Service Calculations - help understanding!!

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starbright28

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Minnesota
I'm getting ready to take my test, and one thing the exam prep classes never went over were some service calculations.

So here I am looking at my books that I have to study off of, and I'm looking at the example of the one-family dwelling calculations. The one the comes at the back of the NEC. (Annex D Examples)

Here's my question.
I don't understand how they get the VA for the small appliance and laundry loads.

How do you get the 3000 VA for the Small Appliance?
How do you get the 1500 VA for the Laundry?
Can someone show me the calculations and/or cite code references?

Then below they have some percentage valuves they multiply by. Where do those come from?

Thanks,
 
How do you get the 3000 VA for the Small Appliance?
Click to expand...

Take a look at 220.52(A) (1500 VA for EACH small appliance branch circuit)

How do you get the 1500 VA for the Laundry?
Click to expand...

Take a look at 220.52(B) (1500 VA for the laundry circuit)

Then below they have some percentage valuves they multiply by. Where do those come from?
Click to expand...

You will need to explain what values you have been given, for me to help answer where they come from.

Chris
 
Amanda,

The small appliance load comes from 220.52(A) the laundry load comes from 220.52(B). The lighting load demand factors for standard calculations come from Table 220.42.
 
raider1 said:
You will need to explain what values you have been given, for me to help answer where they come from.
Chris
Click to expand...

9000 VA - 3000 VA = 6000 VA at 35%

Those values are direct from the example problem in the code book, the example one family dwelling.

I'm trying to understand the whole concept of thiings, since they don't go into much detail back there.
 
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starbright28 said:
9000 VA - 3000 VA = 6000 VA at 35%

Those values are direct from the example problem in the code book, the example one family dwelling.

I'm trying to understand the whole concept of thiings, since they don't go into much detail back there.
Click to expand...
That's applying the demand factors of 220.42. See 220.52(A) and (B) for why it is applied to non-lighting loads also...
 
Okay - now I understand where those values are coming from

Continuing to work on in the D1(b) example from the Annex D examples of the NEC 2005 book... I find myself wondering "how did they get that" with the following below:


Example D1(b) one family dwelling, adding the AC, dishwasher and disposal onto the loads.

1) why for the 1 - 115V AC and 120v dishwasher did they only put the 10 amps on the load and not also on the neutral for amps? They put the 12 amps from the AC on both the line and the neutral, so why not the dishwasher amps? EDIT: I understand that you use the larger of the 2 appliances for unbalance, but why in the first place is the dishwasher on with the AC for this calculation?

2) it says 25% of largest motor (see 430.24)
What did they use to get 3 - 3 - 2 for amps?
The largest motor to take 125% on is using the motor with the highest amps.

In the example they give the following:
1 - 6A, 230V AC unit (room)
1 - 12A, 115V AC unit (room)
1 - 8A, 115 V waste disposer
1 - 10A, 120V dishwasher.

EDIT:
You would take the 12A * 25% = 3A. but where does the 2A come from?
 
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Star...

You need to multiply the largest load 12 amps by 1.25 = 15 amps
the 12 amps will be counted as 15 amps to make it simple, then add your other loads....... the 2 on line B must be a mistake Onitram
 
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starbright28 said:
You mean the examples in the code book in the back are wrong?
Click to expand...
The 25% is per line. The largest on Line B is 10A, of which 25% is 2.5A. 2A is used because the decimal portion is discarded. The NEC is consistently inconsistent from a math standpoint.
 
Here is where I wish the NEC would come up with some sort of standard. I didn't think the NEC would be wrong.

I think I understand how to do a dwelling unit (example a & B) haven't pursued the calcs any further yet. will let you know if I got questions.
 
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