formula
    or the exponent. Can’t tell if they are + ,-,* they all look the same blob.
    A friend has an Airfoxx carpet dryer 1HP. The cap got fried along with most of the wires. Can replace wires but cannot read anything on the Cap as is too badly damaged. The manufacturer has apparently gone out of business. It has three speeds and a 4 pole motor. I am assuming it was a start capacitor as it was plastic I was told most start caps are plastic and running are metal. Any help would be appreciated.
    Thank you.
    The manufacturer plate only gives amps 8,9 and 11 amps and 1HP. Says 4 pole motor.
    It runs on 120volt ac.

  9. Please,I want to see the clear.cnnection Of the of the three phase motor with.capacitor

  10. Gigi Moore says:

    Could u please send me sizes if capacitor fir 220 volt ac electric motors. Starting and running capacitors for all different kw/hp motors? Thank u

  11. Ayyasamy says:

    can u send me on CLEAR copy of capacitance motor formulla! – See more at:

  12. kavuru srinivasarao says:

    sir i need formula for capacitor value in terms of winding parameters.i.e. winding resistances and reactances and frequency etc.please send me mail

  13. if i use 30mfd capacitor instead 25mfd capacitor require single phase ac motor what happen ?

  14. I want know what much capacitor need to buy ,the electric motor 220volt /1hp 60Hz/single phase.pls send an idea how to chose a correct capacitor

    • Firstly try to see the amps rating on name plate of motor and do this calculation.
      If amps is 8 amps
      By formula shown above for starting cap.
      C = 8 x 1000000 / 2 * 3.14* 60*220
      = 8000000/82896
      = 86.5 mfd

  15. Joe Baisch says:

    Hello… I own a Craftsman Table Saw 113299410… I am replacing the Start capacitor and the run capacitor … The installation of the run capacitor eliminated the UF number…
    What is the UF number of that capacitor? Joe Baisch, Brinnon Washington

  16. S.N.Sharma says:

    I want to know formula for decide capecitor’s capecity in mfd. For single phase motor, as culling fan and. 5hp ,1hp, 2,3,5hp also.

  17. Hung Yew Kim says:

    I want know what much capacitor need to buy ,the electric motor 240volt /1hp 50Hz/single phase.pls send an idea how to chose a correct capacitor

  18. What is that rating of 370W single phase motor?

  19. Rasheed says:

    Dear sir
    Starting capacitor value for 3 HP single phase induction motor ?

  20. Mwandu Singoyi says:

    I want to know the size of capacitor I can use to a single phase water submissive pump

  21. Gaurav says:

    which capacitor is use in 10hp motor????
    i mean how MFD capacitor is use in 10 hp motor???
    reply me fast

  22. Kim Poindexter says:

    I have a 1/3HP motor. It says to use a 7.5uF, 370V capacitor. This will be used for a disc sander, so will never run more than about 5 minutes at a time, any typically under 1 minute. The only thing I can get locally is an 8uf 440V capacitor. Should I be concerned about the capacitance difference?

  23. Question: Can you use an 130 mf electrolytic DC capacitor for a motor run capacitior?

  24. mark Ammons says:

    hello, i have an air compressor that has had the capacitors removed can someone suggest the start and run caps for it……EMERSON LR63596 T63BXBYF1133
    240v 15.0amp LR amps93 output 2.98KW PH 1 SF 1.0 3450rpm
    Thanks for any help

  25. Djuned Arie Prasodjo says:

    On table for “Running Capacitor”, mF mean mili Farad or micro Farad ??

  26. moses says:

    does a water pump motor
    use start capacitor or running capacitor or Both?


  1. […] single phase electric motor help. from here Single phase capacitor sizing 80-110 microfarads, if this is for the lathe, I would say go big, as IIRC you will get more […]

Speak Your Mind