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Thread: Electrical subpanel for new addition

  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.

  7. #7
    DIY Junior Member seattle_steve's Avatar
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    Thanks. It would be helpful to know how you arrived at that number. Seems like you must have taken some fraction of the dryer load into consideration?

  8. #8
    Electrical Contractor/Instructor jwelectric's Avatar
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    3 watts per square foot or 2562 plus 1500 for the washer and the dryer at 6240 equals 10302 then divide that by 240 volts and what do we have 42.925

  9. #9
    DIY Junior Member seattle_steve's Avatar
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    Ok, got it now. Looking back at your original reply, what confused me was "... the calculation is over." I took "over" to mean that my calculation was "way too high". You were just saying that the calculation "is complete" at that point (and no need to add the outdoor receptacle load).

    So, seems like a 50amp sub-panel? Is 6-3 NM the right feeder cable?

    Thanks.

  10. #10
    DIY Junior Member seattle_steve's Avatar
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    As an alternative to the 6-3 NM (Romex) cable, I'm considering running conduit (to protect the wire). If I were to go that route, what would be the minimum size conduit? And would it be the same size for the individual conductors (#6)?

  11. #11
    Electrical Contractor/Instructor jwelectric's Avatar
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    #6 copper NM cable is good for 55 amps and could be protected with a 60 amp breaker as long as the load on the conductors does not exceed 55 amps.

    #4 SE-R is cheaper and the above applies to it also.

  12. #12
    DIY Junior Member seattle_steve's Avatar
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    Thanks, I'll take a look at se-r as well.

    Regarding the conduit route: I'm not having much luck finding references for conduit use. As an alternative to using NM cable for the feeder, can I route individual #6 THHN wires through conduit? If so, do you know what the minimum size conduit would be? And does it matter if it's metal or plastic?

    Thanks.

  13. #13
    Electrical Contractor/Instructor jwelectric's Avatar
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    EMT 1 inch
    RMC 1 inch
    PVC 80 1 inch
    PVC 40 1 inch

    Although these sizes will work I have always found it easier to install the conductors if I installed one size larger pipe than what was called for. I have even been known to go two sizes larger if there was more than two 90 degree pulls and very much distance between the two.

  14. #14
    DIY Senior Member kreemoweet's Avatar
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    Steve, I strongly suggest you have a separate circuit for each bath. Whenever I have seen baths done on a shared circuit,
    there have invariably, and I mean without exception, been problems.

  15. #15
    Retired Defense Industry Engineer jadnashua's Avatar
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    Since the bath receptacles must be GFCI, if you trip it, you'll also mess with th eother bathroom. And, if the users are both trying to use a high wattage appliance (say two hair dryers or curling irons) at the the same time, you'd trip the breaker, too. Much nicer to have them separate. Also, if you were to do any work on one bathroom, you wouldn't be cutting the power to the other...a nice thing as the days get shorter and it gets darker earlier.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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