Tap Conductors and VFD Question

[t_ray7]

Hello all. I have a question that came up among our engineers that I'd like some outside feedback on.

We are building a control panel that is to be UL 508A listed. The main fused disconnect is connected to a power distribution block using the appropriate sized wire based on the fuse. Out of the distribution block we then feed an Allen-Bradley 755 VFD with a conductor sized 125% of the drive input rating. This conductor however is smaller than the feeder conductor and is considered a tap conductor. I'm of the opinion that since this is a tap conductor that it has to terminate in a branch circuit over current device before connecting to the drive. The main disconnect fuse size is the fuse recommended by the Allen Bradley for this specific VFD.

There is also a small 1kVA transformer and a 24VDC/10A power supply also connected to the distribution block and those tap conductors all terminate in BCPDs. I would think this entire panel would be considered a branch circuit since the main disconnect is technically the last OCPD before the load. Any one have experience with something like this?

 

[petersonra]

I don't think it is a tap conductor because the motor article specifically allows conductor ampacity in motor circuits to be less than the rating of the OCPD. How is this any different than running the wire direct from the main disconnect to the VFD?

 

[t_ray7]

I wouldn't have an issue if the main disconnect wasnt supply additional loads. I'm aware that conductors can be smaller than the BCPD.

This main panel disconnect feeds 3 (maybe 4) distinct loads, 2 (or 3) of which have their own BCPD, but it is also the OCPD for the drive. I suppose I'm having a hard time determining whether the main disconnect is considered feeder or branch at that point.

Looking at the panel it seems odd to have 3/0 wire (200A fuses) feeding the PDB, but 1/0 AWG feeding the VFD (117.4A input) from the PDB without additional protection.

Do I even need the 3/0? Thanks for the response.

 

[Jraef]

Since the conductors don’t exit the panel, and the fuse size is within the maximum listed for the drive, the concept is that since the only thing that could overload those conductors, and the fuses are already approved, it’s fine.

 

[t_ray7]

I appreciate the response and I now agree that the conductor size can be sized the way it is. A couple of questions came up and I'd like some additional feedback on them, if possible.

In reference to the attached picture, this is the way the design currently stands (dashed lines are external to the panel).

The questions are as follows:

1. Since this entire panel is considered a branch circuit, does UL 508A 31.4 - Sizing of Branch Circuit Protection for Motor Groups apply?

2. If it does apply, I think method B is the appropriate way to calculate, and the 12 and 14AWG conductors feeding the additional protection would need to comply with the tap rules defined, which would result in no less than 1/3rd the ampacity of the branch circuit conductor, so 6AWG @ 60C.

3. Would this be a common practice for something like this? Reason I ask is that this is the only time I've seen something like this. Any other design or panel build I have looked at or built would have a branch circuit protective device in front of everything PLUS a feeder OCPD.