600 Amp Main Breaker, 400 Amp Bus

Status
Not open for further replies.
J

JDBrown

Senior Member
Location
California
Occupation
Electrical Engineer
Came across an existing installation this morning. It's a distribution board with a 600 amp main breaker, but the bus is only rated 400 amps.

Obviously, this is not generally allowed. But is there some specific application where the NEC would allow a main breaker's trip setting to be higher than the rating of the bus? Or is it just wrong, no matter what?
 
JDBrown said:
Came across an existing installation this morning. It's a distribution board with a 600 amp main breaker, but the bus is only rated 400 amps.

Obviously, this is not generally allowed. But is there some specific application where the NEC would allow a main breaker's trip setting to be higher than the rating of the bus? Or is it just wrong, no matter what?
Click to expand...

I would think it is wrong. a 400 amp buss is good for 400 amps. A 600 amp trip unit won't trip till 600 amps are exceeded. Has someone changed out the trip unit? But even then I would not think a 600 amp trip unit would fit in a 400 amp frame.
 
JDBrown said:
Came across an existing installation this morning. It's a distribution board with a 600 amp main breaker, but the bus is only rated 400 amps.

Obviously, this is not generally allowed. But is there some specific application where the NEC would allow a main breaker's trip setting to be higher than the rating of the bus? Or is it just wrong, no matter what?
Click to expand...

You can change the CT's or rating plug to 400A and be good, you can also change the pickup setting but I think there is a caveat to that where the settings cannot be easily accessed or something (Password protected, locked access cover). Don't recall exact wording, one of the code gurus here will know I am sure.
 
zog said:
You can change the CT's or rating plug to 400A and be good, you can also change the pickup setting but I think there is a caveat to that where the settings cannot be easily accessed or something (Password protected, locked access cover). Don't recall exact wording, one of the code gurus here will know I am sure.
Click to expand...

240.6 (b) or (c) 2008 NEC

Thanks Zog. Made me look for it! Good exercise.
 
Thanks for the input, guys. Now I'm having trouble finding where it says you can't feed a switchboard with a breaker whose trip setting is higher than the rating of its bus. I found it for panelboards:
408.36 Overcurrent Protection
In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard.
Click to expand...
Is there a similar section for switchboards that I'm missing, or do the rules for panelboards also apply to switchboards for some reason?
 
The 600 amp OCPD is irrelevant if the OCPD at the origin of the feeder is rated for 400 amps.
 
infinity said:
The 600 amp OCPD is irrelevant if the OCPD at the origin of the feeder is rated for 400 amps.
Click to expand...
Sorry, I should've included that. This switchboard is the main service for the building, and is fed from the Utility transformer. The feed goes from the utility transformer directly into the switchboard's meter/main section.

So, does 408.36 apply to switchboards as well as panelboards (even though it's in the panelboards section), or is there a similar section that applies to switchboards? I can't believe the NFPA would leave out something this basic (although, to be fair, anybody designing or installing this service should have known better).
 
JDBrown said:
Came across an existing installation this morning. It's a distribution board with a 600 amp main breaker, but the bus is only rated 400 amps.

Obviously, this is not generally allowed. But is there some specific application where the NEC would allow a main breaker's trip setting to be higher than the rating of the bus? Or is it just wrong, no matter what?
Click to expand...

a 600A frame breaker is ok if it has a 400 trip unit installed. I have been known to do that kind of thing now and then.

IMO, if the bus is rated for 400A is has to be protected at its rating.

having said that, is i possible that the switchboard has internal fuses? I have seen some oddball installations like that where they put in what looks like a CB but it really is just a molded case switch and installed fuses downstream inside where you cannot easily see them.
 
petersonra said:
a 600A frame breaker is ok if it has a 400 trip unit installed. I have been known to do that kind of thing now and then.

IMO, if the bus is rated for 400A is has to be protected at its rating.

having said that, is i possible that the switchboard has internal fuses? I have seen some oddball installations like that where they put in what looks like a CB but it really is just a molded case switch and installed fuses downstream inside where you cannot easily see them.
Click to expand...
I suppose anything's possible. It looks like a breaker, and the only visible label says 600A. It looks like we're going to just have an Electrician go out, take the cover off and verify what's actually there. Then we'll be able to start assessing what should be done without so much guesswork.

Thanks for all the info, guys.
 
JDBrown said:
I suppose anything's possible. It looks like a breaker, and the only visible label says 600A. It looks like we're going to just have an Electrician go out, take the cover off and verify what's actually there. Then we'll be able to start assessing what should be done without so much guesswork.

Thanks for all the info, guys.
Click to expand...

let us know what you find please.
 
Also make sure the 400 amp isn't a max branch rating even though the bus may be a higher overall rating.

I installed an I-Line panel recently that is 800 amp bus but was only rated to accept a maximum of 600 amps for any individual branch, or in this particular case a main breaker would have to be a back feed and you were still limited to 600 amp main breaker because of that.
 
Status
Not open for further replies.
Top