avaiable fault current

[wireman1]

from the main distribution system . a feed going to a 480/277 panel with a 400 amp main this is a full rated system. the available fault current lets say is 40,000 amps. does the brakers which feed 277 volts for the office areas and 480 volts for hi bay lighting carry the same rating of the main breaker if is rated for 40.000 amps ques 2 what about a series rated system panel a 60,000 amps panel b 40,000 amps panel c 20,000 amps. would the manufacturer under engineered design specify what the rating of the breakers would have to be for each panel in the series rated system.to make it a safe operating system.

 

[bphgravity]

1. Yes - this is what maintains the fully rated system. All overcurrent devices installed in the panelboard must be 40k rated.

2. Yes - the engineered system will specify the exact type and rating of every overcurrent device installed.

 

[winnie]

I have a slight disagreement with Bryan's answer, but only because the original post is confusing. From the way the post is worded, I don't know if there is some sort of 'main distribution panel' with 40kA of available fault current, supplying a feeder which goes to another panel, or if the '277V' breaker being asked about is in a panel that has 40kA of available fault current.

To be 'fully rated' the requirement is that each OCPD (fuse, breaker, whatever) be rated for the available fault current at its location.

In general, if you have 40kA available at a panel, you will treat the available fault current everywhere in that panel as being 40kA. To be fully rated, both the main breaker (if present) and all the branch breakers would need a 40kA rating. (Of course, this is an approximation, since the panel bus itself has some impedance, meaning that the available fault current is different at different parts of the panel. This change in available fault current is probably insignificant. )

However a feeder to another panel probably has enough impedance to make a significant difference in fault current. The downstream panel probably has lower available fault current, and thus may not need 40kA breakers to be fully rated.

-Jon