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  1. #1
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    Jun 2013
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    30 amp fused discon. for indoor unit off a 50 CB

    Have a 30 amp fused disconnect for an indoor ahu on a 50 amp main circuit breaker. One fuse keeps blowing and plastic around disconnect clip is partially melted. HVAC guy came out last night as said problem is in disconnect box (bldg. also has aluminum wiring) and box needs to be replaced and can be done so with a 60 amp non-fused disconnect. Is this correct? Calling electrician this morning.

  2. #2
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    Dec 2002
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    SouthEast NC ICW & Piedmont Foothills
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    not enough info
    1-what is the connected load?
    2-what size conductor?
    hopefully the electrician will be more knowledgeable than hvac guy
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  3. #3
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    Jun 2013
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    Quote Originally Posted by mennian10 View Post
    Have a 30 amp fused disconnect for an indoor ahu on a 50 amp main circuit breaker. One fuse keeps blowing and plastic around disconnect clip is partially melted. HVAC guy came out last night as said problem is in disconnect box (bldg. also has aluminum wiring) and box needs to be replaced and can be done so with a 60 amp non-fused disconnect. Is this correct? Calling electrician this morning.
    Mennian, I'm thinking that you'll get the same answer here as you did on HVAC.

    The solution is staring you in the face (or screaming that you should not be working on this!)

    Not trying to be an aze, but this is a safety issue.

  4. #4
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    edit

  5. #5
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    edit

  6. #6
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    Nov 2006
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    I think we addressed this in the HVAC forum, but it bears repeating.

    Start with the equipment label.

    If it says "fuses," and not "breakers/fuses" or "overcurrent devices" (sometimes you must interpret what is on the label) then fuses it is! If the equipment is listed for fuses, fuses must be your last overcurrent device.

    If the disconnect is damaged, it should be replaced.

    The unit should have a minimum circuit ampacity that is associated with whatever accessory equipment is installed. Carrier gives you a list of various MCA's for the array of potential heat packages that can be installed.

    ALL of the rules for bundling and ambient temps must be followed in determining the actual conductor sizing needed, along with temp limits on lugs. The 2011 code allows NM cable to be used in more situations, but it must be kept from contact with combustibles like building insulation.

    A licensed electrication should be brought in, because if I recall, this is in an apartment building, meaning that anyone who works on this has liability in the event of a catastrophe.
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  7. #7
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    Mar 2011
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    Nw.Connecticut
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    I just would like to say, I work for a mechanical co, I work on A/C AHU's .I work a lot with potential. My main focus is commercial /industrial repair and service I have seen CB / fuse combos. I am responsible for from the end of which the equipment is on . If my amp load is in spec. and potential is in spec.(voltage correct + - 10%) Electrician is necessary period. My training is also in generators (cert generac) and I do have a good understanding of electricity 12vdc 24vac and up to 480 3ph. But something's are BEST left to those who Know. mennian10 I suggest you follow the pros advice here. You need an Electrician. If you are not trained ( and it sounds like your not )then you will most likely need an ambulance / morgue .Fix your leaky faucet or that closet door PLEASE. I don't know you, I'm just reading posts and want to see you and your neighbors live on.

  8. #8
    Join Date
    Jun 2013
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    As has been mentioned , the first 2 magic numbers are on the name plate of the HVAC equipment .

    MCA - minimum circuit ampacity

    MOP - maximum over current protective device or max fuse / HVAC circuit breaker

    These will in turn dictate wire size and fuse and / or circuit breaker rating .

    The big red flag , to me , is the aluminum wiring . Much more caution and expertise is needed . I , personally do not use Al wiring & am VERY cautious when I deal with it . Best solution id to replace it , if practical .

    And , yes , by all means , replace the disconnect with the proper type .

    God bless
    Wyr

  9. #9
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    May 2014
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    Nassau, Bahamas
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    Simply. To solve your problem let the electrician install a 50 amp disconnect and use #6 wire for it.

  10. #10
    Join Date
    May 2014
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    Nassau, Bahamas
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    Your disconnect and your circuit breaker should have the same amperage because if your unit is pulling 40 amps on a 30 amp disconnect and the circuit breaker is rated at 50 the breaker cannot protect the the disconnect because its amperage is too low

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