Main Lug Vs Main Breaker PanelBoards.

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When a main breaker or disconnect is required you could use a main CB panel. For a subpanel that has an OCPD at the origin of the feeder a MLO may be all that's required.
 
It?s a design issue. When I design a large building that has electric rooms on each floor and any number of panels scattered throughout the hallways, I usually call for MLO panels. But when a panel is on a different floor from the DP that serves it, I often make it an MCB panel. When the panel is in a different building, or when it is directly fed by a transformer, or when it needs a disconnecting means for any other reason, then I usually make it an MCB panel, just because that is the easiest way to provide a disconnecting means. Sometimes the client will have a requirement one way or the other, so be sure to ask.
 
You may even come accross a situation where you need a MCB panel, but due to available physical constraints, you need to use a MLO panel, and put an enclosed CB next to the panel.
 
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The requirement for GF on breakers 1000 amps or more (on systems above 150 v t gr) can influence the decision also. For example a MB panelboard (480Y/277) wold require the main to have GF protection, whereas a MLO board with up to 6 service disconnects less than 1000 amps would not.
 
charlie b said:
It?s a design issue. When I design a large building that has electric rooms on each floor and any number of panels scattered throughout the hallways, I usually call for MLO panels. But when a panel is on a different floor from the DP that serves it, I often make it an MCB panel. When the panel is in a different building, or when it is directly fed by a transformer, or when it needs a disconnecting means for any other reason, then I usually make it an MCB panel, just because that is the easiest way to provide a disconnecting means. Sometimes the client will have a requirement one way or the other, so be sure to ask.
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Charlie, Why provide a MCB on a panel that is directly feed from the DB? Isn't that expensive? The only time I provide a MCB is if there are more than six disconnects in my incoming and on the secondary from the transformer. Other than that I always use MLO.
 
necnotevenclose said:
Charlie, Why provide a MCB on a panel that is directly feed from the DB? Isn't that expensive?
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I said "often," not "always," nor even "most of the time." There are other considerations than cost. One is ease of maintenance or new installations. If a panel is on a different floor than the DP, it is easier to turn off the panel and add a breaker, if the panel is an MCB. It really is up to the owner.
 
charlie b said:

I said "often," not "always," nor even "most of the time." There are other considerations than cost. One is ease of maintenance or new installations. If a panel is on a different floor than the DP, it is easier to turn off the panel and add a breaker, if the panel is an MCB. It really is up to the owner.
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I was thinking about this. if you took the cover off the PB to install another CB, would that not expose live voltage to the worker, whether the MCB was off or not?
 
If the line side of the MCB panelboard is live, then there are live exposed terminals to the worker when the breaker is off (unless you live in Canada ;) ).
 
petersonra said:
If you took the cover off the PB to install another CB, would that not expose live voltage to the worker, whether the MCB was off or not?
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Yes, of course. But not in the area in which the worker will be working. Is that a "safe working practice"? I do not really know. Nor do I know what percentage of electricians would consider it safe enough to turn off the MCB and install a new branch circuit breaker, knowing that there is voltage present at the line side of the MCB. All I am saying is that I have had clients insist on having MCB panels throughout the building. And I have complied, whether or not that requirement made sense to me.
 
I would think that with the big push towards complying with 70E and other safety standards that we may see panels such as they have in Canada. Then you will see what Charlie has stated as it may be we will see more main breakers in panels for safety/convienence reasons.
 
Pierre C Belarge said:
I would think that with the big push towards complying with 70E and other safety standards that we may see panels such as they have in Canada. Then you will see what Charlie has stated as it may be we will see more main breakers in panels for safety/convienence reasons.
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Pierre you and I agree on this one. :cool:

I think that ultimately 70E will result in a total rethink on how all kinds of electric equipment are designed.

I think we will see much more guarding and the complete separation of 'harmless' class 2 and 3 circuits from line voltage circuits in machine control cabinets.
 
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