Range Calcs when to use...what?

[rhamblin]

I am preparing for my Master's exam and am stuck on the Range calculations. My first question is when do I use the column A or B vs C. I understand that you consult the notes. Note 3 says it is permissable to use columns A and/or B for certain conditions, so how do I know when to use A/B or C?

 

[rhamblin]

It seems to me that there are times when we have multiple colums/options to figure out a calculated load for ranges, is it safe to say that we are always looking for the smallest calculated load?

 

[mivey]

It seems to me that there are times when we have multiple colums/options to figure out a calculated load for ranges, is it safe to say that we are always looking for the smallest calculated load?

Yes

add: see this thread also http://forums.mikeholt.com/showthread.php?p=1028928

 

[rhamblin]

Alright another question for you. Again I am working through Mike Holt's Master Prep book. In the book they refer to column C of T220.55 as being over 8 3/4 kw. My problem is that I don't see any documentation to this effect. Am I missing something?

I thought the reason might be that column B (and A) would always be less than column C (when they apply accordingly). But that doesn't seem to be the case either.

For example; figure the feeder/service calculated load for 20 ranges at 8 3/4 kw. According to column C, we scroll down to find 20 appliances and follow it over to find 35 kw maximum demand.

If we use note 3 which allows column B to be used instead. We scroll down to 20 appliances and follow it over to 28% and multiply that by our connected load of 20x8.75kw. I come up with 49kw. In this case column C allows for a smaller demand factor. Which you would want to use right?

Which brings me back to why does the book (pg. 292 step 5 example) state "Over 8 3/4, and less than 12kVA, look up the kW Demand Load in Table 220.55, Column C.

If given the choice which I believe the code book offers in this situation, you would want to use the column C methodology.

As always any input is appreciated.

 

[RonPecinaJr]

You always want to use the minimum demand load. Minimum being the keyword here. Occasionally they will throw in a question where you'll have to use column B (and sometimes A too) and the actual demand load will be higher than the kilowatt rating in column C.

For instance, What is the feeder demand for a 4kw oven and a 12kw oven?

Column B: 4kw x 80% = 3.2kw
Column C: 12kw = 8kw
Both add up to 11.2kw for the demand.

But column C says for 2 ovens the demand factor is only 11kw. So here the answer is 11 Kilowatts, not 11.2 kilowatts. Always remember to use the minimum, and always, always check with column C to compare kilowatt ratings.

Simple, right?

 

[rhamblin]

Thanks, Ron for the clarification. :smile:

That is exactly what I thought, so it must be a mistake in the book where it states that column C is only for 8 3/4 and above.

 

[RonPecinaJr]

Thanks, Ron for the clarification. :smile:

That is exactly what I thought, so it must be a mistake in the book where it states that column C is only for 8 3/4 and above.

I'm not so sure about that. Check your private message box.

 

[resistance]

Thanks, Ron for the clarification. :smile:

That is exactly what I thought, so it must be a mistake in the book where it states that column C is only for 8 3/4 and above.

I'm really late on this, but I tend to agree. I have the book, and some things aren't lucid enough. No need to go private to be honest!!! All and all the book is great, but not easily understood.

Yet, it want stop me from buying more books--seeing that we get the site and clarification from a ton of well rounded team members on this site for free!!!