panelboard overcurrent protection 2

[Reesh14]

It is my understanding that overcurrent protection of a panelboard can be done inside of the panelboard itself(with a main circuit breaker) or a panel with main lugs only will have a disconnecting means outside of the panel (circuit breaker in the panel serving it) or a disconnect switch.

I often see a panelboard with a main circuit breaker inside of it as well as having a feed breaker from the supply panel. Is there a reason for this or special situation?

If I had an MDP serving a 100A panel, I can put a 100A main circuit breaker in the panel or a 100A breaker in the MDP to feed it, or do I have to have both?

 

[dpc]

You must have overcurrent protection at the source MDP (unless you meet the tap rule requirements). The breaker at the panelboard is optional. If the panelboard is in the same room as the MDP, there's not much reason for the main bkr in the panelboard. But if the panelboard is five floor up or 1000 feet away, a main disconnect at the panel can be really handy and be safer, since the electrician will often not want to take the time to walk back to the MDP to kill the power, even though they know this is the right thing to do.

 

[alehman]

Another reason is where the branch devices of the panelboard do not have adequate interrupting rating and series rating with an upstream breaker is required. An upstream device of the same manufacturer as the branch breakers can be used, but where the feeder device is of a different manufacturer and series rating is required, a main breaker may be needed.

 

[bjenks]

You are probably looking at a lighting & appliance panel board (Read 408.36 for most situations). NEC 408.36 mentions with exceptions that a lighting and appliance panel shall have a a main circuit breaker in it. Many purchase don't know all the rules so they just put it in or if the panel is being tapped off the feeder then you will usally see the breaker in the panel too.

 

[dpc]

True, but Exception No. 1 in 408.36 is a pretty big loophole. As long as the upstream feeder breaker rating does not exceed the panelboard rating, no local main breaker is required. That's true much more often than not.