How to Size a Run Capacitor for a Compressor

Running capacitors are used to continuously adjust the current or phase shift of the motor winding to optimize the torque and efficiency performance of the motor. Because it is designed for continuous operation, the failure rate is much lower than the starting capacitors

Common causes of capacitor damage:

1. Large room with low power air conditioner. Because the small air conditioner is difficult to meet the temperature control requirements, and the air conditioner does not reach the indoor temperature you set, it has to continue to work. The air conditioner continues to work at high temperatures, and the capacitor is easily damaged at high temperatures for a long time. If it is a small room with small air conditioners, there is a way to increase the air conditioning cooling temperature, do not barely want to achieve a very cold effect.

2. The air conditioner is frequently activated in the case of high temperature of the chassis, which also makes the capacitor susceptible to damage. The air conditioner outside the machine still needs to have better heat dissipation and ventilation conditions, so that the chassis is not directly exposed to the summer sun.

3, the external machine should actively add refrigerant, the refrigerant is too small, the air conditioner needs more time and more power to reach the cooling setting, the energy is wasted, the machine is also hurt, and the fault is coming soon. This also makes the external machine more prone to heat. Therefore, when the external refrigerant is too small, the capacitor is also easily broken.

Capacitors are sized to suit the motors starting and running needs, they need to be replaced with ones of the same mfd rating, you can go higher on the voltage rating, but not lower. 

The nearer they run to their maximum voltage rating the quicker they wear out. 

A too high a mfd rated cap will cause too high a current flow in the start winding, if its high enough it will damage the windings insulation and lead to failure of the motor.

Following is method has worked for me as experience : 

(2650 x full load amperage) divided by supply voltage = mfd 

ex: 2650 x 12 = 31800/240= 133 

oops thats for start capacitors but still good to know 

2 ton 20-25mfd 

3 ton 30-35mfd 

4 ton 40-45mfd 

and so on 

it will be close to where it needs to be but check amperage and adjust to try to get FLA (full load amperage) HIgher mfd to lower amperage and lower mfd to raise amperage 

the suction pressure also has a big part in FLA try to make condtioned space has hot has it will every be in normal operating conditions and then adjust. That is talking ac, refrigeration same concept unless unit has cpr (cankcase pressure regulator) than its easy to adjust. 

This is not set in stone or very accurate according to some but has always worked for me. the best thing to do is call manufacture or take a crash course on electrical phisics.