CB coordination study

[anbm]

If we have 75kVA transformer is fed out of 480V/3PH panel by 125A CB in the panel.

The transformer feeds 208Y/120V panel with 225A main breaker. Do we need to provide coordination study between 125A and 225A breakers above. IMO, they are kind of at the same protection level for Xfmr and can be exempted frm coordination purpose. Anyone object this?

 

[jim dungar]

In 2008, the NEC added an exception for this situation in 700 and 701 locations. The Critical Operations (COPS) section does not contain this exception.

 

[jghrist]

You don't gain any selectivity by having the low side breaker trip before the high side breaker. You lose the whole transformer feed if either trips, so you can set them with no coordinating interval.

 

[dkidd]

Go to NEMA

http://www.nema.org/stds/abp1.cfm

and download the Selective Coordination NEMA ABP 1-2010

On page 35 you will find a good diagram.