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Distribution Switchboard -- 3-phase 4-Wire 120/208V: 1200 A Main Distribution

  1. Jan 7, 2016 #1

    Just looking into some clarrification. If a customer states that they want the following system, what is it they mean (note, it is 3-phase 4W 120/208V:

    1- 600 A house panel feed
    2 - 100 A tenant sections
    2 - 200 A tenant sections

    Basically the 1200 A is split up into a 600 A fusible switch, two 200 A fusible switches, and two 100 A fusible switches?

    Furthermore, I was told that fusible switches had to be used as opposed to circuit breakers. Can anyone describe why?
    Last edited: Jan 7, 2016
  2. jcsd
  3. Jan 7, 2016 #2


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    I'm not an electrical engineer, but I play one at meetings...

    My understanding is that either fused switches or circuit breakers are acceptable (codewise) and it is just a personal preference which one to use --- but today, most people use breakers. I'd ask a superior to clarify, but otherwise ask the client why they have the preference -- only they can know.

    That said, I'm not clear what is the description you were provided and what was your interpretation. To me, a "tenant section" is a panel fed from the main switchgear. I don't think fused switches fit in the same panel as little breakers, so I think you are needing a main switchboard with switches feeding each of the 5 sub-panels.

    Some light reading: http://static.schneider-electric.us/assets/consultingengineer/appguidedocs/0600DB0601.pdf
  4. Jan 8, 2016 #3


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    These specs or requirements are not likely to have been drawn up by an EE (or an experienced one anyway). While a fuse is always superior protection to a circuit breaker, they only work ONCE. Years ago, it was somewhat common to have combination breakers (30-40+ years ago). It is very uncommon today (there are some available). It would not be done for any typical commercial building.
    On another note, a 1200 amp main breaker feeding a distribution board often has 1.5 -2.5X in circuit breaker capacity attached ie the circuit breakers would add up to 2000 amps or so. All of the breakers feed sub panels (tenant sections and one "house" mechanical panel). All of these panels are usually loaded at 40-60% on the initial design. That allows the local panel to accept more load. If significant loads are added throughout the building, THEN you do a service upgrade. But initially, the buildings Main Distribution Panel (MDP) is designed to carry 60% or so of its rated load (often less, but this depends upon the economy of the design).
    Whomever is writing these specs should consult with an Electrical Engineer on what they really need. I have seen distribution systems designed by electricians that were code compliant and certainly safe enough, heavily over designed and using extra transformers (ie they added unnecessary single phase 240/120V transformers) and circuits which if the work had been designed by an experienced EE, the project would have been $15-20K cheaper.
  5. Jan 8, 2016 #4

    jim hardy

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    just a guess here

    1200 amp service is really big compared to what we see in normal residential service.
    When a short circuit appears, how much current can flow from the source during that brief interval before the protective device opens?
    That is called "fault current" and it's a lot more than 1200 amps.
    Typical household breakers are only rated for 10,000 amps of fault current.
    The cost of breakers goes up drastically with increasing fault current rating.

    What i think is behind the statement 'gotta have fuses' is the huge fault current available at your service.

    Check out these two articles
  6. Jan 8, 2016 #5


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    Yes, that was my interpretation as well. I'm sensing the spec was provided by a client who is a building owner/developer or his/er maintenance person and I'm hoping the OP is a junior EE or designer who is looking to find some answers him/erself before asking a superior...
  7. Jan 8, 2016 #6
    I'm with jim on this one, this is the most common situation I see where fuses are much more desirable. Reliability would probably be another one, especially if overcurrents are located outside. I've heard that recent UL investigations have proven that over time circuit breakers do not do well in weather and have a high failure rate in a decades timescale. If one wants to avoid an extended outage, make sure you have spare sets of fuses.

    I have seen ridiculous levels of over/under-designed systems from electricians and engineers alike including "experienced" engineers.
  8. Jan 9, 2016 #7
    600A for a house? Is he smelting aluminium in the basement? As per comment above, only time I've specced/used fuses is when we are too cheap to put in a decent breaker.
  9. Jan 9, 2016 #8


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    No, "house panel" is a term of art that doesn't mean an actual single family house. If, for example, you have an apartment building where much of the power is supplied to panels in apartments, the power that supplies the common areas comes from a panel you might refer to as a "house panel". Example:

    The OP appears to be describing a multi-tenant commercial building, but the same terminology is often used.
  10. Jan 11, 2016 #9
    Thanks for the feedback guys.

    The following is information I have found relevant to my work:

    Instances in which Fusible Switches would be used in lieu of Group Mounted Devices (Circuit Breakers) are feeder device amperages over 1200 A, Single Mains over 800 A, or if a Device is a Double Ended Main in a Service Entrance Lineup, Single Main Lug-In Lug-Out, or Submain or Feeder combined with a Utility Metering Compartment, or if the Fusible Device requires ground fault protection

    In regards to that list, I am a bit confused about the statements "feeder device amperages over 1200 A", and "Single Mains over 800 A"

    You cannot have device amperages over 1200 A unless the Single Mains is over 800 A, so why are both of these on the list?
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