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Thread: 100 amp sub panel load question

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  1. #1
    DIY Junior Member apparentgenius's Avatar
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    May 2008
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    Default 100 amp sub panel load question

    Hi,

    I am wiring my new house and have installed a 200A main panel with 36 slots. This particular panel does not accept double breakers so I am limited to 36 slots. I have installed a 100 amp sub panel in the adjacent stud bay.

    My question is how much can I run on this panel. I would like to put the entire garage/shop area plus the electric furnace on this panel. It would include:
    2 60A circuits for the 20KW furnace
    2 20A circuts for a 3HP Tablesaw and 1 HP dust collector
    1 20A Circuit for the 3/4 HP garage door opener
    1 20 A circuit for the septic system (pump and small compresser)
    1 20 A outlet circuit
    1 15A lighting circuit

    With this configuration I would have 4 slots left in the main panel and could move a few circuits over, but I figure it would be cleanest to arrange things this way.

    Is this too much for the 100A sub?

    Thanks

    Glen

  2. #2
    Plumber in Previous Life sixlashes's Avatar
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    Default

    Even if you don't take into consideration the fan motor in the 20Kw furnace, these two circuits will be drawing 83 amps between themselves while the furnace is running. The math says - leave the furnace out of the subpanel and it will work great.

    Move over a couple of other 15A or 20A circuits to give yourself the room in your main panel. I had to go the route of two 200A sub panels feeding off of a 320 amp meter/main panel combination. It cost a little more, but I have all the room in the world for future expansion.

  3. #3

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    Quote Originally Posted by sixlashes View Post
    Even if you don't take into consideration the fan motor in the 20Kw furnace, these two circuits will be drawing 83 amps between themselves while the furnace is running. The math says - leave the furnace out of the subpanel and it will work great.

    Move over a couple of other 15A or 20A circuits to give yourself the room in your main panel. I had to go the route of two 200A sub panels feeding off of a 320 amp meter/main panel combination. It cost a little more, but I have all the room in the world for future expansion.
    Yes, rather than getting involved in a boring calculation and asking you for more information about the heating unit, just leave the heat in the original panel and you should be fine unless there are some surprises in the main 200amp panel that would require you to upgrade your entire service to a larger one.

    I am seeing more and more 320a continuous services with 2 200amp panels these days and some 400a services with 2 200a panels in residedential areas.
    http://www.inspectpa.com/forum/forum.php
    My answers are based mostly on the ICC codes. Advice given is my personal opinion and every person performing work should acquire a permit from his/her jurisdiction and get the work inspected. My opinions are not directions to follow for DIYs or professionals

  4. #4

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    Is this too much for the 100A sub?
    I say yes.

    Put your furnaces on the 200A panel. Even if they only draw 30 amps, you are still using 60% of your available power.

    Move all the lighting/recep loads to the 100

  5. #5
    DIY Junior Member apparentgenius's Avatar
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    Default

    Thanks for all the help. Looks like I have a little re-arranging to do and I'll be all set.

    glen

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