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!


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Comment by macsolomon on May 11, 2016 at 6:29am

what does the 2652 stands for in the given equation above?

Comment by Bob Blanchette on May 25, 2012 at 7:54pm

I suppose "trial and error" could be used to determine resonance. Using several capacitors that are close to what you think you need determine which one has the highest voltage across it. That will be the capacitor closest to resonance which will give a 90 degree phase shift to the start windings.

As far as thermostats go, any digital stat that doesn't have the ability to be common wire powered (must have batteries) are mostly junk. There are a few exceptions, but not many. About 80% of the digital thermostats I see in the field are NOT common connected, go figure...

Comment by Bob Blanchette on May 25, 2012 at 7:46pm

Your test works great to figure the actual working value of the installed capacitor, but I'm not seeing it as a way to verify if it's correct value for the unit. The only way to verify correct capacitor without having manufacturers data is to test the phase of the power going into the start winding vs the run winding. They should be 90 degrees apart under "typical load", phase will vary based on actual compressor load. Unfortunately this requires a dual channel oscilloscope, something few techs are likely have on their truck, much less know how to use properly. I've thought of trying to get the voltages equal across the start winding and capacitor to verify resonance. However there is no easy way to separate the inductive, resistive, and load transfer components of the start winding. It's best to look it up on the manufacturer's website if you aren't lucky enough to find a readable label on the unit.


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