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Thread: 25 foot tap rule

  1. #1
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    25 foot tap rule

    Hi all,
    in a 10 story building there's an existing 1200 amp riser that feeds multiple floors (riser fused at 1200 amps). on the 4th floor, we have a 100 amp disc. sw (fused at 100 amps) that taps off this main riser. however, due to architectural revisions, the 100 amp disc. sw. has to be relocated, and it will be about 20 feet away from the riser. In which case i follow the 25 foot tap rule. So my tap conductors now have to be sized at 1/3 of the feeder fuses, which would make it 400 amps, so i would use 600MCM tap conductors. my question is:

    1) can the 600 MCM conductors terminate in the 100 amp disc. sw. that is 20 feet away? would there be an issue with the 600mcm not fitting in the lugs of the 100 amp disc. sw.? should i be specifying a 400 amp disc. sw. instead, and use fuse reducer clips to keep the fuses at 100 amps? a 400 amp switch is pretty big. is there another way around this?

    thanks!

  2. #2
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    The 25' tap needs to be sized 1/3 all the way to the line side of the OCPD. The Lugs that the feeder tap land on need to be fully rated to the ampacity of that tap. I think that you're stuck with a 400 amp with the 25' rule. With the 10' rule, you would me good. Can I build a case with code sections? Not without a lot of time digging.


    Sent from my iPhone using Tapatalk

  3. #3
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    Quote Originally Posted by Grouch1980 View Post
    Hi all,
    in a 10 story building there's an existing 1200 amp riser that feeds multiple floors (riser fused at 1200 amps). on the 4th floor, we have a 100 amp disc. sw (fused at 100 amps) that taps off this main riser. however, due to architectural revisions, the 100 amp disc. sw. has to be relocated, and it will be about 20 feet away from the riser. In which case i follow the 25 foot tap rule. So my tap conductors now have to be sized at 1/3 of the feeder fuses, which would make it 400 amps, so i would use 600MCM tap conductors. my question is:

    1) can the 600 MCM conductors terminate in the 100 amp disc. sw. that is 20 feet away? would there be an issue with the 600mcm not fitting in the lugs of the 100 amp disc. sw.? should i be specifying a 400 amp disc. sw. instead, and use fuse reducer clips to keep the fuses at 100 amps? a 400 amp switch is pretty big. is there another way around this?

    thanks!
    You may not like this either, but you could extend the 1200 amp conductors to within 10 feet and then use the 10 foot tap rule.

    If you can find a way to run the tap conductors outside the building there is no minimum size or length.

  4. #4
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    Quote Originally Posted by kwired View Post
    ...
    If you can find a way to run the tap conductors outside the building there is no minimum size or length.
    The tap has to originate outside the building to use 240.21(B)(5).
    Don, Illinois
    The code only applies to T&M work....it does not apply to contract work.

  5. #5
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    thanks everyone for the feedback. yeah, the taps originate inside, so I wouldn't be able to use 240.21.b.5. I'm stuck with the 400 amp switches. we'll just have to build a larger closet!

  6. #6
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    Quote Originally Posted by Grouch1980 View Post
    thanks everyone for the feedback. yeah, the taps originate inside, so I wouldn't be able to use 240.21.b.5. I'm stuck with the 400 amp switches. we'll just have to build a larger closet!
    Unless you can get a 100A disc with the ability to field replace lugs that accept 600's.
    Ron

  7. #7
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    Quote Originally Posted by ron View Post
    Unless you can get a 100A disc with the ability to field replace lugs that accept 600's.
    I'm not so sure that the NEC is very clear on this, why wouldn't the switch need to be rated for 400 amps? Do the tap conductors end at the terminals on the switch or at the fuse?
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  8. #8
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    Quote Originally Posted by Yetibiker View Post
    The 25' tap needs to be sized 1/3 all the way to the line side of the OCPD. The Lugs that the feeder tap land on need to be fully rated to the ampacity of that tap. I think that you're stuck with a 400 amp with the 25' rule. With the 10' rule, you would me good. Can I build a case with code sections? Not without a lot of time digging.


    Sent from my iPhone using Tapatalk
    Where does it say that in the code?

    It might be impractical to do it otherwise, but I don't recall any code requirement for this.
    Bob

  9. #9
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    Quote Originally Posted by petersonra View Post
    Where does it say that in the code?

    It might be impractical to do it otherwise, but I don't recall any code requirement for this.
    I think that it was discussed here years ago that the conductor ends at the fuse clip, it's at that point the tap conductors end not at the terminals.
    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

  10. #10
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    Quote Originally Posted by infinity View Post
    I think that it was discussed here years ago that the conductor ends at the fuse clip, it's at that point the tap conductors end not at the terminals.
    the lug is not a conductor IMO. it is a terminal, since UL lists them that way.
    interesting that the code does not define conductor.
    Bob

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