Selective Coordination

[Platinum]

Hey guys,

My question deals more with what devices should coordinate rather than how to coordinate. Say I have an I-Line distribution panel with a 1200A Main Breaker feeding:
250A breaker
300A breaker
450A breaker
600A breaker
(All these breakers are static/electronic trip)

Obviously those feeder breakers have to coordinate with the 1200A main. But do those feeder breakers have to coordinate with each other as well? (ie devices in parallel with each other).
I typically set up a tcc graph with the main breaker and the largest feeder breaker to make sure they coordinate. Then I set up a tcc graph for each feeder breaker with all its downstream devices and make sure they coordinate.

 

[charlie b]

But do those feeder breakers have to coordinate with each other as well? (ie devices in parallel with each other).

No. The presumption is that one fault occurs somewhere on the system. The breaker closest to that fault, and not one further upstream, should trip, to clear the fault. A breaker parallel to the one closest to the fault is not going to see the fault current, and will not trip.

 

[Platinum]

Thanks Charlie. So devices in series should coordinate.

Now how do I coordinate an electronic feeder breaker, say 300A, which feeds a thermal mag main breaker (300A) of a panelboard? Its difficult to 100% coordinate and it seems they always mis-coordinate in the Long Time Region...i max out my LTD to 24 and it still intersects. I guess that's just the way it is when you deal with devices of the same size....

 

[jim dungar]

Thanks Charlie. So devices in series should coordinate.

Now how do I coordinate an electronic feeder breaker, say 300A, which feeds a thermal mag main breaker (300A) of a panelboard? Its difficult to 100% coordinate and it seems they always mis-coordinate in the Long Time Region...i max out my LTD to 24 and it still intersects. I guess that's just the way it is when you deal with devices of the same size....

You can not.

The 2008 NEC removed the requirement for two devices directly in series (with nothing in between except a transformer) from needing to be coordinated.

 

[kingpb]

There's no need too, look at your circuit, does it really matter which one trips first?

 

[Platinum]

There's no need too, look at your circuit, does it really matter which one trips first?

I guess it doesnt matter. I was thinking of convenience. I would rather have the panelboard trip first than having to walk downstairs to the main switchboard to reset the feeder breaker.

 

[Platinum]

You can not.

The 2008 NEC removed the requirement for two devices directly in series (with nothing in between except a transformer) from needing to be coordinated.

Did not know that. Would by chance have the article# ?
Thanks

 

[ryan_618]

Did not know that. Would by chance have the article# ?
Thanks

Off the top of my head its 700.27 and 701.18, or really close to those sections.

 

[charlie b]

Correct on both counts, Ryan. Well done you.