Correctly Sizing a Capacitor

  I don't know how many times a technician has said that they installed a part based on what was on their service truck.  I have heard of technicians wasting money over-sizing contactors, cutting down air filters, and even using controlled substances to clear condensate drains!  Of course, these scenarios all get the job done, but I would argue the many reasons why not to do these.  The one thing that gets to me is when a technician doesn't verify they are installing the correct size dual run capacitor.  Believe it or not, there is a simple method to figuring the correct size capacitor, without waiting on hold for the distributor's guru.  Of course, you could use a multimeter that reads microfarads (uf), but this will only tell you if the existing capacitor is weak - not the correct size!
Testing Volts / Amps
With the condensing unit operating under a load, you will need to measure the total volts between the HERM and COMMON terminals on the run capacitor (i.e. 345 VAC).  Next, measure the amperage on the wire leading from HERM to START on the Compressor (i.e. 4 amps).

  Use the equation below to verify the size of the capacitor. The resulting microfarad (uf) should match the size of the installed capacitor.

  An over or under-sized capacitor will cause an imbalance in the magnetic field of the motor.  This hesitation when operating will cause noisy operation, an increase in power consumption, a drop in motor performance and eventually overheating or overloading motors like compressors.  The run capacitor should have the exact microfarad (uf) that the motor is rated for.  Capacitors rated above 70uf are considered Start Capacitors and are generally removed from the circuit electrically during operation.  This is where the rule of +/- 10% of the rating came from, for Start Capacitors ONLY!  The voltage rating should be no less than the listed amount for the motor, for central heat pumps and air-conditioners this is usually a minimum of 370VAC.  Most new condensing units are specified for 440VAC capacitors, and are more durable during fluctuations in power supply.  I have seen some universal type dual run capacitors rated for as much as 700VAC, as this voltage rating does not impact uf performance.  The changes in uf will impact amperage draw and reflect on kilowatt hour usage though.
  When completing your early season maintenance this year for your customers, do them a service by testing capacitors and verifying their size.  You could just increase your service ticket total and save some all important power.  You may even prevent a call-back during the next heat wave!


  1. OK, so what can one do when one wants to install a capacitor to help a compressor starting when it has not previously had one? How can you then go about calculating and or to use a run or start cap? All I know is I have a freezer rated to use 90watts and is single phase 1/6th horsepower compressor motor. It is a common house freezer 240volt in Australia.

  2. This article is misleading in that it will tell the effective capacitance of the capacitor that is installed. If the wrong cap was installed it will be replaced by a wrong capacitor.
    The correct capacitance value to be used is normally stated on the name plate of the compressor. The measurements/ calculations have to match the spec of the motor label.

  3. Using the capacitance on the name plate is a good general starting point. However, if the motor or compressor has been in operation for some time (years); chances are it has changed its coil characteristics due to heat and/or unbalanced loads (especially when one cap was bad or a power surge/brown out occurred). Chances are you don't have a means to measure the coil inductance. If the motor won't turn over and you suspect the cap you should never go for the smaller cap. The formula above will help to reign in on what capacitance is there (actual value - not labelled value). If the motor has seen some use and efficiency is key; a value slightly (emphasis on slightly) higher or lower value may improve performance.