Page 1 of 2 12 LastLast
Results 1 to 10 of 16

Thread: Main Vs. MCB

  1. #1
    Join Date
    Sep 2012
    Location
    NJ
    Posts
    1,393

    Main Vs. MCB

    I've never seen this before where it lists the main as 600A but the MAC 1200A.
    Attached Images Attached Images  

  2. #2
    Join Date
    Aug 2004
    Location
    Northern illinois
    Posts
    15,270
    maybe it is a typo. or a copy and paste error.
    Bob

  3. #3
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
    Posts
    6,545
    Quote Originally Posted by petersonra View Post
    maybe it is a typo. or a copy and paste error.
    Yeah, otherwise it would make no sense and nobody could build it. You can't put an MCB in a panel with bus bars rated for 1/2 of the MCB rating.
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  4. #4
    Join Date
    Feb 2003
    Location
    Wisconsin
    Posts
    9,764
    Quote Originally Posted by Jraef View Post
    Yeah, otherwise it would make no sense and nobody could build it. You can't put an MCB in a panel with bus bars rated for 1/2 of the MCB rating.
    It is possible to build a panel with a 1200A frame main circuit breaker dialed down to a LTPU = 50%, with wire bending space for 600A conductors and put a 600A nameplate on it.
    Depending on the breaker, it is probable that the protective functions are based on the Sensor and not the LTPU.
    Just because you can, doesn't mean you should.

  5. #5
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
    Posts
    6,545
    Quote Originally Posted by jim dungar View Post
    It is possible to build a panel with a 1200A frame main circuit breaker dialed down to a LTPU = 50%, with wire bending space for 600A conductors and put a 600A nameplate on it.
    Depending on the breaker, it is probable that the protective functions are based on the Sensor and not the LTPU.
    Sure. But as an MCB for a panel, what's the point behind using a 1200AF breaker with a 600A rating plug, when you cannot increase the size of the rating plug because your panel bus is only rated 600A? It's an automatic limitation, so why not just use a 600A breaker? The difference in the breaker frames is huge!
    __________________________________________________ ____________________________
    Many people are shocked when they discover I am not a good electrician...

  6. #6
    Join Date
    Feb 2003
    Location
    Wisconsin
    Posts
    9,764
    Quote Originally Posted by Jraef View Post
    Sure. But as an MCB for a panel, what's the point behind using a 1200AF breaker with a 600A rating plug, when you cannot increase the size of the rating plug because your panel bus is only rated 600A? It's an automatic limitation, so why not just use a 600A breaker? The difference in the breaker frames is huge!
    To achieve coordination between similar sized devices, like a 600A Main with a 500A Feeder. Instantaneous settings are usually based on the sensor size, so a 1200AF might start at 6000A, while a 600AF probably stops there.
    Just because you can, doesn't mean you should.

  7. #7
    Join Date
    Sep 2012
    Location
    NJ
    Posts
    1,393
    Thanks

  8. #8
    Join Date
    Jul 2017
    Location
    Concord, NC USA
    Posts
    1

    use of 1200 MCB and 600 amp mains

    Quote Originally Posted by mstrlucky74 View Post
    I've never seen this before where it lists the main as 600A but the MAC 1200A.
    I have seen this happen before. Assume that the panel is a long distance from the source. Due to significant voltage drop, the feeder cables need to be significantly oversized. Even thou you only need 600 amps of service. Typically the oversized cables will not fit in the standard 600 amp frame. The 1200 amp main circuit breaker will have large enough lugs to handle the over sized feeder cables on the incoming side. I have used this trick myself. Just set the trip on the 1200 frame to 600 to handle the 600 amp bus downstream. Hope this is helpful

  9. #9
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    32,105
    Quote Originally Posted by Tom Zacharias View Post
    I have seen this happen before. Assume that the panel is a long distance from the source. Due to significant voltage drop, the feeder cables need to be significantly oversized. Even thou you only need 600 amps of service. Typically the oversized cables will not fit in the standard 600 amp frame. The 1200 amp main circuit breaker will have large enough lugs to handle the over sized feeder cables on the incoming side. I have used this trick myself. Just set the trip on the 1200 frame to 600 to handle the 600 amp bus downstream. Hope this is helpful
    Can't say I have seen it but was going to suggest that kind of scenario as one possibility.

  10. #10
    Join Date
    Feb 2013
    Location
    India
    Posts
    1,898
    Quote Originally Posted by jim dungar View Post
    Instantaneous settings are usually based on the sensor size, so a 1200AF might start at 6000A, while a 600AF probably stops there.
    If 600A bus bars are not suitably brazed to withstand the higher fault current of 1200AF, then such MCB not to be used, IMO.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •