Table 220.55 – Demand Factors for Household Electric Ranges; Using Column C after using Column B & C...???

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bvanderwerf

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I'm studying for the Journeyman test I will be taking tomorrow and need some help on a Mike Holt question. The question is "What's the calculated feeder demand load for one 12kW range and 6kW cooktop?" [Electrical Exam Preparation, Unit 9 Challenge Question #15].

The options are:
(a) 11kW
(b) 11.2kW
(c) 12.80kW
(d) 18kW

The answer key says it's 11kW, I would say 12.80kW

Per the table, in column C, I would size the larger, 12kW range right at 8kW. The 6kW falls under cooktop falls under column B, which would be sized 80% of 6kW = 4.8kW. 8kW + 4.8kW = 12.8kW.

The closest I could find to how to combine these numbers is end of note 3 to Table 220.55, where it says "Where the rating of cooking appliances falls under both Column A and Column B, the demand factors for each column shall be applied to the appliances for that column, and the results added together." I know it says Column A and Column B, but what about when you have a solution from Column B & C?. The answer key says 11kW, which pulls the second 4.8kW "demand-factored" cooktop into Column C, and treats it as an "over 8-3/4kW to 12kW" cooking unit and then sizes both the range and cooktop as two (2) column C units, making it 11kW.

To summarize my question: Where in the 2017 NEC is it justified to demand factor a Column B rated cooking unit as a Column C unit?

Thanks!
 
david luchini

david luchini

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Number of Appliances = 2.

Table 220.55, Column C says 11kW for two appliances.

I would say 11kW is the correct answer. Read the heading of T220.55..."Column C to be used in all cases except as otherwise permitted in Note 3."

(Column C isn't "over 8-3/4kW to 12kW. Column C is over 1-3/4kW to not over 12kW. And then see the Notes for over 12kW.)
 
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bvanderwerf

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Cassopolis, MI, USA
david luchini said:
Number of Appliances = 2.

Table 220.55, Column C says 11kW for two appliances.

I would say 11kW is the correct answer. Read the heading of T220.55..."Column C to be used in all cases except as otherwise permitted in Note 3."

(Column C isn't "over 8-3/4kW to 12kW. Column C is over 1-3/4kW to not over 12kW. And then see the Notes for over 12kW.)
Click to expand...

David,
Thank you! That was very helpful. Pointing out the title of the Table 220.55 helped me understand what I was missing.
 
Dennis Alwon

Dennis Alwon

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I think that 95% of the people answering that question would get it wrong. It is exactly as David said-- but... it is easy to think Col C is 8 3/4- 12kw. IMO, I think either answer could be correct since it didn't ask for the lowest demand one could apply....
 
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bvanderwerf

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david luchini said:
The question (as presented) doesn't specify that 220.55 must be used.
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It does say "What's the calculated feeder demand load". Is that not referring to the demand factors that fall under the Standard method of calculation? It was my understanding that if it doesn't explicitly say to use the Optional method, the Standard method applies, which would put this question in the scope of Table 220.55.
 
david luchini

david luchini

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bvanderwerf said:
It does say "What's the calculated feeder demand load". Is that not referring to the demand factors that fall under the Standard method of calculation? It was my understanding that if it doesn't explicitly say to use the Optional method, the Standard method applies, which would put this question in the scope of Table 220.55.
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You are permitted, but not required, to use 220.55. If you don't use it, then the calculated feeder demand load would be 18kW.

But of course, the question not only expects you to use the demand factors in 220.55, but it expects you to find the smallest permitted load using 220.55.
 
Dennis Alwon

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Dsg319 said:
Is this correct?

12kw @ column c 2 appliances = 11kw
11kw+6kw=17kw x 65%= 11,050w?
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As David said, NO

If you use Col C then any range less than 12 must be converted to a 12Kw range. So know the 6kw cooktop is consider 12kw and the 12 kw range is 12kw.

This gives us 2- 12kw ranges. If you look at the 2nd row for 2 ranges under col. C you get 11kw.
 
Dennis Alwon

Dennis Alwon

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The brain doesn't really think like that unless you're smart like David... seriously. One tends to use col b for 6kw and col c for the 12kw. The notes specifically say to use both Col A and Col B when you have ranges that fall in that category but it does not say to use Col. B and col. C which gave 12.8kw. That obviously would work but it is not the lowest demand.
 
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