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Thread: Electrical subpanel for new addition

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  1. #1
    DIY Junior Member seattle_steve's Avatar
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    Default Electrical subpanel for new addition

    I need to wire a new 2nd floor addition. Since I'll have several circuits on the second floor and the (200amp) main panel is in the basement, I'm planning to have a sub-panel on the 2nd floor feed the new circuits. I'm wondering if the following plan makes sense?

    Here are the stats:

    - 3 bedrooms
    - 2 baths (one new one the second floor plus a remodeled bath on the main floor -- I plan to use one dedicated circuit for both baths)
    - 1 laundry in second floor bath
    - small balcony to include lighting and outlet
    - 854 total sq ft (including the main floor bathroom)

    For circuits I'm thinking:

    - 1 15-amp lighting circuit
    - 1 (or 2?) 20-amp circuit(s) for bedroom/office outlets w/AFCI breaker(s)
    - 1 20-amp dedicated laundry circuit
    - 1 30-amp dryer circuit
    - 1 20-amp bathroom circuit (serving 2nd floor & main floor baths)

    From a DIY book worksheet, I calculated that I'll need a 60amp breaker, fed via a #6-3 copper cable. Here are the details from the worksheet:

    Basic Light/Receptacle Load (854sq ft x 3w): 2562w
    Laundry circuit: 1500w
    Dryer: 6240w
    Outdoor (1 receptacle x 180w): 180w
    Gross load: 10482w (total of above)
    NEC safety adjustment (10482 x 1.25): 13102.5w
    Convert to amps: (13102.5 / 230): 56.97amps (= total load)

    Does all of the above make sense?

    Thanks.

  2. #2
    Electrical Contractor/Instructor jwelectric's Avatar
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    No sense at all.

    Square footage time 3 watts per square feet plus 1500 for laundry plus nameplate on dryer and calculation is over. No need for the 125% or the 180 VA for the outside receptacle.

    The conductors supplying would depend on the type of conductors and how installed but #6 copper NM cable is only good for 55 amps

  3. #3
    DIY Junior Member seattle_steve's Avatar
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    Well, I guess that response was somewhat helpful.

    So:

    >>"Square footage time 3 watts per square feet plus 1500 for laundry plus nameplate on dryer and calculation is over."

    Is there a better rule of thumb here?

    >>"No need for the 125% or the 180 VA for the outside receptacle."

    Removing those from the calculation results in a 44amp load.

    So, a 50amp panel fed by 6-3 cable?

    Thanks.

  4. #4
    Electrical Contractor/Instructor jwelectric's Avatar
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    When doing a residential calculation it is 3 watts per square foot and no additional continuous load or the need of 180 watts per device.

    When doing a commercial calculation then the 180 per device and 125% for continuous loads is required.

  5. #5
    DIY Junior Member seattle_steve's Avatar
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    Ok, I missed your original point about the dryer. So, based on that and your most recent post, would that mean the total load is just 11amps (2562 / 230)? Or do I need to add some load for the dryer, just not the full 6240?

    Thanks.

  6. #6
    Electrical Contractor/Instructor jwelectric's Avatar
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    I got 42.925 amps for the panel that will be supplying those circuits in the original post.

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