What are and how do you size capacitor switching contactors?

The modern ones (over the last 20 years or so) use a specific configuration of main and auxiliary contacts that allow resistors to be switched in series with the capacitor before closing the mains and then instantly removing them after the circuit is energized.

If you have a 480 VAC capacitor, you get 1.2 amps per KVAR. I use the guideline of 1.50 times the nominal current of the cap to size the contactor:

20KVAR at 480 VAC gives us 24 amps and x 1.5 gives us 36 Amps to size the contactor minimally.

Multiple step capacitor banks and other designs can be considerably more cumbersome and difficult to calculate. The good news is the folks at ABB  can assist us with double checking our numbers to verify sizing and duty cycle in addition to providing the caps, contactors and additional components to solve the most complex problems. They even have a software program called CAPCAL that can make this process extremely simple.


In addition to our partnership with ABB as an ABB Authorized Value Provider, we prefer ABB’s UA and RA contactor lines because they actually go up to 80KVAR and have a wide range of coil voltages available. We like the packaging and the reliability with hundreds in the field.

Last Words

Safety. Safety. Safety. In the Navy, I remember using a megger to charge a small cap and then throw it to someone to get a little shock. Everyone would have a good laugh and life went on. Sometimes I think that it is a wonder that someone did not die back then. Capacitors are one of the most dangerous components in industrial power electronics. When you have a power capacitor in a VFD, as an example, after power the drive off, the caps are still charged for a period of time. Please make sure the caps are discharged before handling or servicing anything.  Think about why they are used in defibrillator units and taser guns.