Proper sizing of motor run capacitors

Thread Starter

Mark Alan

Joined Mar 26, 2010
Hi everybody,

Please advise the effect to energy efficiency and other ill effects if a motor start capacitor is oversized similiar to the following scenario.

single phase blower motor, original factory cap was 370vac 10mf.

replaced with a 440vac 10 mf motor run capacitor.

I have found some posts that state energy consumption will be higher. however, could someone tell me how to calculate this if possible.

Please also advise if motor or system will experience any ill effects of the oversizing.

This was used in a residential a/c system and would appreciate any and all assistance that maybe given.


Mark Alan


Joined Apr 2, 2009
When replacing these caps, one should bear in mind about the type of di-electric used.
When comes to capacitor voltage, always remember one rule, never go below the original value, too much is never bad, but will tend to be bigger and costlier.
Capacitance and type should always be the same.

Only change in capacitance will effect the motor operation. Provided the voltage ratings are normal.


Joined Mar 12, 2010
As a State Certified Air Conditioning designer of 25 years, I can definitely say, the motor doesn't think you changed anything. It's the microfarads that matter. The voltage only has to be "enough". Use a marker to write inside the air conditioner that the original cap was 370 volts. That way you'll know the information if you ever need it.