100 amp sub panel load question

Discussion in 'Electrical Forum discussion & Blog' started by apparentgenius, Nov 10, 2008.

  1. apparentgenius

    apparentgenius New Member

    May 5, 2008

    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?


  2. sixlashes

    sixlashes Plumber in Previous Life

    Aug 6, 2008
    Retired USAF Aircraft Maintenance Superintendent,
    Pensacola. FL
    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.:D
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  4. jar546

    jar546 In the Trades

    Feb 20, 2008
    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.
  5. Alectrician

    Alectrician DIY Senior Member

    Jun 15, 2007
    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
  6. apparentgenius

    apparentgenius New Member

    May 5, 2008
    Thanks for all the help. Looks like I have a little re-arranging to do and I'll be all set.

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