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Thread: duplex calculation

  1. #1
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    duplex calculation

    looking for advice on following service size for duplex and let me know if I'm doing it correct or missing anything

    3300 square feet
    3,300 by 3va = 9900va
    Small appliances load (2 per unit) = 6000va
    Laundry load (one per unit ) = 3000va
    Range (one per unit) = 12,000 va
    Water heater (one per unit) 9000va
    Dishwasher (one per unit ) = 3000va
    Dryer (one per unit ) = 10,000va
    Disposal (one per unit ) = 2000va
    Microwave (one per unit ) =3000va

    General load =51300 va
    First 10,000 at 100%
    Remainder at 40%
    26520va general load
    ac load 4 hp = 2982 by2 =5964
    heat pump load I do not know

    32484 by 240= 136 amps

    is this correct

  2. #2
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    You did not double the sq.footage
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
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  3. #3
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    Generally speaking the heat pump has auxiliary back up heat. If the heat pump is wired independently of the strips then you only need the heat strip load as that is generally larger than the cooling load. If the heat pump and strips run together then you need both in the calculation.

    I assume you are using the optional calculation...
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  4. #4
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    Quote Originally Posted by Dennis Alwon View Post
    You did not double the sq.footage
    It appears he's giving totals.
    I will have achieved my life's goal if I die with a smile on my face.

  5. #5
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    It is good that you are doing calcs, dont get me wrong. Some AHJ's will want to see them, and good for a sanity check now and then. From a practical standpoint, its typically going to be 100 amps to each unit, and a 200 amp-ish riser for this sort of thing. I just started almost the exact same thing as this minus AC yesterday, and that is what I am doing (thankfully, I had to talk him down from wanting 150 or 200 for each unit. I should print out your load calc and show it to him )
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  6. #6
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    If the 3300 is the sq.ft for both then your calculation looks good. I did not do the math but the heat will be the big factor. I would also have 100 amps for each unit
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  7. #7
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    Quote Originally Posted by Rodney27 View Post
    looking for advice on following service size for duplex and let me know if I'm doing it correct or missing anything



    is this correct
    You appear to be using 220.82 for your calculation. 220.82 applies to A dwelling unit, not multiple dwelling units.

    You should use Article 220, Part III, or alternately, 220.85.

  8. #8
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    Quote Originally Posted by Dennis Alwon View Post
    Generally speaking the heat pump has auxiliary back up heat. If the heat pump is wired independently of the strips then you only need the heat strip load as that is generally larger than the cooling load. If the heat pump and strips run together then you need both in the calculation.

    I assume you are using the optional calculation...
    Generally, generally speaking one would not "waste" the money on a heat pump if the weren't going to use it for heat, so it is likely that if it is a heat pump it will need to be used in conjunction with the auxiliary heat strips for calculation.


    I know what I don't know, and I know where to go to find it!

  9. #9
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    Quote Originally Posted by Strathead View Post
    Generally, generally speaking one would not "waste" the money on a heat pump if the weren't going to use it for heat, so it is likely that if it is a heat pump it will need to be used in conjunction with the auxiliary heat strips for calculation.
    Well, sometimes the units are wired so that the heat pumps come on first and then the heat strips take over if the heat pump can't handle the job. They can be wired so that when the strips come on the heat pump goes off or they can be wired so that both run at the same time. Not sure why you would have the heat pump on if the heat strips can handle it. Maybe if they use larger heat strips then the heat pump doesn't have to run. I don't know.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  10. #10
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    Quote Originally Posted by Dennis Alwon View Post
    Well, sometimes the units are wired so that the heat pumps come on first and then the heat strips take over if the heat pump can't handle the job. They can be wired so that when the strips come on the heat pump goes off or they can be wired so that both run at the same time. Not sure why you would have the heat pump on if the heat strips can handle it. Maybe if they use larger heat strips then the heat pump doesn't have to run. I don't know.
    There are two conditions that normally call for aux heat.
    One is when the heat pump cannot produce its rated output, or any useful output, because of low temperature at the outside coils.
    There is not much point in keeping the heat pump on under those conditions.
    A second is when the interior set point temperature is higher than the current interior temperature by more than a set differential.
    The heat pump may be able to reach the set temperature just fine, but very slowly. The controls may turn on both heat pump and aux resistance heat under those conditions until the room temperature is close to the set point.

    The latter is one reason that conventional timed setback thermostats may not be economical when used with heat pumps, unless they are able to start the heat pump early enough to slowly ramp up to the desired temperature at the set time.

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