Dwelling unit service calc.

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brownout

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I am scheduled to take the Michigan masters exam in about a week. Up here we are still using the 2002 NEC. I need to be able to perform service load calculations. I have gone through Mike Holt?s ?Electrical Exam Preparation? and I understand it. The trouble is that ANNEX D, particularly example D1(b) has got me stumped. Example D1(a) makes perfect sense, but if I add together all of the loads from examples D1(a) and (b) and start from scratch I get an entirely different solution.


1500 square ft. + Service load Neutral load
laundry +
sm. appl. x demand factors = 5,100 5,100
12kVA range 8,000 5,600
5.5kVA dryer 5,500 3,850
6A 230V A/C 1,380 0
12A 115V A/C 1,380 1,380
8A 115V disposal 920 920
10A 120V dishwasher 1,200 1,200

Totals
Volt Amps 23,480 18,050
Amps 97.8 75.2


I guess that my questions can be summarized as follows:

1. Why are the loads in D1(a) computed in volt amps as a total but in D1(b) in amps? If this were all one problem which way should I start?

2. According to article 220.22 the neutral load shall be the MAXIMUM UNBALANCE between the neutral and ANY one ungrounded conductor. My interpretation of this is that we must allow for someone to come along and move all of our single pole breakers to one phase. In any case, 220.22 was adhered to in D1(a), but in D1(b) the additional loads were balanced between phases. This significantly changes the service neutral load. What happened to 220.22?

3. And finally, where in the heck did the reference to 430.24 come from? The Mike Holt book never mentioned it in any of the service load calculations. If a house has central A/C do I add 125% of the condenser load? What if it has 7,000 VA of electric heat and a 6,440 VA A/C, do I add 7,000 VA or 8,050 VA?
 
I'm confused!!!

Are you asking why?? d1 b yields a larger feeder demand than d1a???


charlie
 
All right, bare with me, I'm new to this. My math was all in neat columns but for some reason it didn?t come out that way. My net service load came out to 23,480 volt amps or 97.8 amps with 18,050 volt amps or 75.2 amps on the neutral.
 
I realize that D1(b) yields a higher demand because 2 A/Cs, a dishwasher, and a disposal were added. However, if I do a service load calculation for the whole thing [the loads in D1(a) and D1(b) combined] I get a 100 amp service with 75 amps on the neutral. Annex D shows a 110 amp service with 84 amps on the neutral. I guess I’m just trying to figure out where I screwed up the math.
 
Try 23480 VA plus 25% of largest motor load (1380VA) = 23825 / 230 =103.58 A

If you are having a problem with the requirement to consider article 430.22 or 24 in the 02 refer to 220.15 ( I think) motor loads are considered as contiuious duty unless the nature of their load will prevent them from operating so. see the small print to table 430.22

Does this help


Charlie
 
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I still get 99.3 amps, dividing by 240 volts instead of 230 as per article 220.2(A). So I’m still getting a 100 amp service instead of 110 as in D1(b).
 
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