Article 220.55

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dre120

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I have a question from one of my study guides(studying for the limited license exam). The answer that the study guide has is not matching with what I have.
Here is the question:
Calculate the minimum loads which will be used for sizing the feeders supplying the following 240 volt household ranges:

8.75 kw 10 kw, 10.5 kw
They answer is 18 kw

I get 12 kw.
 
dre120 said:
I have a question from one of my study guides(studying for the limited license exam). The answer that the study guide has is not matching with what I have.
Here is the question:
Calculate the minimum loads which will be used for sizing the feeders supplying the following 240 volt household ranges:

8.75 kw 10 kw, 10.5 kw
They answer is 18 kw

I get 12 kw.
Click to expand...

I guess I am no help I get 19 kw
 
dre120 said:
8.75 kw 10 kw, 10.5 kw
They answer is 18 kw

I get 12 kw.
Click to expand...

I think you got 12 by giving 12kw to each range adding them up and dividing by three. Can't do that because the 8.75 is not over 8.75 kw. You can take the 2 units 10.5 and 10 accept them as 12 kw and divide by 2-- you get 12.

Then you take the one unit at 8.75 and multiply by 80% and get 7 kw. 12 + 7 = 19kw
 
dre120 said:
I have a question from one of my study guides(studying for the limited license exam). The answer that the study guide has is not matching with what I have.
Here is the question:
Calculate the minimum loads which will be used for sizing the feeders supplying the following 240 volt household ranges:

8.75 kw 10 kw, 10.5 kw
They answer is 18 kw

I get 12 kw.
Click to expand...

Is that minimum loads or minimum demand?
I get 14kw.
Looking at the title of the table T220.55 .......(Column C to be used in all cases except as otherwise permitted....
We are permitted to use the other columns, not required.
Under column C, 3 appliances (none of which are over 12kw) lists a demand of 14kw for the feeder.
(even using footnote 2 I still get 14kw)
 
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I got it now-- My first part was right. 8.75*.8= 7 (Table 220.55 1 appliance Col. B)

2 ranges not over 12kw -- (Table 220.55 2 appliances- col C 11kw)

11kw + 7 KW = 18 kw.
 
The answer of 18 kW is wrong.

The interpretations of Table 220.55 in other posts are incorrect, unnecessarily complex, and lead to the wrong answer.

The heading of Table 220.55 says in part: . . . (Column C to be used in all cases except as otherwise permitted in Note 3.)

Note 3 refers to cases where the maximum rating is 8.75 kW. The example doesn't meet that requirement.

For ranges of 8.75 kW, 10 kW, and 10.5 kW, none over 12 kW, it is only necessary to go down to the row for 3 appliances and read the maximum demand in Column C.

The maximum demand is 14 kW. End of problem.


This approach is proven by two examples in Appendix D of the Code.

See examples: D4(a) Multifamily Dwelling (From my NEC 2005 Code).

There is a case with 20 units where 10 have ranges. The assigned load is 25,000 Watts and they cite Column C of Table 220.55.

There is another case of 40 units where they have 20 ranges. The assigned load is 35,000 Watts and they cite Column C of Table 220.55.
 
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I believe Dennis is right because you would not put all of the ranges under one specific note.

8.75 would follow note 3, 10.5kw and 10kw range would follow note 2. Because for the 10.5kw and 10kw would average out to 12kw, so you would follow column C(Which is the minimum).
 
ray cyr said:
Quite alright :smile: I'm just glad that my reasoning was confirmed :cool:
Click to expand...
Why is that Table so difficult. I have never seen the same answer gotten from a question on it.

I am not sure why my method is wrong but it appears to be so. If you had 3 ranges at 12kw then the demand would be 14kw as stated so why would it be 18 using the method I employed with ranges less than 12 kw.

I hate that Table and it hates me. :grin:

I will go eat some crow now.
 
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