×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

MLO(Main Lug Only) vs MCB(Main Circuit Breaker)
4

MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

(OP)

Hello,

I am working on the panel schedule, and I noticed that some panel has MLO, and some panel has MCB.

I believe that the omission of a main breaker in many of these cases is due to cost and perhaps in some cases available panel real estate. Is there any rule for MLO or MCB?

I am not quite sure the reason why.

RE: MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

Service entrance will generally have a main breaker.
Sub panels often don't have a main breaker.
Actually, adding a main breaker is often due to cost.
If the code requires a disconnecting means or overload/over current protection, a main breaker is often cheaper that a stand-alone
disconnect/protection.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

With the case of "sub-panels" in the same building as the feeder OCPD, that OCPD provides the overcurrent protection that is required by 408.36 for the panel. In those cases, it is a design and/or cost issue.

RE: MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

In some locations there is a requirement for a main disconnect, outside of the building, so a box with a main only may be specified, and on the inside the box would not have a main.
Or as said above, sub panels would not have a main.

Actually, if you really looked at the curves, you may not want a main, but opt for a fuse as a main.

Also, if you use a stepdown transformer within the building, such as a 480 to 240, you may have a breaker on the high side, and you may not want a main on the low side.

In most homes the builder will install a panel with a main, as this cost's less than having that in a separate box.

RE: MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

@cranky108
While the transformer may not required an OCPD on the secondary side, the secondary conductors will almost always need an OCPD per 240.21(C).

RE: MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

While the transformer may not required an OCPD on the secondary side, the primary disconnect is seldom accessible to the consumer so even if overcurrent protection is not needed, a disconnect is needed.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

Having done a heck of a lot of commercial design in a former life (over 1,000,000 sf of office space for just one client was a small bit of it) pretty much every branch panelboard in that realm is fed from a bigger distribution board upstream.

So, if this 225 panelboard is fed from a 225A breaker in that distribution board (that might be only a few feet away, why would I put in a 225A main breaker instead of going with a 225A MLO panel? If there’s ever a fault that needs to be cleared by tripping at the 225A level isn’t one breaker enough? If there’s two in series and the fault is after the second one they’ll both trip anyway, isn’t one enough?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

I agree David. However there are cases where a visual disconnect may be required, and to some people a breaker is easer than a pull out plug. Mainly in times when the local distributer does not have any visual disconnect devices, and customer wants there service right now.

I have something like that on my well pump. A breaker in the panel, and a box with a breaker near the pump house (underground vault).

RE: MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

I agree with Cranky108, sometimes it is a matter of convenience. I just did that sort of project at a large plant that has a long long processing line. The MDP was at one end, my project was at the other end, it was about 1/3 mile between them. I put in a technically redundant 400A MCB in the local panel so that the electricians don't have to take the time to go all the way back to the MDP if they need to add a branch breaker to it. It COULD have been a 400A non-fused disconnect, but then I would have had to deal with SCCR issues etc. and besides, adding the 400A MCB to the panel was less work for the installer and less expensive all around.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: MLO(Main Lug Only) vs MCB(Main Circuit Breaker)

That example is a perfectly good use of a panel with a main breaker. But I've done a lot of projects where an electrical room would have a main distribution board with multiple 225A breakers each feeding a 225A MLO panel in the same room. No need to waste the money or the panel space to have a main breaker within reach of the same sized breaker feeding it. Do what makes sense for the project.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close