Nec 220.44

[erickench]

Hi Guys, just started reading my book titled "Stallcups Electrical Calculations Simplified". I came across a calculation procedure which Stallcup did not complete. Here is the problem:

What is the load for 40 duplex receptacles used for continuous and 40 duplex receptacles used for noncontinuous duty in illustration 1-6(a)?

The procedure that Stallcup used was this:

Continuous duty load: 180VA x 40 x 125% = 9000VA
Noncontinuous duty load: 180VA x 40 = 7200VA

In applying the demand factors of NEC Table 220.44 Stallcup applies the 100% demand factor to the noncontinuous loads. See the problem? We usually only apply the 125% when the demand factor is 100%. If I were calculating this load I would apply the 100% to the first 9000VA of continuous load and then add on 1000VA to make it 10000VA and to the remaining VA I would apply the 50% demand factor which in this case would be noncontinuous load as it should be. Thus we have

9000 + 1000 + (7200 - 1000) x 50% = 13100VA

The only problem now is what would happen if the continuous load went over 10000VA? How do we bring ourselves to apply both a 50% demand factor and 125% to the same quantity of VAs? Opinions gentlemen!

 

[Dennis Alwon]

Maybe its me but this question does not make sense. How can you calculated continuous load on a receptacle that is continuous. If it is continuous then what load is being utilized by the receptacle? To assume 180va for continuous seems odd.

I must admit I am not a student of calculations but this I am puzzled by.

 

[erickench]

That's the way Stallcup has the problem written out in the book. I find mistakes all the time in books. Look's like he completely muddled this one. Unless he was assuming that lamps would be plugged into the receptacles. Can we assume such a thing?:roll:

 

[Dennis Alwon]

That's the way Stallcup has the problem written out in the book. I find mistakes all the time in books. Look's like he completely muddled this one. Unless he was assuming that lamps would be plugged into the receptacles. Can we assume such a thing?:roll:

In my opinion no but again I am not very good with this. I just don't understand the concept of why he is doing this.

 

[charlie b]

I think the book is wrong. 220.44 says to take the receptacle load that you obtained from 220.14(H) and (I) (with ?H? not being relevant to this problem), and apply the demand factor (i.e., 100% of the first 10K and 50% of the rest). But if you go to 220.14(I), you are not directed to separate continuous from non-continuous, when you assign 180VA per duplex receptacle. Therefore, I would disregard that part of the question, and do the calculation this way:
? 180 x 80 = 14,400
? First 10K counts at 100%
? Next 4,400 counts at 50%
? Total is therefore 12,300.

 

[erickench]

Yes I believe your right Charlie. But it's always troubled me on how continuous loads would be calculated if they were plugged into a receptacle. Stallcup makes mention of receptacles rated for noncontinuous duty. Is there such a thing?