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


Lemau is Owner of .Involved in Electrical & Automation Engineering since 2001. Passionate to shared all experience & knowledge about Electrical.

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  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

  6. GOOD


  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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