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Thread: Fused main

  1. #1
    Jerome W. Young's Avatar
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    Default Fused main

    This house had a fused main. Each was was rrated at 300 amp. The main panel was rated for 400 amp max. The two non service panels next to it each had 200 amp breakers. The main is over fused correct?

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  2. #2
    Join Date
    Mar 2007
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    Default Re: Fused main

    "The main is over fused correct?"

    Correct.

    The service equipment main overcurrent devices should be the same size as, or smaller than, the rating of the conductors they are protecting or the panels they are protecting. Unless, of course, there is another layer of protection downstream which is properly protecting the lesser rated components.

    I.e., you could have a 300 amp main, with 300 amp rated conductors, going into a 300 amp rated panel with two 200 amp breakers which then protect the smaller panels fed from them.

    But that was not quite what you described that setup as being.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  3. #3
    James Duffin's Avatar
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    Default Re: Fused main

    I would also verify that that lugs in the panel are rated for two conductors in a terminal.


  4. #4
    Jerome W. Young's Avatar
    Jerome W. Young Guest

    Default Re: Fused main

    The main has 2 300 amp fuses which is 600 amp total or over the 400 amp rating of the main panel. The two adjacent downstream non service panels each have 200 amp breakers. I am alittle confused obviously. The main is over fused , but?


  5. #5
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    Default Re: Fused main

    Quote Originally Posted by Jerome W. Young View Post
    The main has 2 300 amp fuses which is 600 amp total
    Nope, that's 300 amps fore each leg, for 300 amps total.

    The two adjacent downstream non service panels each have 200 amp breakers. I am alittle confused obviously.
    I am also confused.

    Those 200 amp breakers, are they the breakers that feed the downstream panels, or are they mains in those panels?

    The main 300 amp fuses are not overfused for the 400 amp rating of the main, however (and this depends on how the lower rated downstream panels are protected) the downstream panels *may* be ... or ... *may not be* ... overfused at the main.

    Look at it this way:

    Start at the down stream end, what is the rating of those panels and the conductors feeding them, and is the rating of the overcurrent device protecting them the same as, or smaller than, those ratings.

    Next, move upstream to the next panel and repeat the above.

    Then move upstream to the main.

    As long as the overcurrent protection is the same as or smaller than the rating of the equipment it is protecting, then 'that part' is okay.

    Also remember that there are two hot phase legs with a single phase service, and that each phase leg must have *the same* size overcurrent protection, and that the total rating *is not the sum of* those two overcurrent protective device, but *is the same as* each identically protected phase leg. In this case, think of it not as '1 + 1 = 2', but as '1 in & 1 out = 1 total'.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  6. #6
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    Reno, Nv. - Now St. Louis, Mo.
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    Default Re: Fused main

    Are the wires sized for 300 amps? The don't look like it.

    Are the lugs intended for multiple conductors? I don't think they are.

    I can't tell if the 'tap rules' apply, from the information given.

    Finally, is that disconnect listed for use as service equipment? I have my doubts ....


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