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Old 10-04-2010, 04:46 PM   #1
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Default Single family dwelling service load calculation help!

Hi all,
I am about to take my Journeyman electrical test on Saturday. I have been going over the NEC and trying to figure out these calculations. I really need help on this one I just cant figure out where some of the numbers came from or how they determined what to use. I am sure I am just over looking something.




Single-Family Dwelling Service Load Example
The dwelling has a floor area of 2000 ft 2, exclusive of an unfinished cellar not
adaptable for future use, unfinished attic, and open porches. Appliances are a 12-kW
range and a 5.5-kW, 240-V dryer. Assume range and dryer kW ratings equivalent to
kVA ratings in accordance with 220.54 and 220.55.
Calculated Load [see 220.40]
General Lighting Load: 2000 ft 2 at 3 VA per ft 2 = 6000 VA
Minimum Number of Branch Circuits Required [see 210.11(A)]
General Lighting Load: 6000 VA 120 V = 50 A
This requires three 15-A, 2-wire or two 20-A, 2-wire circuits.

Small Appliance Load: Two 2-wire, 20-A circuits [see 210.11(C)(1)]

Laundry Load: One 2-wire, 20-A circuit [see 210.11(C)(2)]

Bathroom Branch Circuit: One 2-wire, 20-A circuit (no additional load
calculation is required for this circuit) [see 210.11(C)(3)]

Minimum Size Feeder Required
[see 220.40]

General Lighting = 6000 VA

Small Appliance = 3,000 VA

Laundry = 1,500 VA

Total = 10500 VA
3000 VA at 100% = 3,000 VA (I dont get this one!)

9000 VA - 3000 VA = 6000 VA at 35% = 2625 VA ( or this one)

Net Load = 5625 VA
Range (see Table 220.55) = 8,000 VA
Dryer Load (see Table 220.54) = 5,500 VA
Net Calculated Load = 19125 VA
Net Calculated Load for 120/240-V, 3-wire, single-phase service or
feeder
19125 VA 240 V =
79.7 A

Sections 230.42(B) and 230.79 require service conductors and disconnecting means
rated not less than 100 amperes.
Calculation for Neutral for Feeder and Service
Lighting and Small Appliance Load = 5625 VA

Range: 8000 VA at 70% (see 220.61) = 5,600 VA

Dryer: 5500 VA at 70% (see 220.61) = 3,850 VA

Total = 15075 VA
Calculated Load for Neutral
15075 VA 240 V = 62.8 A

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Old 10-04-2010, 05:23 PM   #2
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From T. 220.42 we see that the first 3000 va is at 100% and the up 120,000 at 35%.

Let's say we have a calculated lighting load demand factor of 11,500 va.

This means the first 3000 are at 100% which equals 3000va
Now the rest of the load is at 35%. So we have 11,500- 3000= rest of the load = 8,500. 8500 X 35%= 2975va. So now we have a total of 3000va+2975va= 5975va. Now you calculate the range, dryer, etc. Look at Annex D in the back of the NEC for a good example-- example D(1)

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Old 10-04-2010, 07:54 PM   #3
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Default ahhh

Man I really sat and stared at annex "D" for ever. This is the example from annex D only square feet changed. I could not get this at all. I got it now. Thanks a ton for the information I really do appreciate it. I am taking the trst on Saturday, and I am just not getting the small stuff!
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Old 10-04-2010, 08:03 PM   #4
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One more thing, why does it only add up the small appliance and the kva per square foot to get the 9000. is it because the laundry circuit isnt considered because it isnt part of the lighting load. or am I way off?
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Old 10-05-2010, 10:55 AM   #5
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Originally Posted by dadtodc View Post
One more thing, why does it only add up the small appliance and the kva per square foot to get the 9000. is it because the laundry circuit isnt considered because it isnt part of the lighting load. or am I way off?
The laundry circuit can be included in the lighting loads and be subject to the demand factors of Table 220.42. Take a look at the last sentence of 220.52(B).

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Old 10-05-2010, 12:18 PM   #6
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The laundry circuit can be included in the lighting loads and be subject to the demand factors of Table 220.42. Take a look at the last sentence of 220.52(B).

Chris
Ok, I read the sentence and I see that it can be used. I read that before and that is why I asked. So if that is the case then in myexample why is the laundry load not used in the calculation? Ihave 6000kva, for the sqft, and 3000kva for the 2 1500kva circuts. That is where the 9000 comes from right. I alsohowever have a single 1500kva from laundry that isnt used. That I think is what keeps throwing me off.
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Old 10-05-2010, 12:39 PM   #7
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Originally Posted by dadtodc View Post
Ok, I read the sentence and I see that it can be used. I read that before and that is why I asked. So if that is the case then in myexample why is the laundry load not used in the calculation? Ihave 6000kva, for the sqft, and 3000kva for the 2 1500kva circuts. That is where the 9000 comes from right. I alsohowever have a single 1500kva from laundry that isnt used. That I think is what keeps throwing me off.
The problem is that after you take to 3000 VA at 100% you used 9000 instead of 10,500 to subtract the 3000 VA.

Chris
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Old 10-05-2010, 08:18 PM   #8
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The problem is that after you take to 3000 VA at 100% you used 9000 instead of 10,500 to subtract the 3000 VA.

Chris
Ok I am with you now. The reason I was doing that is because the example I gave you was from a prep CD I bought. It differed from the annex "D", but made absolutly no sense to ma at all. Thanks so much for clearing all of this up!

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