motor control cabinet

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richr777

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I have a custom built skid for 8 5 hp motors the cabinet contains seperate overload protection for each motor and they will controlled individually by VFD's.
Do VFD's affect the size when calculating for OCPD Because of the soft start
Feature? Or is it just the same as any other multiple motor calculation.

Any advice would be appreciated thanks!
 
more info

more info

I already sized the conductors and the OCPD. but the Nec is a little vague in Part 10. 430.120 there will be no bypass installed.

I sized the OCPD @150A and the conductors @ 3#4 w/#8 ground. 480 volt motor 5hp 6 FLA. I sized the conductors acording to 430.6 and 430.250 7.6 amps
 
thank you 76nemo I didn't think so but the engineer asked why a 150 amp breaker? the inrush current will not be as much with the drive's ramp up over 60 seconds. I replied that the OCPD prtects against groung fault or short circuit for the conductors.
 
If you are referring to the feed to the VFD, you must size the OCPD and conductors as per the NEC and VFD manufacturer's recommendation, not the motor. The rules on VFD circuit sizing are different because a VFD can be significantly larger than the motor it is connected to. This is specifically addressed in Article 430, although I don't have my copy here.

But one area of confusion for me is, do you have ONE 40HP+ VFD and it is feeding 8 x 5HP motors, each with it's own OL relay downstream from the VFD? If so, that's a whole different kettle of fish. EACH motor circuit then becomes separate from whatever is feeding the VFD, because now the motor circuits (and windings) are significantly smaller than the VFD overall circuit.
 
the feeders are coming into a distribution terminal block 3 #4 to 12 #12s 12 for each phase. from there to overload protection, one for each drive, then from the overload protecton to each drive's line side, the load side of the drive to each motor.
 
What is the Overload Protection device then? If it is just an OL relay, then it is redundant to the OL protection that is built in to the VFD. It is NOT providing short circuit protection for each motor / VFD circuit. You would still need a properly sized CB or fuses in front of the VFDs.

If it is an IEC style Motor Starter Protector (IEC manual Motor Starter), then the situation can be much more complicated than what most people think. In a nutshell though, you can ONLY use those devices as OCPDs for a VFD if the VFD manufacturer has specifically listed their unit to be protected by them. So in your case with A-B drives, it's possible that A-B has listed their drives to be protected by THEIR MSP device, but not likely any other brand. I work for Siemens, and it is that way for us; OUR drives can be protected by OUR MSPs, but not by one from A-B or Schneider, because we would have had to test and list them that way and why would we? So bottom line, you need to check.

All this by the way is assuming you are in the US, which I assumed is the case because you used AWG and HP units...
 
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