UL508A FLA Calculations Motor NP vs UL 50.1 Table

[JovialBulge]

If I have a few components, say two VFDs and a 3-phase to 24vdc PSU within an enclosure. The VFDs are connected to motors which have FLA's of 7.2A and 0.92A and the PSU's operating input current is 0.69A. The UL table 50.1 has the FLA's for the motors based on their HP at 7.6A and 1.1A respectively. Which amperage should I use when adding everything up to put on the placard that determines the panels FLA? Additionally, if I were to want to include on the placard a space to show the Largest Single FLA, would I the 7.2A or 7.6A based on the answer to my previous question or should that really be set as the maximum output current that the largest VFD COULD produce in a locked motor scenario?

 

[Jraef]

Label must be base on Section 50 values, so Table 50.1.

You never use locked rotor current for any of this...

You are not required to list the largest FLA on the label, but if you want to, I would use the table values, because you never know when someone might change the motor (which is why those table values are higher than what you find in the wild).

 

[JovialBulge]

Label must be base on Section 50 values, so Table 50.1.

You never use locked rotor current for any of this...

You are not required to list the largest FLA on the label, but if you want to, I would use the table values, because you never know when someone might change the motor (which is why those table values are higher than what you find in the wild).

Thank you sir.

Table 50.1, or really anything I’ve seen in UL, doesn’t take into account the possible output current that a rated VFD for said motor HP could produce. Is that not considered just because there could be so many variables at that point? Versus just using the AC motor’s current.

I never have used locked rotor current for anything UL related or otherwise in the past, but why don’t they take that into account at all since it could feasibly happen?

 

[Jraef]

Because the data is for use in selecting conductor sizes and protective devices, but brief transient events like LRA are irrelevant to the conductors.

A VFD cannot “force” a motor to draw more current than it will need to do its job.

 

[JovialBulge]

Great point that the LRA time would be a very brief event if it ever did occur. Thanks for the other answers.