SCCR marking for dual voltage panel

[SJBatTCE]

I've been asked to evaluate a panel for SCCR labeling. The panel is fed from a 575V source. The 575V source feeds some 600V equipment, leaves the panel to an external 50kVA 575/480V transformer, then the 480V circuit comes back into the same panel and feeds 480V equipment.

So, how do I label this panel for SCCR? Should the panel be labeled for each voltage separately? The external transformer is not attached to the panel and its selection is not under control of the panel fabricator.

Thanks.

 

[djs]

I would say yes, it needs to have two SCCR labels.

 

[jraef]

How is the power going out to the transformer primary? Do you ahve a circuit protective device in your panel to feed it?


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[SJBatTCE]

Yes, there is a fused switch before the conductors exit the panel to the transformer.

 

[jraef]

Then the amount of available short circuit current involved is finite; limited by the primary current coming FROM your already listed panel. Nothing is adding fault energy to this circuit, in fact the transformer impedance and additional wire is actually removing some of it.

Also, your transformer cannot deliver much energy and the minimum SCCR rating for untested systems is 5kA, so there is no way that a 50kVA transformer can deliver anywhere near 5kA at 480V anyway.


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[SJBatTCE]

Good point, jraef! I hadn't thought about that. Assuming an infinite primary bus, a 50kVA dry-type with approx. 5.0%Z, only about 1200A can be delivered to a bolted 3-phase fault.

Still, I have no control over the source of the 480V; customer may decide to feed from a separate 480V utility secondary. I'm going to label for both voltages per UL 408A SB to CMA, but I won't be too concerned about the SCCR of the 480V portion as the current system as designed can not deliver significant fault current.

Thanks to you both for your input!