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Thread: Distribution Blocks/Tap Rules

  1. #1
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    Distribution Blocks/Tap Rules

    Hello,

    I have a question about the application of distribution blocks. I am a greenhorn for sure so please bare with me. After looking at the feeder tap rules and watching Mr. Holt's video on feeder taps I am wondering if the feeder tap rules apply to distribution block in a control panel. I have an 800 amp disconnect feeding a distribution block with two 500 MCM conductors.

    Out of that distribution block are three circuits being fed out five terminals on the load side, one of which is a 500 amp 2/0 parallel feed protected with a 500 amp breaker and two others are #2 AWG 90 amp feeds protected with 90 amp breakers. The continuous load for the system is 480 amps and the non-continuous load is 120 amps.

    My question is....... Is this design ok or is something gonna burn up?

    I appreciate any feedback. I'm just trying to learn as much as I can.

  2. #2
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    2/0s with a 500 amp breaker seem really odd to me. Both in that the rating of the wire is only 350 and a 500 amp breaker is non standard. I’ve not seen one but that doesn’t mean much.

    Burn up? That can or not happen purely on your skills at installation. Proper torque would be the primary one.

    Are are you building this yourself?
    Tom
    TBLO

  3. #3
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    Parallel #2/0's is only good for 350 amps if you have two sets. You would need 2 sets of parallel 250 kcmil for 500 amps or 3 sets of #2/0's.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  4. #4
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    I appreciate the response!!

    Yes sir, I am trying to understand how it should be done first though. The breaker I mentioned is actually a 600 amp frame with a 500 amp trip unit installed so you were definitely right about that. The 2/0 parallel feed would be three conductor per phase. I'm not sure exactly how that works though. Sizing parallel conductors is not something I am very familiar with "as you can probably already tell". I looked on the 75c table and saw that 2/0 has 175 for it's ampacity. What I am not sure of is if that means I can use three of them per phase for a 500 amp feed.

    Something else I don't understand is if I can tap those parallel feeds of the load side of a distribution block being fed in parallel by an 800 amp fused disconnect. Any suggestions or insight on this would be greatly appreciated.

    Thanks again!

  5. #5
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    Your tap conductors, among other things, need to be sized according to the OCPD at the end of the tap. Since you have a 500 amp OCPD you need a minimum of 500 amps worth of conductors. 3 sets of #2/0 {175*3=525 amps} will work. Personally I would prefer 2 sets of 250 kcmil {255*2=510 amps} due to the terminal configuration on the 600 amp frame.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  6. #6
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    Good deal!!

    Thank you for the info!

    I originally wanted to use the 250's but the distribution block options I could find wouldn't support it and the other circuits that would be tapped off of it as well.

    What I will be doing is tapping the 500 amp circuit off of the main P.D.B. to feed a line reactor that will be filtering the line side voltage for multiple VFD's. I was planning on coming out of the Line reactor with parallel set's of 250 to another distribution block that would feed the VFD taps. This setup I'm hoping, if it's ok, will give me a filtered and non-filtered P.D.B. There are also two soft starts and a control XFRMR that will tap off of the non-filtered P.D.B.

    Does this sound ok to you??

  7. #7
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    What is the load on the parallel 2/0 set? If a motor there may be better chance it is acceptable.
    I live for today, I'm just a day behind.

  8. #8
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    Quote Originally Posted by ccokz615 View Post
    Hello,

    I have a question about the application of distribution blocks. I am a greenhorn for sure so please bare with me. After looking at the feeder tap rules and watching Mr. Holt's video on feeder taps I am wondering if the feeder tap rules apply to distribution block in a control panel. I have an 800 amp disconnect feeding a distribution block with two 500 MCM conductors.
    Probably ok as long as the calculated load < 760 A

    Out of that distribution block are three circuits being fed out five terminals on the load side, one of which is a 500 amp 2/0 parallel feed protected with a 500 amp breaker
    Potentially Ok if it is a motor being fed. Otherwise you are protecting 350A worth of conductor with a 500A CB.

    and two others are #2 AWG 90 amp feeds protected with 90 amp breakers.
    I can't think of any circumstance off hand where this would not work.

    The continuous load for the system is 480 amps and the non-continuous load is 120 amps.

    My question is....... Is this design ok or is something gonna burn up?

    I appreciate any feedback. I'm just trying to learn as much as I can.
    I don't think anyone can tell you if the design is "OK"or if something will "burn up". Just not enough information available, but from what you have stated it seems quite possible that the design is acceptable.
    Bob

  9. #9
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    Quote Originally Posted by kwired View Post
    What is the load on the parallel 2/0 set? If a motor there may be better chance it is acceptable.

    It will be feeding to and through a line reactor to another distribution block to supply input power to eight VFD's. The continuous load would for all of the drives would be 420 amps.

  10. #10
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    Quote Originally Posted by petersonra View Post
    I don't think anyone can tell you if the design is "OK"or if something will "burn up". Just not enough information available, but from what you have stated it seems quite possible that the design is acceptable.

    Great! This is actually the first panel I've had the opportunity to build from the ground up so my main concern is not creating a hazard. I know I have a lifetime of learning left to do. I'm glad this forum is here for sure though! I work on night shift so I really don't have any other resources to reach out too when I have any questions. What I was having trouble understanding I guess was if I could add a parallel feed off of the bottom of a distribution block. I knew that the total load wouldn't exceed the ampacity rating of the block but just had never seen it done before. I appreciate all the feedback! Thanks!

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