Notes for table 220.55

[bolognaman]

My 4th year apprenticeship class has been having a debate over how to interpret the notes from table 220.55, mostly over the following problem which was written by the instructor:

3 kw cooktop
6 kw wall oven
14 kw range

What is the demand factor?

One arguement is that you can only use one note for a calculation, so you would add the 3 kw and 6 kw together (note 4), get the demand factor for that number and then separately get a demand factor for the 14 kw.

The other side is to add the 3 kw and 6 kw together (note 4) and then take the two values, 9 kw and 14 kw and use note 2 to figure the demand factor.

Then the random guy in the back said to just get the demand factors for all three separately then add them together, since one is column 1, another column 2 and then column 3.

I tried to find a relevant thread in the forum, but none quite hit this conundrum. I'm looking for the method, not an answer to the problem, although if you need to work it out to prove the method, be my guest. Thank you for your input.

-Jason

 

[nyerinfl]

First of all, Note 4 applies to branch circuits, so it would not apply to this question. Notes 1 and 3 would be used. Note 1 would be used for the range, 14kw = 8 kw x 10% = 8.8 kW. Next Note 3, the second half tells you to do each one individually and then add them together, 3kw cooktop is column A, 3kW x 80% = 2.4 kW, and 6kW oven is 6kW x 80% from Column B, = 4.8 kW + 2.4 kW = 7.2 kW for both cooking, + 8.8 kW for the range gives you 16 kW. The guy in back was pretty much right on this one. If you had 2 cooktops of equal ratings they would have been added together and used '2' # of applicances demand factor.

 

[George Stolz]

I'm looking for the method, not an answer to the problem, although if you need to work it out to prove the method, be my guest.

Jason, welcome to the forum.

The answer you seek is in the back of the book, titled "Proposals for the 2011 NEC" - you just found one that needs to be written, IMO. There is nothing in Table 220.55 that covers your installation as far as I can tell.

Edit to add: I'd say 15 kW: 3 appliances, Column C, plus the 10% for the 14 kW item. However, that doesn't match the question too well, because the instructor asked for a "Demand Factor" not a load.

Edit x 2: Actually, I think that's the only right answer, come to think of it. I retract the proposal comment - I think it works, the way I just laid it out.

 

[nyerinfl]

George, I have to disagree with you. First, essentialy Column C is the Demand factor for ranges over 8 3/4 kW. It may not be a percentage, but it is a Demand Load. I don't think you can assign a demand factor percentage to this problem, it doesn't work. If you have 3 pieces of cooking equipment, one falls in Column 'A', one in 'B' and one in 'C' you can't just add them together and divide by 'x'. You can only add nameplates if they fall into the same column. I'm not sure what you're referring to about the 2011 proposals, I don't have the 08 yet.

I'm not entirely understanding how you figured '3' applicances under column 'C'. It doesn't make sense to me to do it this way.

Also I missed the part about not looking for the answer to the question.

 

[Bob NH]

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

Use of Note 3 is permissible but not required.

Note 1 starts with the statement: Over 12 kW through 27 kW ranges all the same rating. Therefore, Note 1 doesn't apply to the stated case.

Note 2 provides the best answer that is also the easiest to apply. It allows the use of Column C for all of the ranges by using 12 kW for any range that is less than 12 kW to yield the following result: (14 kW + 12 kW +12 kW)/3 = 12.67 kW as the average for all (Range + cooktop + oven). The maximum demand in Column C for 3 appliances is 14 kW, but must be increased by 5% for the major fraction of a kW that the average exceeds 12 kW, so the total demand for the ranges is 14 kW x (1.05) = 14.7 kW.

Not only is the Note 2 calculation the simplest and least ambiguous; it also yields a lower demand than any of the other approaches.

The demand factor for all three ranges (the stated condition) is the calculated maximum demand divided by the sum of the nameplate ratings:
14.7 kW/(14+3+6)kW = 14.7 kW/23 kW = 63.9%.

 

[George Stolz]

First, let me say that I think Bob from NH is correct in every aspect on this - I skimmed over the wording of Note 2 and missed the averaging part.

First, essentialy Column C is the Demand factor for ranges over 8 3/4 kW. It may not be a percentage, but it is a Demand Load.

IMO, that's two different things. Look closely at the layout of the headings of the columns, and then consider the definition of factor: [SIZE=-1]"any of the numbers (or symbols) that form a product when multiplied together." You're not supposed to take the numbers in Column 3 and multiply them with anything else, they're not a percentage; they're standing in for a "rating multiplied by a percentage".[/SIZE]

I'm not sure what you're referring to about the 2011 proposals, I don't have the 08 yet.

Well, I have a copy sitting on my desk.

If you were to submit a proposal, it would have to be to amend the 2011 NEC, and the deadline for proposals is only a little over a year away.

 

[nyerinfl]

Note 2 provides the best answer that is also the easiest to apply. It allows the use of Column C for all of the ranges by using 12 kW for any range that is less than 12 kW to yield the following result: (14 kW + 12 kW +12 kW)/3 = 12.67 kW as the average for all (Range + cooktop + oven). The maximum demand in Column C for 3 appliances is 14 kW, but must be increased by 5% for the major fraction of a kW that the average exceeds 12 kW, so the total demand for the ranges is 14 kW x (1.05) = 14.7 kW.

Not only is the Note 2 calculation the simplest and least ambiguous; it also yields a lower demand than any of the other approaches.

The demand factor for all three ranges (the stated condition) is the calculated maximum demand divided by the sum of the nameplate ratings:
14.7 kW/(14+3+6)kW = 14.7 kW/23 kW = 63.9%.

Again I have to disagree, Note 2 applies to Over 8 3/4 through 27 kW, if you have a 3 kW and 6 kW cooktop they do not apply to Note 2 as they don't fall in that range. I'm not seeing your guys argument, because when I read Note 2 I see something completely different than what you guys are showing. I see 'Over 8 3/4 kW through 27 kW Ranges'. Please explain how a 3 kW or 6 kW cooktop could possibly fall into this category. I still think Note 1 applies, as there is only 1 range in the category. The 12 kW for all ranges would only apply if the additional cooking units were 8 3/4 kW or higher, which they're not. The key word is Ranges, which the 3 kW and 6 kW are not, nor do they in any way fall into the 8 3/4 kW+ category.

 

[Bob NH]

The caption of Table 220.55 says: Demand Factors and Loads for Household Electric Ranges, Wall-Mounted Ovens, Counter-Mounted Cooking Units, and other Household Cooking Appliances Over 1 3/4 kW Rating (Column C to be used in all cases except as otherwise permitted in Note 3.)

When the calculation of 16 kW in the second post of this thread is compared to the Note 2 Calculation of 14.7 kW that results from assuming that the oven and cooktop are each 12 kW, then the 16 kW result fails a test of logic.

If the maximum demand for a 14 kW range and two 12 kW ranges is 14.7 kW, then the demand can't be any greater if the two 12 kW ranges are replaced by a 3 kW cooktop and a 6 kW oven.

The caption of Table 220.55 says that Column C shall be used in all cases except as otherwise permitted in Note 3. Note 3 is clearly identified as an alternative to the Column C method and does not supersede it.

Edited - Another view. (I was working on and posted this part when the next reply was posted, so I didn't see the next until the edit was posted.)
Another way to look at it, which is not supported by the language of the code but could be inferred from Note 4, would be to consider the cooktop and the oven together as one 9 kW range, and then apply Note 2 for the case of one 14 kW range and one 12 kW range.

The average per Note 2 would be 13 kW, which would require the 11 kW demand for two ranges to be increased by 5% or 0.55 kW. The resulting demand would be 11.55 kW. That is probably closer to being consistent with the case of two real ranges.

The demand factor for that case would be 11.55/23 = 50.2%.

 

[nyerinfl]

I believe the question falls under the last sentence of Note 3. Note 2 does not. We may have to agree to disagree, because I can't agree with your argument, I've never seen it done that way, and now that I have I don't think it makes any sense. In your first sentence you quote the title of the Table, particularly (Column 'C' to be used in all cases except as otherwise permitted in Note 3.) and I believe that Note 3 in this instance does apply.