Circuit breaker usage

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jez85

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I work in the water/wastewater industry. So, I have a 250A breaker in my MCC. This Circuit goes down stairs into a control cabinet and lands on a 400A breaker that feeds 4 VFD controlled pumps. The 400A and 250A breakers are my only means of disconnect for my VFDs. We are wanting to put each vfd on its own breaker so that they can be worked on individually. I am new to this facility and have never seen a setup quite like this. What would be the reasoning for putting a 400A breaker after a 250A breaker in a circuit? Wouldn't the 400A breaker need to be at the MCC to actually protect this circuit. Thank you for any guidance that can be provided.
 
In all likelihood the 400 amp breaker is simply serving as a disconnecting means and the conductors are protected by the 250 amp breker.
Each VFD should have short-circuit-ground -fault protection based on the individual VFD nameplate.
 
augie47 said:
In all likelihood the 400 amp breaker is simply serving as a disconnecting means and the conductors are protected by the 250 amp breker.
Each VFD should have short-circuit-ground -fault protection based on the individual VFD nameplate.
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I agree unless the trip characteristics are set in a weird way. The 400 is set to instantaneous and the 250 has a delay? Just thinking out loud??? That still does not make sense though.
 
are you sure it is even a CB? Might just be a molded case switch.

or someone did not want the filed CB to trip first so made the rating big enough he figured it would not trip first.

it seems unlikely to me that 4 VFDs could be fed off a 250A CB and meet code though while being sized appropriately.
 
Are you breakers feeding the VFDs in an MCC? Is there room in the MCC for additional buckets?
Or its often less expensive to buy an entire MCC section, use the buckets, scrap the section, Or put the new MCC next to the existing.
 
How big are the VFDs? Does each one have its own set of fuses? If not, how are they being protected?
As the others have pointed out, this seems a little odd and MAY be a violation as it is now, which means there may be no way to fix it as it is, resulting in an entirely new setup anyway. Details matter.
 
I am thinking it is a pre-packaged setup and the 400A breaker is what the manufacturer standardized on for this cabinet.
 
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