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Thread: MCC - Path for Feeder Wiring

  1. #1
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    Oct 2006
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    MCC - Path for Feeder Wiring

    A new 800A MCC with a 400A feeder bucket is located in the lowest bucket position. The 400A feeder breaker is to supply parallel 500kcmil feeders. The bending space for the conductors appears inadequate for the conductors to be installed. The conductors must be top fed - bottom fed is not possible.

    Question is if the space behind the subpanel (which the 400A breaker is mounted on) can be used for field installed feeder conductors. This is the same space that the horizontal and vertical bus run between MCC sections. Why can it or why can't it be used?

    Thanks all!

  2. #2
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    Since parallel 500's are way beyond what's required for a 400 amp breaker, I would most likely install a j-box above the MCC and either reduce down to one set of 500/600mcm or switch to DLO cable.

    I would never consider chasing through the factory buss compartment. The fact it is even being brought up on a new install screams poor design to me.

  3. #3
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    Hi Screaming Cow -
    What makes this a poor design?

  4. #4
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    Is the 400A breaker able to accept parallel 500s? If so, the UL required bending space would be part of the MCC design and listing process. If the breaker was not able to accept 2x500s per phase and you changed the lugs to make it work, the onus is on you for not telling the MCC supplier that you needed larger conductors, or for swapping the breaker lugs for something it was not approved for.

    But no, the bus compartment is not approved as a raceway for conductors, you have no way to properly secure the conductors inside, so during a fault event they will flap around in the bus compartment and damage the insulation, the bus supports, the bus, or all of the above. It’s an Arc Flash event just waiting to happen... That’s not to say it isn’t done, but usually if someone intends on doing that they will order the MCC with “isolated bus”, meaning the bus bars are in barriered areas so that the conductor insulation can’t touch them. I still don’t recommend it though, I’m just saying that I’ve seen it done.
    Last edited by Jraef; 07-10-18 at 10:59 AM.
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  5. #5
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    Quote Originally Posted by Natfuelbilll View Post
    A new 800A MCC with a 400A feeder bucket is located in the lowest bucket position. The 400A feeder breaker is to supply parallel 500kcmil feeders. The bending space for the conductors appears inadequate for the conductors to be installed. The conductors must be top fed - bottom fed is not possible.

    Question is if the space behind the subpanel (which the 400A breaker is mounted on) can be used for field installed feeder conductors. This is the same space that the horizontal and vertical bus run between MCC sections. Why can it or why can't it be used?

    Thanks all!
    It can certainly be used for running wires. It can also be used to store your lunch. Whether it is a good idea or safe to do so is something else.

    These does not seem like a well thought out design. No reason you cannot move the feeder bucket into a more conveneint place. Or just run smaller conductors until you get out of the MCC and than splice in bigger ones.
    Bob

  6. #6
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    Typical lugs for a 400A feeder in an MCC (from looking at A-B, C-H and Sq. D catalogs that I have) will accept 2 x 250MCM/phase max. So you must have changed the lugs in the field. BUT, those same breakers have an option for up to 1 x 600MCM cable with no change in the unit space, so the unit space design must have wire bending space suitable for up to 600MCM and UL wire bending space does not care how many cables you have per phase. I would not want to have to make that bend (my hands hurt just thinking about it), but it is likely possible.
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