Load Calculations: the controversy

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tomwible

Member
I am currently studying to take the NH Masters test. I am studying the codebook and using the Snapz course.

Seems like there is some discrepancy with article 215 and the course. The Snapz has several questions in which the given answer says that continuous loads are calculated at 125% for load calcs. The article seems to say that continuous loads have their feeders sized at 125%.

Which is it??? Loads calculated at 125% or 100%. Seems to me the code says circuits are sized at 125% but not for load calcs.

Please help!!!!! Thanks in advance.

-Tom
 

jumper

Senior Member
I am currently studying to take the NH Masters test. I am studying the codebook and using the Snapz course.

Seems like there is some discrepancy with article 215 and the course. The Snapz has several questions in which the given answer says that continuous loads are calculated at 125% for load calcs. The article seems to say that continuous loads have their feeders sized at 125%.

Which is it??? Loads calculated at 125% or 100%. Seems to me the code says circuits are sized at 125% but not for load calcs.

Please help!!!!! Thanks in advance.

-Tom
You are going to have to deal with this at some point.

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not
less than required to supply the load as calculated in Parts
III, IV, and V of Article 220. The minimum feeder-circuit
conductor size, before the application of any adjustment or
correction factors, shall have an allowable ampacity not
less than the noncontinuous load plus 125 percent of the
continuous load.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I would not use the 125% for service calculations unless specifically mentioned as in the largest motors, etc.

Another example would be if the lights are continuous you would still just use the watts/sq ft and not multiply by 125%
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Derek has correctly pointed out that the 125% only applies to continuous loads. But there are two relevant articles, and both are saying the same thing. There is no conflict. Keep in mind that article 220 is all about calculations, and 215 is all about feeder sizing. They address separate things, but both make us account for 125% of the continuous loads.

That said, if this is the reason for your confusion, let me clarify that you don't calculate the load by adding an extra 25%, and then take that figure and add another 25% when you select the feeder size.
 

tomwible

Member
The controversy for me is that the Snapz is telling me 125%. I see now that this is wrong for load calcs. I see Mike H. is selling Snapz though.

Thanks everyone.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
The controversy for me is that the Snapz is telling me 125%.
I don't understand your issue. I have no copy of Snapz (I had never even heard of it before), so I can't look it up myself. So please tell me,

  • WHAT is Snapz is telling you to take 125% of?
  • At WHAT stage of the process is this supposed to happen?
  • And WHAT are you supposed to do with that number, once you calculate it?
  • In the context of the Snapz statement, are you dealing with NEC article 215 or 220?
 

tomwible

Member
Here is a link to the Mike H. course which includes the Snapz disc I am using.

http://www.mikeholt.com/productitem.php?id=904&year=2008&from=All&type=Book

I will paste an example of what I am talking about, which is taken directly from this Snapz course:

What is the minimum lighting load for an auditorium with 10,240 sq ft?

Ans:
1) see 220.12
2) Which says use 1 va/sq ft.
3) therefore 10,240 va
4) 215.2 a says "feeders must be computed at 125% of continuous load.
5) 10,240va * 125% = 12800va

This seems wrong in light of what is being said on the code and here. Isn't this a load calc? Not an ampacity queston?

-Tom
 

tomwible

Member
Here is another example from the Snapz course which seems wrong to me:

220.12

What is the computed load for a store room with the following? 2300 sq ft of floor area, 70 receptacles, 15 feet of show window, 24a three phase air conditioner, 120volt 1500 watt water heater. The service will be 120/208 volt wye.

1) look up ?loads? in the index and find the sub index for ?branch circuits calculations? which refers to annex d and article 220
2) Section 220.12 requires a unit load of not less than that given in table 220.12 be used for the minimum lighting load.
3) Table 220.12 requires a minimum of 3va per sq ft for lighting loads. Section 215.2a feeders requires this load to be increased by 125% as it will continue for more than 3 hours.
2300 sq ft x 3va = 6900va
6900va x 1.25 = 8625va
4) There are 70 receptacles section 220.14l requires 180va to be used to calculate the loads for these.
70 x 180 = 12600va
Section 220.44 permits the receptacle loads to be derated according to table 220.44. The first 10000va is used at 100% with the remainder at 50%.
12600va ? 10000 VA = 2600va
2600va x .50 = 1300va
1300va + 10000va = 11300va
5) Determine the load for the show window lighting. Sections 220.14g2 and 220.43a requires a load not less than 200va per linear foot.
15 ft x 200va = 3000va
Section 210.19a requires branch circuits be not less than 125% of the continuous load
3000va x 1.25 = 3750va
6) Section 220.14a requires the ampere rating of a specific appliance in this case a water heater be added at the nameplate current rating.
1500w = 1500va
7) Section 220.14c requires the a/c load to be calculated per section 430.22 430.24 and article 440. Section 440.6a hermetic refrigerant motor compressor requires the use of the nameplate rated load current for the air conditioning for branch circuit feeder calculations.
24a x 208v x 1.732 = 8646va
8) Calculate the total service load by adding the loads calculated above.
General lighting 8625va
Receptacle load 11300va
Show 3750va
Appliance 1500va
Air 8646va
Total 33821va
9) To find the service size divide the total service load by the service voltage. For a 3 phase service the service equals 208v x square root of 3.

208v x 1.732 = 360v
33821va / 360v = 93.94a

Note step 5 whci is using the 125% again for a load calc?

-Tom
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
The controversy for me is that the Snapz is telling me 125%. I see now that this is wrong for load calcs. I see Mike H. is selling Snapz though.

Thanks everyone.
Mike Holt also sells the NEC but he didn't write it. He sells the Snapz because along with being an electrician/inspector,instructor,etc. he is a businessman, and Snapz is a popular course. You should check out Mike's exam prep materials, they are excellent.
In you examples, they are talking about branch circuit and feeder loads which would be sized for the 125% loads involved. The auditorium lights would be a continuous load because they are considered on for more than 3 hours, therefore 125%. I don't have time right now to go into more detail, but someone else will surely add to (or correct) what I have tried to show you here.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
OK Tom. Now I understand the situation. And I agree with you that the book answer is wrong. But I seem to vaguely recall that this issue has been debated here before (not recently). I do not recall whether the membership arrived at a consensus.

First I’ll comment on your first example (Post #7). The question was, what is the minimum lighting load. To get that answer, you look in 220, not in 215. Anything 215 has to say about sizing feeders will have nothing to do with the question of the amount of lighting load. The answer is 10,240, not 12,800.

Now suppose we were to say that all of the lighting load, and no other load, was going to be supplied by a single, separate, panel. Suppose we then asked what size the feeder to that panel had to be. Now you are into 215. Here we must pause and consider whether to use the 10,240 value or the 12,800 value. This is where the debate came in.

? Some members (myself included, I seem to recall) believed that you never add 25% to a load value that originated in the “watts per square foot table." The notion is that that table represents a lighting allowance, not an actual value of lighting load. If you instead arrived at a lighting load value by counting the actual fixtures that were installed, then that would be a “continuous lighting load,” and the 125% factor would be applied.

? Other members believed that lighting is continuous in most real-life applications. That is, lights are going to be on more than three hours in a row in most offices, warehouses, theaters, banks, and other buildings. They conclude that the “watts per square foot” value of lighting load needs to be treated as a continuous load, and they apply the 125% factor.

So depending on which side you take in this debate, the answer to my proposed question would be one of the following two values:

? You size the feeder for a minimum ampacity corresponding to 10,240 VA, based on a lighting load of 10,240 VA, or

? You size the feeder for a minimum ampacity corresponding to 12,800 VA, based on 125% of a lighting load of 10,240 VA.

But whichever of these you choose, the lighting load is still 10,240 VA. I say again, the book is wrong.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
Tom, My reply in my previous post was assuming they were asking for a branch circuit and feeder sizing because they specifically said branch circuit and feeder. I didn't see step 9 in your second example asking for service size until now. To me, it appears they are inter mixing service and feeder/branch circuit calculations. Look in your book and see if there is a place (usually a web site ) to check for errors found after the book printing. Or an email where you can ask them directly and get some guidance. I know in Mike Holt's material, there were errors and when found, a list of the errors and corrections was listed on his website. You could also report any new errors that you might find. Also, make sure it isn't a trick question, putting in info that has nothing to do with sizing a service. I found some of them on the actual exam, got to watch for that.:mad:
 

Smart $

Esteemed Member
Location
Ohio
Here is a link to the Mike H. course which includes the Snapz disc I am using.

http://www.mikeholt.com/productitem.php?id=904&year=2008&from=All&type=Book

I will paste an example of what I am talking about, which is taken directly from this Snapz course:

What is the minimum lighting load for an auditorium with 10,240 sq ft?

Ans:
1) see 220.12
2) Which says use 1 va/sq ft.
3) therefore 10,240 va
4) 215.2 a says "feeders must be computed at 125% of continuous load.
5) 10,240va * 125% = 12800va

This seems wrong in light of what is being said on the code and here. Isn't this a load calc? Not an ampacity queston?

-Tom
Correct.

The load, a minimum as determined by floor area served, is 10,240VA... period. The only time the lighting load would be larger is if the actual connected load was greater than the calculated minimum load.

When the question asks for a "load" value, it is the load at 100% regardless of being a continuous or non-continuous load.

Even in instances, such as when sizing conductors, where you multiply the continuous load or largest motor load by 125%, the load is still the 100% value!!!
 
Last edited:

tomwible

Member
Thanks so much for the detailed analysis of my questions. Of course my main concern now is that my test reflects my understanding of this issue. My second concern is how many people get the Snapz course and are taught the wrong method.

-Tom Wible
 

tomwible

Member
Correct.

The load, a minimum as determined by floor area served, is 10,240VA... period. The only time the lighting load would be larger is if the actual connected load was greater than the calculated minimum load.

When the question asks for a "load" value, it is the load at 100% regardless of being a continuous or non-continuous load.

Even in instances, such as when sizing conductors, where you multiply the continuous load or largest motor load by 125%, the load is still the 100% value!!!
I think motor load is calculated at 125% as per 430.

-Tom
 

Smart $

Esteemed Member
Location
Ohio
I think motor load is calculated at 125% as per 430.

-Tom
125% of largest motor for service or feeder calc'.

But even so, the motor load is still the 100% value. Calculating at 125% is just that, calculating at 125%. If you had two motors of different "size" on two different feeders, they may be calculated at 125% on their respective feeder, but only the larger would be 125% on the service while the other would be at 100%!!!

And in most instances, the 125% factoring is never applied twice. I am only aware of one place in the NEC which compounds the 125% factoring... and that's in Article 690 when sizing photovoltaic source conductors.
 

skeshesh

Senior Member
Location
Los Angeles, Ca

? Some members (myself included, I seem to recall) believed that you never add 25% to a load value that originated in the “watts per square foot table." The notion is that that table represents a lighting allowance, not an actual value of lighting load. If you instead arrived at a lighting load value by counting the actual fixtures that were installed, then that would be a “continuous lighting load,” and the 125% factor would be applied.
Charlie,

I think I agree with that as a general approach but I believe there are other times when it's appropriate to apply the 125% factor. For example, I've worked on some projects where pre-fabricated modular buildings are being used. In this case, due to it being a gov't project and having an open bid process, I could not specify a manfuacturer and design based on their specs. What I ended up doing to estimate the lighting load was to calculated the maximum allowable load (per Title 24 out here in Ca) and used a 125% factor for that load in feeder sizing for the panel. In other words if I can't define the fixture input and count as is regularly done, I think it's prudent to assume that maximum allowable load IS the installed load in order to account for the worst case possible.
 

tomwible

Member
Thanks again for taking the time to clarify. I do greatly appreciate it.
I am continuing on in my studies and I find yet another example where they seem to confuse load with ampacity. Seems like Snapz is taking the 125% and applying it to continuous loads whether you are talking about feeders size or computed load.

I pray my Masters Exam has it correct!

-Tom
 

tomwible

Member
Update from Snapz

Update from Snapz

Here is the response from Snapz:

Tom

Thank you for bringing this to our attention. I've looked at it and agree with you, the question is not asking about the feeder, but the lighting load. We'll review it further to see what the intent was and how best to correct it.

If you find other questions that are questionable please feel free to send me their QIDs and I'll check them out.

Appreciate the help.

Cordially

James Vafakos
Snapz Software, Inc.


James seems to respond and care about his product!

-Tom
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
Here is the response from Snapz:

Tom

Thank you for bringing this to our attention. I've looked at it and agree with you, the question is not asking about the feeder, but the lighting load. We'll review it further to see what the intent was and how best to correct it.

If you find other questions that are questionable please feel free to send me their QIDs and I'll check them out.

Appreciate the help.

Cordially

James Vafakos
Snapz Software, Inc.


James seems to respond and care about his product!

-Tom


That's why I told you to check with them, because that's the way Mike Holt handles courses. It never hurts to ask. I questioned a few things that turned out I was wrong and they were very good to show me why. I also found some mistakes that were their fault, and they apologized and corrected them. Humans will err.;)
 
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