Testing A Capacitor Under Load (VIDEO)

In this video I walk you through how we check capacitance on run capacitors at Prestige Air. 

Too many companies and techs will just turn their meter on to the uf (microfarads) reading and put their leads on the capacitor and accept the result. This is called bench testing, since it is a test you can do at a bench. It’s a way to test the capacitor when there is no power and no way to test it in the circuit. This is also the only time you should ever bench test a capacitor, when there is no way to test it in the circuit. 

One example of this is when the capacitor has failed completely and you cannot get the circuit to come on (since it requires a capacitor). In this instance you would bench test to confirm the failure of the component. 

In all other scenarios you need to do a run test. Why? Because that’s how it operates…while running. That’s the capacitance we need to confirm. No one cares about the capacitance when it’s off, only while it’s under load in the circuit.  

In order to do a run test we need two measurements: 

  1. The voltage from the start lead to the common lead on the capacitor
  2. Then the amperage on the start lead, all while the system is running. 

Then we pull out our phone and open the calculator and use this formula:

Amps x 2652 / volts = run capacitance

So if I have 1.1 amps on the start winding for my condenser fan motor, and I get 383 volts between start and run on the capacitor, the formula looks like this:

1.1 x 2652 / 383 = 7.61 uf. 

If this is 7.5uf capacitor then we are right where we need to be, but if it is supposed to be 10uf we are out of range for manufacturer specs. 

The only caution I would offer when doing a run test for capacitance is to remember that this is live and the wires are hot, usually to the tune of 370+ volts across the capacitor. 

At Prestige Air all of our techs do run tests, and we check this on every call. Every tune-up or repair get’s taken through a full system check which includes the run test on all capacitors. This way we can help find problems before they cause a breakdown on your system. 

If you experience problems with your heating and cooling call Prestige Air. Our expert service professionals have the training and experience to keep your HVAC system in tip-top shape all year long. Call (817) 200-7215 to make sure your heating and cooling are ready for the season!

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