1. Junior Member
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Aug 2016
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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

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

32484 by 240= 136 amps

is this correct

2. You did not double the sq.footage

3. 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...

4. Originally Posted by Dennis Alwon
You did not double the sq.footage
It appears he's giving totals.

5. 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 )

6. 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

7. Originally Posted by Rodney27
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. Senior Member
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Ocala, Florida, USA
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Originally Posted by Dennis Alwon
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.

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.

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Originally Posted by Dennis Alwon
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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