Calculating Load for a motor control center (MCC) containing FVNR starters and VFDs

[Oly]

How do you determine the NEC (and therefore AHJ) accepted load for a buss limited MCC (800A, 1200A or 1600A bus, take your pick) that is populated with standard duty (SD) or heavy duty (HD) rated variable frequency drives (VFD) and FVNR contactors.

NEC 430.122 requires that non by-passed, adjustable speed drive power conversion systems (or VFDs as I will use here) are to have feeders rated at 125% of the VFDs rated input current and not 125% of the motor FLC.

In the case of a multiple motor situation where the feeder will be matched and protected to the bus rating of the MCC, how is the total loading of the MCC justified?

(consider all MCC loads continuous).

Are the rated VFD input current values :

1. Summed with all FVNR motor FLCs, plus 25% of the largest single load value; whether the largest single value is a FVNR motor FLC or a rated VFD input current?
2. Summed with all FVNR motor FLCs, plus 25% of the largest single VFD input current plus 25% FLC of largest single FVNR motor?
3. Summed with all FVNR motor FLCs, plus 25% of the sum total of rated VFD input current plus 25% FLC of largest FVNR motor?

Can someone help blow the smoke off of this?

 

[Jraef]

NEC 430.122 requires that non by-passed, adjustable speed drive power conversion systems (or VFDs as I will use here) are to have feeders rated at 125% of the VFDs rated input current and not 125% of the motor FLC.

Read it again. It does not say anything about the feeder OCPD, only the CONDUCTOR sizes. The size of feeder OCPD is related to what the VFD was listed with when given an SCCR rating. Youi need to get that from the VFD mfr, it's important.

Load is load. You don't need to size the service based on how you size individual feeder conductors or OCPDs. MCC bus bars are rated for continuous current, and if necessary, the Main CBs on them can be ordered as 100% rated (with the usual caveats regarding incoming cables etc.). But the VFD input current is usually LESS than the motor current. So Version 1, except don't use the VFD input current, just use the largest MOTOR current. And remember, this is for sizing the FEEDER to the MCC, not the MCC bus itself. You don't need to add the 25% to the bus size, technically that can be sized at 100% of the connected load, although it's not a good idea to push anything to the edge and leave no room for expansion. Think about this: when sizing cables, future additions might mean pulling new cables, but changing an MCC bus size after the fact is almost impossible. Over sizing now with the future in mind is prudent.

 

[Oly]

Read it again. It does not say anything about the feeder OCPD, only the CONDUCTOR sizes. The size of feeder OCPD is related to what the VFD was listed with when given an SCCR rating. Youi need to get that from the VFD mfr, it's important.

Load is load. You don't need to size the service based on how you size individual feeder conductors or OCPDs. MCC bus bars are rated for continuous current, and if necessary, the Main CBs on them can be ordered as 100% rated (with the usual caveats regarding incoming cables etc.). But the VFD input current is usually LESS than the motor current. So Version 1, except don't use the VFD input current, just use the largest MOTOR current. And remember, this is for sizing the FEEDER to the MCC, not the MCC bus itself. You don't need to add the 25% to the bus size, technically that can be sized at 100% of the connected load, although it's not a good idea to push anything to the edge and leave no room for expansion. Think about this: when sizing cables, future additions might mean pulling new cables, but changing an MCC bus size after the fact is almost impossible. Over sizing now with the future in mind is prudent.

Thanks for the response Jraef.

I perhaps was not clear. The basis of the question was not meant to be directed toward OCPDs or bus ratings although they are ultimately in the picture. I am only attempting to determine the correct process of justifying the NEC total load on an MCC containing both across the line and VFD controlled motors. Your answer would imply that I ignore any reference to NEC 430.122 in the load totalization process and only use 430.24 items 1 & 2 (only motor load present).
In other words I am only adding the connected Motor FLC plus 25% of the largest motor FLC for the total MCC load.

From this value then i can justify bus rating and then feeder size and then OCPD.

Is there anyone else out there that wants to weigh in on this confusion with 430.122 and group motor MCC load calcs that lead to feeder sizing?