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Thread: Overcurrent Protection Question

  1. #1

    Overcurrent Protection Question

    We are replacing an air handler on a high rise building. The existing air handler is fed off of a 208V switchboard with an 800A breaker. However, the wires that were installed have an ampacity of just 500A. I am assuming this is because of motor load rules that allow for the overload in the air handler starter to act as the overcurrent protection.

    The new unit we are providing has an MOCP of 450A and the owner was planning on just reusing the existing circuit. Question is can we simply install a 450A fuse ahead of the air handler or must the breaker at the switchboard be replaced?

    I had thought the OCPD protects the feeder so I personally did not think the 800A breaker at the switchboard would be acceptable.

  2. #2
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    Quote Originally Posted by Machiavelli999 View Post
    We are replacing an air handler on a high rise building. The existing air handler is fed off of a 208V switchboard with an 800A breaker. However, the wires that were installed have an ampacity of just 500A. I am assuming this is because of motor load rules that allow for the overload in the air handler starter to act as the overcurrent protection.

    The new unit we are providing has an MOCP of 450A and the owner was planning on just reusing the existing circuit. Question is can we simply install a 450A fuse ahead of the air handler or must the breaker at the switchboard be replaced?

    I had thought the OCPD protects the feeder so I personally did not think the 800A breaker at the switchboard would be acceptable.
    Does the CB have a dial to change the setting at which it trips? maybe you don't need to add a fuse at all.

    240.6 Standard Ampere Ratings.
    (C) Restricted Access Adjustable-Trip Circuit Breakers. A
    circuit breaker(s) that has restricted access to the adjusting
    means shall be permitted to have an ampere rating(s) that is
    equal to the adjusted current setting (long-time pickup
    setting). Restricted access shall be defined as located behind
    one of the following:
    (1) Removable and sealable covers over the adjusting means
    (2) Bolted equipment enclosure doors
    (3) Locked doors accessible only to qualified personnel
    A 450 A Cb might be more cost effective than a 600 A fusible switch with fuses.
    Bob

  3. #3
    Join Date
    Jul 2017
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    MD, USA
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    Quote Originally Posted by petersonra View Post
    Does the CB have a dial to change the setting at which it trips? maybe you don't need to add a fuse at all.



    A 450 A Cb might be more cost effective than a 600 A fusible switch with fuses.
    You say more cost effective. How do you know? Is there a place, book, link etc you guys can look up cost of all electrical items?

    Sent from my SM-G935U using Tapatalk

  4. #4
    Join Date
    Feb 2005
    Location
    Charlotte, NC
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    6,787
    Quote Originally Posted by Machiavelli999 View Post
    We are replacing an air handler on a high rise building. The existing air handler is fed off of a 208V switchboard with an 800A breaker. However, the wires that were installed have an ampacity of just 500A. I am assuming this is because of motor load rules that allow for the overload in the air handler starter to act as the overcurrent protection.

    The new unit we are providing has an MOCP of 450A and the owner was planning on just reusing the existing circuit. Question is can we simply install a 450A fuse ahead of the air handler or must the breaker at the switchboard be replaced?

    I had thought the OCPD protects the feeder so I personally did not think the 800A breaker at the switchboard would be acceptable.
    What type breaker and trip unit?

  5. #5
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    Aug 2004
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    Northern illinois
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    Quote Originally Posted by codequestion View Post
    You say more cost effective. How do you know? Is there a place, book, link etc you guys can look up cost of all electrical items?

    Sent from my SM-G935U using Tapatalk
    I said it might be more cost effective. It depends on a lot of factors including what your company pays for things, which can vary wildly from distributor to distributor and among companies buying from them.

    OTOH, shutting down the switchboard to change out a CB might not be real cost effective or conveneint even if the parts cost is less.
    Bob

  6. #6
    Join Date
    Jan 2007
    Posts
    3,660
    Quote Originally Posted by codequestion View Post
    You say more cost effective. How do you know? Is there a place, book, link etc you guys can look up cost of all electrical items?

    Sent from my SM-G935U using Tapatalk

    A 600 amp 600v Fused 3r disconnect with fuses could easily run 3 grand.


    JAP>

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