# Cb feeding a vfd.



## tates1882 (Sep 3, 2010)

Would you use a trip curve d or k breaker to feed a drive?


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## JRaef (Mar 23, 2009)

When discussing "curves" on circuit breakers, you can only be referring to MCBs, (Miniature Circuit Breakers). If you are using those here in the US, I do not know of any VFDs that are rated to be protected by MCBs. Most require Molded Case Circuit Breakers (MCCBs), and a few are listed behind MPCBs (Motor Protection Circuit Breakers). So the answer to your question, again if you are here in the US, would be irrelevant, because you are required to use something other than those cheapest-of-all breakers. One reason is, MCBs are never more than about 18kAIC, some are 10kAIC and others as low as 5kAIC, which means they are basically useless for most industrial installations, which is where most VFDs are used. Most reputable brands of VFDs will have, in the installation manual, a list of approved SCPDs that you can use ahead of the drive. That's the best approach. 

Side note: Some of the cheaper drives are only UL listed when protected by fuses, so even IF you use a CB ahead of it, you will need fuses as well.

If you are outside of the US, you would have to refer to your local codes.

All that aside and you insist on using one of those, a B curve would be best, because the VFD will ensure that the breaker never sees the inrush or high starting current from the motor. "K" curves are only applicable to ABB MCBs, they made that one up, nobody else uses that except a few people who either brand-label the ABB MCBs, or are cloning them.


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## tates1882 (Sep 3, 2010)

That just killed that idea.


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## John Valdes (May 17, 2007)

I would follow the manufacturers instructions.


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## tates1882 (Sep 3, 2010)

John Valdes said:


> I would follow the manufacturers instructions.


I haven't chosen one yet still in the concept phase.


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## Big John (May 23, 2010)

Just an FYI, if the breaker is available in multiple letter trip curves, it's also most likely only acceptable as supplementary overcurrent protection per the NEC. 

They have a different listing, UL 1077, versus UL 489 for a branch or feeder breaker. A very few miniature breakers are 489 listed, and can be used in place of a 1077 breaker, but not vise versa


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