Adjustable-Speed Drive System vs Industrial Control Panel

[Badams]

I design electrical systems for industrial installations and I'm not clear on how to size a feeder for a control panel containing multiple VFD's. If I consider that control panel to be an Adjustable-Speed Drive System, 430.122 says Branch/Feeder conductors must be sized 125% of the input current of the power conversion equipment. Does that mean I need to add up the total VFD input current and add 25% for the feeder size?

If I consider the control panel to be an Industrial Control Panel, 409.20 says the supply conductor must be sized for 125% of the full-load current of the largest motor plus the sum of the full-load current of all other motors. There's no mention here of VFD's or sizing the conductor based on VFD input current rather than motor full-load current.

This gets to be an issue where a panel shop puts several over-sized drives in a panel. I recently had a project where there were 15 VFD's in a panel serving 1HP - 5HP motors, and the shop put in all 7.5HP drives.

Thanks,

 

[Jraef]

It's a good question, but I don't over think it.

If sizing the feeder TO the ICP, then 409.20 applies, regardless of what is inside of the ICP.

INSIDE of the ICP, you must have some sort of distribution block separating out the circuits for each individual VFD. The conductors from that distribution block to each individual VFD would be sized per 430.122.

As I posted somewhere else earlier, I researched the reasoning behind the addition of what is now 430.122 (it was first introduced in the 2002 code under 430.2 as 115%), and it was because people were sizing conductors feeding the VFD as per the motor FLA tables as they had in the past. But after the fact, others would see the VFD rating being higher and increase the motor size to match the VFD, changing the motor leads, yet leaving the conductors feeding the VFD too small. The point of it was to ensure that the size of the VFD feeder conductors are capable of the worst case scenario of what the VFD could draw, nothing more.

In your case with the ICP, if the group of VFDs are all bigger than the loads are now, and someone someday increases the motor sizes, they will end up violating 409.20 anyway, whether they have VFDs or not.

 

[Badams]

I think your approach to size the feeder to the control panel based on 409.20 makes a lot of sense. The panel shop is responsible for sizing the individual circuits to each VFD in the panel.

Thanks,