UL508A Feeder / BCPD

[Alexal]

Hi,
I have a question.
I'm designing UL508A panels.
Main panel, includes feeder circuit (main breaker + pdb) and a few branch circuits (fuses/breakers).
One of the BCPD feed external VFD (mounted near the panel on the wall). Load side of the VFD connects to a subpanel that feeds 4 motors..
The subpanel located next to the VFD.
It conains: main disconnect switch, 4 overload relays, 4 motor disconnect switches.
Can I consider a subpanel's main DS as a branch circuit or feeder circuit (because it has main disconnect switch)?

 

[petersonra]

The last bc ocpd determines where the transition from feeder to branch circuit is.

A word to the wise. Some overloads are not listed to be used on the load side of a vfd. Some are.

 

[Alexal]

The last bc ocpd determines where the transition from feeder to branch circuit is.

A word to the wise. Some overloads are not listed to be used on the load side of a vfd. Some are.

Thanks.
Could you please see the attached picture, am I correct regarding Feeder / Branch boundary?

 

[petersonra]

Thanks.
Could you please see the attached picture, am I correct regarding Feeder / Branch boundary?

I think you are correct.

This kind of arrangement is not a common practice here though.

I am curious why you have a main ds downstream of the VFD and then individual ds for each motor.

 

[Alexal]

I think you are correct.

This kind of arrangement is not a common practice here though.

I am curious why you have a main ds downstream of the VFD and then individual ds for each motor.

Thanks! Customer's request, I'll try to convince him that he don't need it. It's a little bizarre to have UL508 DS as main disconnect....

 

[Alexal]

I think you are correct.

This kind of arrangement is not a common practice here though.

I am curious why you have a main ds downstream of the VFD and then individual ds for each motor.

Hello

The main disconnect switch downstream of the VFD functions as the main disconnecting means for the secondary panel, which contains the PDB and OLs. Does the NEC permit omission of the main disconnect switch in this scenario?

The main panel, VFD and secondary panel are in close proximity to each other.

Thanks a lot!

 

[petersonra]

The nec would not require a separate disconnect for the panel with the pdb at all.

You can use whatever disconnect is designated for the VFD to disconnect that panel. In fact, in most cases you can use the VFD disconnect as the motor disconnect as well.

 

[Jraef]

The nec would not require a separate disconnect for the panel with the pdb at all.

You can use whatever disconnect is designated for the VFD to disconnect that panel. In fact, in most cases you can use the VFD disconnect as the motor disconnect as well.

The NEC, no. NFPA 79 would require a disconnect for that panel. NFPA-79 is not an "enforceable" code, but many facilities require adherence.

Some other things that should be checked:

• MOST (if not all) bi-metal OL relays REQUIRE short circuit protection that is coordinated with their ratings and in the case of circuit breakers, most OLRs are ONLY listed with circuit breakers of the same manufacturer. In this case, the BCPD that is in the Main panel may not count anyway, because it is going to have to be sized per the requirements of the VFD, not the OL relays.
• With no fuses (or breaker) in the secondary panel, the SCCR that you can apply to that secondary panel will be at most, 5kA. You might be able to make a case that the VFD is incapable of delivering EVEN 5kA, but technically, the SCCR rules don't factor that in.
• For that matter, double check the SCCR details of the VFD when protected by that BCPD in the main panel, it will likely need to be the same brand, otherwise the VFD may need fuses.

As a suggestion, if you use IEC Style Motor Protection Switches instead of bimetal OL relays, they could serve AS the local disconnects AND the OL AND the Short Circuit protective devices, AND carry a higher interrupt capacity. But like the bi-metal OL relays, not all of them are rated to be used downstream of VFDs. Some are, most are not.

 

[petersonra]

• . You might be able to make a case that the VFD is incapable of delivering EVEN 5kA, but technically, the SCCR rules don't factor that in.

Read the rules very carefully. You might be surprised to find that they do.