Main breaker vs main lug only

[Joed384]

i am installing 100 amp breaker in a 480v panel to feed a 75 kva transformer. This xfmr will feed a 200 amp 120/208 panel. the 2 panels and xfmer are all located next to each other in the same electric room.

The question is....can I use MLO panel on the 120/208 panel. Or do I need a Main breaker?

 

[iwire]

i am installing 100 amp breaker in a 480v panel to feed a 75 kva transformer. This xfmr will feed a 200 amp 120/208 panel. the 2 panels and xfmer are all located next to each other in the same electric room.

The question is....can I use MLO panel on the 120/208 panel. Or do I need a Main breaker?

You can use an MLO panel but you would have to add some sort of overcurrent protection between the transformer and the panel to protect the conductors and panel.

Take a look at 240.21(C) for a start.

 

[Jraef]

You can use an MLO panel but you would have to add some sort of overcurrent protection between the transformer and the panel to protect the conductors and panel.

Take a look at 240.21(C) for a start.

And the cheapest and simplest way to add that protection would be a main CB in the panel...

 

[Joed384]

You can use an MLO panel but you would have to add some sort of overcurrent protection between the transformer and the panel to protect the conductors and panel.

Take a look at 240.21(C) for a start.

240.21(C)(2) looks like if the secondaries are under 10 ft, Im ok without overcurrent protection?

 

[iwire]

240.21(C)(2) looks like if the secondaries are under 10 ft, Im ok without overcurrent protection?

Under that section you can only run a maximum of 10' of conductor before you must have a OCPD.

 

[Joed384]

Under that section you can only run a maximum of 10' of conductor before you must have a OCPD.

I think that will work. I plan to exit the side of the xfmr and right up to the panel (which is right alongside looking at the wall) into the bottom lugs of a bottom fed panel

 

[charlie b]

240.21(C)(2) looks like if the secondaries are under 10 ft., I'm ok without overcurrent protection?

Bob already responded, but let me put it a different way. That article is telling us we don't have to put overcurrent protection AT THE SECONDARY ITSELF, if you follow the rest of the rules. You still must have an OCPD. And as has already been mentioned, the easiest way to do that is to have an MCB panel.

 

[charlie b]

I think that will work. I plan to exit the side of the xfmr and right up to the panel (which is right alongside looking at the wall) into the bottom lugs of a bottom fed panel

I think I know where you are trying to go. So please let me, at the risk of being repetitive, say clearly, NO :happysad: , you cannot have an MLO panel fed from a transformer, unless you install an OCPD in between them.

 

[mayanees]

I think I know where you are trying to go. So please let me, at the risk of being repetitive, say clearly, NO :happysad: , you cannot have an MLO panel fed from a transformer, unless you install an OCPD in between them.

I think the exception is in 240.21(C)3(2) Industrial Installation Secondary Conductors Not over 25 feet, which allows protection at the sum of the OCPDs in the panel. Of course the Industrial Installation criteria as defined in 240.2 is very limiting.

 

[jumper]

I think that will work. I plan to exit the side of the xfmr and right up to the panel (which is right alongside looking at the wall) into the bottom lugs of a bottom fed panel

Assuming you meet the requirements of that section for 10' or less of secondary conductors, what OCPD is protecting the panel?

Your transformer primary OCPD cannot since you do not meet the exception of 408.36(B) which requires you meet the rule in 240.21(C)(1).

 

[truck41trouble]

Just get a main breaker panel. Its not much more expensive , and it offers piece of mind. The second panel can be mlo, fed from the first.

Sent from my VS986 using Tapatalk

 

[Joed384]

Assuming you meet the requirements of that section for 10' or less of secondary conductors, what OCPD is protecting the panel?

Your transformer primary OCPD cannot since you do not meet the exception of 408.36(B) which requires you meet the rule in 240.21(C)(1).

I just read in 240.21(C)(1) that it would need to be a delta delta (single voltage output) for the secondaries to be covered under the primary OCPD. I guess I missed that part earlier.