am I doing something wrong ? optional method calculations

[curious_one]

Hi, can anyone please help me with this:given duplex 1600sqft each unit.each unit comes with: 11 kVA range,5A garbage disposal, 11A dishwasher, 4.5kVA water heater, 20KVA heater, 2-23A 240V air-conditioning units.use optional method to calculate feeder.My calcullation :

2units x 1600sqft x 3VA/sqft =9600VA
2x4500VA for 2small.appl+washing machine= 9000VA
then: 2x 11000VA range = 22000VA
2x 5Ax120 garab.disp = 1200VA
2x 11Ax120 dishwasher = 2640VA
2x4500VA(water heater) = 9000VA
-------------
53.440
-10.000 at 100%= 10.000VA
43.440 at 40% = 17.376VA
---------
27.376VA

heating/aircond.
2x20kW heater @ 65%= 26.000VA
2X2x23x240 @100%= 22.080VA omit

27.376+26.000=53.376VA
feeder=53.376/240
=222.4A

since I had this wrong at test where did I make mistake.Appreciate it.
This is second time I'm failing because I miscalculated duplex calculations again on optional method.Please help.

 

[Dennis Alwon]

Did you take into effect 220.85? Otherwise I am not sure.

220.85 Two Dwelling Units.
Where two dwelling units are supplied by a single feeder and the calculated load under Part III of this article exceeds that for three identical units calculated under 220.84, the lesser of the two loads shall be permitted to be used.

 

[Dennis Alwon]

I also think you need to use 100 percent for the heat strips.

 

[nakulak]

oops - i don't see a mistake (except that ac motor should use 230 volt x amps - but its not even used)

 

[nakulak]

ok, it might be that you are supposed to take 220.85 into consideration. if you take 3 identical units of this size and use the 220.84 demand factor, you come up with a smaller feeder, and you are allowed to use this value instead of the one you just calculated.

 

[mlnk]

You added 1500 VA for a laundry circuit, what about the dryer? It is a dedicated circuit, add 5000 VA. I am assuming that the units are all electric. There is no disposal in the NEC example and some AHJsl allow the DW circuit to be shared with the disposal, but I think going strictly with the optional calc. you should to include the FLA under the 40% section as you have done. It is a motor over 1/8 hp. and fastened in place.

 

[mlnk]

Looking at the question again. A 1600 SF duplex has 800 SF per unit, or is each unit 1600? Does each unit have two ACs? Also 20,00VA in heating would be 10,000 per unit- that is a 42 amp heater! If each unit had a 83 amp heater it is not called an apartment; it is called an oven.

 

[mlnk]

The question does not say there is a laundry. But if you include a washer at 1500 VA, then I think it makes sense to include a dryer at 5000 VA Lack of information in the question creates ambiguous answers.

 

[nakulak]

The question does not say there is a laundry. But if you include a washer at 1500 VA, then I think it makes sense to include a dryer at 5000 VA Lack of information in the question creates ambiguous answers.

laundry ckt is required, dryer is not

 

[ray cyr]

On an optional duplex calculation you:
1st, do a general (non-optional) calculation on the two units,
2nd, do an optional calculation on what would be 3 units (the first 2 plus 1 more),
3rd, compare the two calculations.
The calculation that gives the lower feeder demand is the one that you would choose.