Upstream breaker sizing

[Alexm]

Hi guys,

Long time lurker. First time posting. I have a general question with breaker sizing. Say I have a 480V control panel with a main breaker. I want to feed it from an mcc or distribution panel. How do you determine the proper size upstream breaker? Should it be rated higher than the one in the control panel so the breaker closest to the load would trip first? If so, could someone explain determining the size? Thanks in advance!

 

[ActionDave]

Welcome aboard.

You should size your upstream breaker based on the load of panel and size the conductors based on that same breaker. It can be smaller than the main breaker in your sub panel.

 

[Alexm]

Thanks ActionDave,

Say the breaker for the control panel was sized for the load. Is it acceptable to have the upstream breaker the same size (i.e. A 20A feeding another 20A)? Would this cause issues in an over current fault scenario?

 

[petersonra]

Hi guys,

Long time lurker. First time posting. I have a general question with breaker sizing. Say I have a 480V control panel with a main breaker. I want to feed it from an mcc or distribution panel. How do you determine the proper size upstream breaker? Should it be rated higher than the one in the control panel so the breaker closest to the load would trip first? If so, could someone explain determining the size? Thanks in advance!

This is a feeder. You would determine the minimum conductor sizing based on the calculated load per article 220.

By code, the rating of the feeder CB can essentially be whatever you want as long as it protects the conductors unless there are continuous loads. If there are continuous loads you have minimum OCPD ratings per 215.3. As a practical matter, putting a 15A CB on a load that is calculated at 100A is probably not a good idea, even if it is not prohibited by the code.

There is no guarantee that a higher rated CB upstream would not trip before a downstream CB unless careful attention was paid to the TCC for the respective breakers.

 

[Alexm]

Thank you guys,

I appreciate the help. So what I gather is as long as the upstream is sized
for the load, it's relationship to the size of the downstream one doesn't matter. It's just a matter of coordination between the breaker curves in order to ensure the downstream trips first in a fault.

 

[iwire]

It's just a matter of coordination between the breaker curves in order to ensure the downstream trips first in a fault.

True but in most cases there is no reason to care which breaker trips. As long as the circuit is protected the NEC is satisfied.

Things change when you get into a emergency and life safety systems.

 

[kwired]

Even with different trip curves, both may trip anyway if actual fault current is within both curves.

 

[topgone]

Thank you guys,

I appreciate the help. So what I gather is as long as the upstream is sized
for the load, it's relationship to the size of the downstream one doesn't matter. It's just a matter of coordination between the breaker curves in order to ensure the downstream trips first in a fault.

If you intend to install a main breaker on your panel, you don't have to worry coordinating your panel main breaker with that of the source breaker in the MCC--> they are serving the same load! Either breaker tripping means you've lost power to your panel!

 

[JoeStillman]

I always consider coordination to be unimportant when there is one breaker feeding one breaker. If either or both trip, you are just as dead.

Not the same as when one breaker feeds several others.