NEMA Contactor Size for 200kVA Capacitor

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circuitrider

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I have a 200kVA, three-phase capacitor with a current rating of 240A at 480V. I need to control this with a contactor. According to 460.8, the conductors and disconnecting means need to be 135% of that, or 324A (presumably due to +15% tolerance in the capacitor rating). According to my NEMA sizing table, a Size 5 controller is good for 270 ?Continuous Current Rating Amps? or 311 ?Service-Limit Current Rating Amps.? Is the contactor big enough?
 
I have a 200kVA, three-phase capacitor with a current rating of 240A at 480V. I need to control this with a contactor. According to 460.8, the conductors and disconnecting means need to be 135% of that, or 324A (presumably due to +15% tolerance in the capacitor rating). According to my NEMA sizing table, a Size 5 controller is good for 270 ?Continuous Current Rating Amps? or 311 ?Service-Limit Current Rating Amps.? Is the contactor big enough?
Contactors must have a Capacitor Switching rating separate from the "normal" or "motor" rating. Capacitors keep the arc alive longer, so they heat up more when switching. The rating is also dependent upon the available fault current at the capacitor terminals, because that has to do with the instantaneous charging inrush of the caps. NEMA contactor design specifications call out for a minimum required capacitor switching rating for the different NEMA sizes, but it decreases on some sizes as the AFC increases. For example a NEMA Size 5 contactor is rated for 160kVAR max at 460V when the supply circuit is capable of no more than 3kA, but at 22kA it drops to 149kVAR. You said 200kVA, which would not indicate the kVAR rating, but if you MEANT 200kVAR, then a Size 5 would not work for you. But a Size 6 is good for up to 320kVAR all the way to 22kA AFC. NEMA ratings do not go above 22kA ratings though, so you are on your own if you have more than that at the capacitor terminals. For IEC contactors, all bets are off, they have VASTLY different capacitor switching specifications, so don't fall into the "IEC sized for the same HP is the same as NEMA" trap on this issue, it's not even close. For example an equivalent IEC contactor for a 200HP motor, i.e. that "NEMA Size 5", has a capacitor rating of only 98kVAR (compared to 160kVAR for the NEMA).
 
Thanks. That was the answer I needed. I was only considering kVAr = kVA, since the load was purely capacitive, but I wasn't thinking about the physics of a reactive current and the associated arching.
 
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