NEC OCPD Sizing vs MCC OCPD Sizing

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RRoc

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I have a 100HP Pump in a MCC being controlled by a VFD.
The FLA of the Pump is 77Amp.

77Amp *1.25 = 96.25Amp so I would use a 100Amp CB.

When I look at Allen Bradley One Line Diagram for the same pump they put a 200Amp CB.

What am I missing? Do MCC use a different method for calculating OCPD.

I'm trying to understand how they get 200 Amp.
 
RRoc said:
I have a 100HP Pump in a MCC being controlled by a VFD.
The FLA of the Pump is 77Amp.

77Amp *1.25 = 96.25Amp so I would use a 100Amp CB.

When I look at Allen Bradley One Line Diagram for the same pump they put a 200Amp CB.

What am I missing? Do MCC use a different method for calculating OCPD.

I'm trying to understand how they get 200 Amp.
Click to expand...
77A*2.5=192.5A...Next size up is 200 Amp.
 
In a VFD, the VFD will protect the motor from Short Circuit, so the breaker is only there to protect the VFD. Whatever they had the VFD UL listed for is what you use. No need to over think it.

That said, the FLA x 1.25 is for sizing CONDUCTORS. There are different rules for sizing OCPDs in motor circuits, 430.52 outlines it, which is what David Luchini laid out for you.
 
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