UL508A Section 49.2 Clarification

[SteveBB]

I have a panel being built for a project I'm working on, I'm in the role of electrical project engineer, but I have a background in controls, although I've not ever worked for a UL certified shop I have typically designed to UL508A standards and I haven't had an issues when I've kicked my designs out to a UL certified panel shop to build.

The equipment is coming from an international supplier and I'm not convinced they have calculated the panel nameplate FLA correctly, and before I put in a feeder servicing it I want to feel more comfortable.

My particular question revolves around motors that are controlled from a drive (VFD or Servo Amplifier) and whether or not to use the motor FLA or the drive's input current.

From searches on this board, it seems to be somewhat of a split opinion with regards to this for protection and feeder sizing, my concern is strictly regarding FLA calculation.

A) The supplier has provided an panel nameplate FLA that uses the servo motor FLA as a basis
B) The supplier has provided branch protection per the drive's current input values

I'm ok with B above, it's A that I have pause.

They contend that when the servo motor is detected by the servo drive that the drive automatically limits current, which in a base sense is true, except that I've personally been involved with servo applications where mechanical design wasn't great and given the dynamics of a servo, it's amplifier, it's tune, that a servo motor can easily be driven beyond it's nominal rating and in fact it's built to do just that so long as it doesn't do it for sustained periods leading to thermal issues.

Even if the mechanical design was perfect and the chosen motor never went pass 100% FLA, the drive itself has efficiency losses.

Does anyone have any thoughts on this?

E: Is it just me or is the wording of 49.2 (and many other regs) leave things open for interpretation or differing opinions?

 

[GoldDigger]

When the VFD is connected directly to building wire, the VFD nameplate input current must be used in wire sizing. Among other things, the motor might be changed and only the VFD to motor wires updated.
When the VFD and motor are part of a single piece of equipment, UL rather than NEC is going to make the determination, and that is outside my expertise.

 

[petersonra]

I don't see much wiggle room. The fully rated input current of the drive is what the FLA calculation needs to use.

Sometimes there is an issue with the way US stuff is rated versus stuff made overseas, as much a terminology issue as anything.

It is highly unlikely that the input current to the drive will ever be more than the motor current.

The nameplate on the panel might technically be incorrect but it probably just won't matter any. If you want to you could bump the feeder conductor size and OCPD rating upwards to accommodate what the FLA would be if the "correct" calculation had been made. It just is not going to matter all that much in the real world though.

 

[SteveBB]

I don't see much wiggle room. The fully rated input current of the drive is what the FLA calculation needs to use.

Sometimes there is an issue with the way US stuff is rated versus stuff made overseas, as much a terminology issue as anything.

It is highly unlikely that the input current to the drive will ever be more than the motor current.

The nameplate on the panel might technically be incorrect but it probably just won't matter any. If you want to you could bump the feeder conductor size and OCPD rating upwards to accommodate what the FLA would be if the "correct" calculation had been made. It just is not going to matter all that much in the real world though.

Thanks for the feedback. I know it's splitting hairs really, but there are other engineering firms on the project with some potential overlapping scope and I just want to make sure my Ts are dotted and my Is are crossed.

 

[GoldDigger]

Thanks for the feedback. I know it's splitting hairs really, but there are other engineering firms on the project with some potential overlapping scope and I just want to make sure my Ts are dotted and my Is are crossed.

And all these years I have been dotting my Ts and crossing my Is. No wonder I have ended up where I am.