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

    • Peeyush Nirdosh says


      This is Peeyush kumar Nirdosh, I am looking for any formula where i could get motor run and motor start capacitance rating as per HP rating of the motor.

      Thanks and regards

  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?

    • Hi Robert,
      I am trying to figure out what run capacitor I need for a compressor motor (Cornelius 130R1500) which has no labels at all but from literature should be 14.5 amp, 110V. I was able to start it briefly with a 300uF start capacitor and two run capacitors at hand- 10uF and 22 uF which I wired separately and in parallel (this kept it running longest, for 20 sec until smoke started coming out as it got very hot so I shut it down).
      Any help appreciated. Looks like an awesome machine, much more powerful than similar size motors I have but pushes the limits of household power so one has to be optimal with capacitor choice.
      Thank you in advance,

  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

    • hng boon pin says

      240 volt / 1hp motor / 50hz / 1 phase, motor must 4 poles 3.25amp.
      By formula shown above for starting cap.
      C = 3.25amp x 1000000 / 2 * 3.14* 50*240
      = 3,250,000 / 75,360
      = 43.12 mfd

      • Peeyush Nirdosh 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

  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 ??

    • Thomas says

      I noticed that, too. I expect it’s microFarad. I have a four uF capacitor on a blower fan motor.

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

  27. Joseph says

    ratings for a 7.5kw ,10hp singlephase motor?

  28. I have this motor from a DAIKIN Split type AC with the model of FTKC35PVM. The motor has this specs 220-240/220-230V. INPUT 57W, CURRENT 0.27-0.26-0.24/0.27-0.26A.This specs is written on a sticker sticked on the side of the AC indoor. Not on the motor itself.

    The motor itself has 6 wires, brown, black, red, blue, yellow, orange.
    the motor has 6A labeled.
    the motor looks like brushless motor. A motor that looks like a drone motor just a bugger version of it.

    Problem :
    – No idea how many phases the motor has.
    – No idea what the capacitor is or the value since it was just given to me. and also no pcb or controller.
    – I want to know the value of the capacitor on its microfarads and volts.
    – I want to know the supply voltage in AC or DC. (I tried to supply 12vc DC on its wire forgot the color maybe between (brown/black/red) and the other also on between those 3 color and somehow I got reaction from the motor. A reaction that is like a stuck shaft on a pole.

    Purpose :
    I want to make the motor run again in manual way. like a simple fan with low, mid, high control. That is why I want to know the voltage input and the value of the capacitor..

    Thanks for any help/effort!

  29. which capacitor is use in 1/2hp motor????

  30. virendra says

    sir 3hp sp mey runnig aur starting condensor kitne mfd ka lagega

  31. virendra says

    sir 1.5 hp water filed panel bar bar trip kar ja raha hai motor is good condition – vac 220 proper avlaible

  32. Hello, I have one 3 phase motor want convert to single phase.Motor details.0.55kw,1.3A, 20uf enough to convert 400v to 220v??

  33. 5 hp 3.5kw singal phase motor is used than
    Which VAC capacitor controller name and company
    Plz suggest me

  34. Vincent says

    Hello. My name is Vincent from Kenya. I have a problem with 10 hp single phase electric motor use in posho mill. It always damage the capacitors, especially starting capacitor.
    Kindly advice me the best capacitor that I should use to avoid damaging

  35. Adarsh says

    I have 1 phase induction motor
    Amp 0.6/ 230VAC
    Which capacitor has to be use.

  36. sathish says

    for single phase starter capacitor is in series connection r parallel connection

  37. My vilage in siries Voltage comes 1 phase 240v 2 phase 180v 3 phase 30v ac how to start single phase motors

  38. My vilage in siries Voltage comes 1 phase 240v 2 phase 180v 3 phase 30v ac how to start single phase motors capacities values tell me plzz which values 10 HP motors

  39. Mwenda Keith says

    I have a 3 phase water pump which runs on a 3.7 kW and I want to reduce it to a single phase. My question is, what size of a running capacitor should I use?


  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 […]

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