UL 508A: Sizing Disconnect (30.2) vs Sizing Feeder OCPD (32.3)

[BN32]

I'm inspecting a panel for UL 508A compliance and am confused on how to correctly size the main panel disconnect.

The panel is feeding a 480V/120V transformer, a 3phase 24V power supply, and multiple VFDs.

I've calculated a 180FLA.

The customer is using a 400A molded case circuit breaker as the main disconnect.

Based off of 30.2.1 this would be acceptable since its not loaded at over 80%.

The problem I'm having is with 32.3.

If I use this calculation, then the OCPD cannot exceed 210Amps.

The customer is using a lug kit and has multiple 10 awg runs to the branch circuits so using 32.3.1 (b) to an advantage is out of the question. (would this be different if they had 400A conductors run to a distribution block?)

I'm not sure what the correct fix would be here.

Any insight would be appreciated. Thanks in advance.

 

[bwat]

Interesting. I'm surprised I've never come across this before.

I'm guessing it's too much of a stretch to say the lugs count as the "conductors" or "busbar" referenced in 32.3.1 (b). You would certainly expect them to have the ampacity that would allow for a 400A breaker. But if they don't, it's interesting that this could be a violation and you probably don't have an issue if they would have used a separate PDB as you suggested. It's not apparent to me why this would be like this if so.

 

[petersonra]

You might want to submit the question to UL, if you can get in touch anyone in the UL508a group.

 

[BN32]

Interesting. I'm surprised I've never come across this before.

I'm guessing it's too much of a stretch to say the lugs count as the "conductors" or "busbar" referenced in 32.3.1 (b). You would certainly expect them to have the ampacity that would allow for a 400A breaker. But if they don't, it's interesting that this could be a violation and you probably don't have an issue if they would have used a separate PDB as you suggested. It's not apparent to me why this would be like this if so.

This was my thinking as well to justify the 400A breaker.

 

[petersonra]

Your interpretation would make the use of distribution lugs that were specifically made for UL508a panels next to worthless.

 

[BN32]

Your interpretation would make the use of distribution lugs that were specifically made for UL508a panels next to worthless.

Can you elaborate? Forgive me if I sound ignorant, I haven't been doing this for very long.

Also, how would you typically size your main panel disconnect? I don't know weather to use section 30.2 or 32.3. I've seen different examples some using 30.2 and others using 32.3.

 

[Jraef]

After the Distribution Block, do the #10 conductors all terminate on Branch Circuit Protective Devices of some sort, like CBs or fuses for the individual loads?

 

[petersonra]

Can you elaborate? Forgive me if I sound ignorant, I haven't been doing this for very long.

Also, how would you typically size your main panel disconnect? I don't know weather to use section 30.2 or 32.3. I've seen different examples some using 30.2 and others using 32.3.

30.2 gives you the minimum rating of the disconnecting means.

32.3 gives you the maximum rating of the over current device.

Sometimes (like with a MCCB) they are the same device.

But you could also have a switch and fuses. The switch would need to comply with the minimum rating determined by 30.2 and the fuses with the maximum rating in 32.3.

 

[BN32]

After the Distribution Block, do the #10 conductors all terminate on Branch Circuit Protective Devices of some sort, like CBs or fuses for the individual loads?

The main circuit breaker has distribution lugs on the load side. All of the conductors terminate on Branch circuit protective devices.

 

[petersonra]

The main circuit breaker has distribution lugs on the load side. All of the conductors terminate on Branch circuit protective devices.

didn't UL change the procedure so the normal NEC tap rules apply? Like the tap conductor ampacity has to be 10% of the rating of the feeder OCPD?

 

[Jraef]

That’s what I was thinking. But even so, a 400A breaker is too large for #10 wire. It would have to be 300A.