Main Breaker vs MLO for facility main Switchboard

philly

Senior Member
I'm looking at an application to replace an existing service entrance 480 4000A switchboard and downstream ATS both located in an electrical room in a commercial facility. The replacement approach would be to provide a service rated breaker based ATS located just outside the building from the electrical room (accessible through door in electric room) and replacing the existing switchboard in place.

My question is weather the new Switchboard should have a main breaker or can just be MLO given the fact that the breakers in the breaker based ATS upstream will provide adequate protection of ATS load cables. Is there anything in the NEC that requires main on Switchboard in this case or a best industry practice?
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
Until 2020 code kicks in, you can have up to six mains in the switchboard. After 2020 is adopted by your local AHJ, the mains will need to be in separate enclosures, so if your ATS is service rated, then it can be one of the six either way, as long as it does not exceed six.
 

philly

Senior Member
Until 2020 code kicks in, you can have up to six mains in the switchboard. After 2020 is adopted by your local AHJ, the mains will need to be in separate enclosures, so if your ATS is service rated, then it can be one of the six either way, as long as it does not exceed six.
ATS will be SE rated. With breakers in ATS do you typically need a main breaker at the downstream switchboard? My only thought is to be able to isolate the switchboard local to the switchboard itself.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
ATS will be SE rated. With breakers in ATS do you typically need a main breaker at the downstream switchboard? My only thought is to be able to isolate the switchboard local to the switchboard itself.
The downstream switchboard would no longer be the service, so no main(s) needed. Of course the neutrals and grounds would need separated at that point.
 

philly

Senior Member
The downstream switchboard would no longer be the service, so no main(s) needed. Of course the neutrals and grounds would need separated at that point.
Correct and agree that it would not be needed. My question is weather or not it is good practice to include one for means of isolating the Swbd local to the switchboard instead of having to go upstream to ATS just outside the room?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Correct and agree that it would not be needed. My question is weather or not it is good practice to include one for means of isolating the Swbd local to the switchboard instead of having to go upstream to ATS just outside the room?
It is safer where you can isolate the switchboard completely, and that can only happen where the disconnect is not in the switchboard itself.
 

philly

Senior Member
It is safer where you can isolate the switchboard completely, and that can only happen where the disconnect is not in the switchboard itself.
In this case you would still have the upstream ATS breaker to isolate, and the main breaker in SWBD would just be redundant. From what I’m hearing from a cost perspective this redundant main isn’t usually considered?
 
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