Load Diversity

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sherwin

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I searched the archives and have seen postings from charlie b and others regarding the definition of load diversity and table B310.11.

I believe I can convince my local inspector into letting me use B310.11 if I can explain the formula at the bottom of the table.

Can anyone explain the difference between the variables 'N' and 'E'?

I see that if 'E' is twice 'N' then the square root term goes to one and the solution refers back to B310.11.
 

charlie b

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Re: Load Diversity

Originally posted by sherwin: I see that if 'E' is twice 'N' then the square root term goes to one and the solution refers back to B310.11.
I think you mean if "N" is twice "E," or if "N" is bigger than twice "E."

Your question has led me to suspect that the article is using the phrase "load diversity" in the context of "not all wires will be carrying current at the same time." In other words, if you have two pumps, and if all wires are in the same conduit, and if the switch is set up to turn on one or the other (but never both), then only half the wires will be carrying current at any one moment. That would give you (in the apparent context of this formula) a load diversity of 50%.

That is not a standard use of the phrase "load diversity," as you may have seen in my earlier posts on this topic. So the rest is a guess on my part.

Count how many conductors are in the raceway. That is "N." Now suppose that you have control switches to prevent all of the wires from carrying current at the same time. So count up how many might be carrying current at the same time. Please note that it does not matter how much current, or does the current-carrying capability of each wire matter. If it carries current at the same time as any other wire, then count it. This total count is "E." Here is what you do with the information:

</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">If "E" is half of "N," then you have 50% load diversity (again, in this non-standard context). That means that A2 is the same as A1, as you have already pointed out.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">If "E" is less than half of "N" (let us say that "E" is one eighth of "N"), then the load diversity is lower than 50% (in this case, 12.5%), and A2 would be equal to 2 times A1. The rule tells you to use A1, if this happens.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">If "E" is more than half of "N" (let us say that "E" is 3/4 of "N"), then the load diversity is higher than 50%. You would expect to not be able to use the same factors that show up in the table, since they are based on 50% LD. In this example, A2 would be about 81% of A1, so you would have to derate by 81% of Table B310.11.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Finally, if "E" is equal to "N" (it cannot be higher, or you have mis-counted) then you have 100% load diversity. That means that A2 is about 71% of A1, so you would have to derate by 71% of Table B310.11.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">If this makes sense to you, I would be happy, for that would mean that it made sense to at least one of us. :D Good luck.
 

sherwin

Member
Re: Load Diversity

charlie b,

Yes it does make sense to me. Your interpretation of the formula is what I had guessed. However, I think the way the NEC has written the code as it relates to 'load diversity' and ampacity can use some clarification.

In my situation there are no controls to prevent current flow in some circuits while others are energized. Past experience has shown me that these circuits have typically been underloaded and that an estimate of less than 50% loading is reasonable. Past experience, of course, is not an engineering calculation that an inspector might be looking for.

An argument of 50% loading is reasonable for the use of B310.11, although not explicitly backed by the NEC or the formula following the table. My inspector might follow this reasoning as well. If not, the worst that can happen is he can say "No".

Thanks for your insight.

[ July 13, 2005, 04:36 PM: Message edited by: sherwin ]
 

charlie b

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Retired Electrical Engineer
Re: Load Diversity

Originally posted by sherwin: Past experience has shown me that these circuits have typically been underloaded and that an estimate of less than 50% loading is reasonable. . . . An argument of 50% loading is reasonable for the use of B310.11,
Whatever the NEC ultimately defines "load diversity" to be, if it ever gives a definition, I think the one you are suggesting will not be it. That is regrettable, for I think physics would be on your side (aside to physis: I said "physics," not "physis," so I am not presuming to speak for you ;) ).

Suppose each and every conductor in the conduit is carrying 50% of its rated current (i.e., its ampacity from whatever table applies). In accordance with the common Engineering definition of "diversity factor," which may or may not be the same as "load diversity," your "diversity factor" would have a value of 100%. This fits with most of our interpretations of Table 310.15(B)(2)(a), in that if a conductor carries any current at all then it counts, no matter how much current it is carrying.
 

steve66

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Location
Illinois
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Engineer
Re: Load Diversity

I have to disagree with Charlie on this one. If the code doesn't specifically prohibit something, then its allowed, right?? Since the code doesn't define "load diversity", I think engineers are allowed to define load diversity as necessary to apply to your situation.

As engineers, we should be able to calculate the maximum heating that will occur in the wires, and calculate what is needed to keep this under the limits of the wire.

If you are willing to do the calculations, stamp them, and submit them to the AHJ, I don't think the AHJ has any right to challange the definition of "load diversity" that you choose to use.

Steve
 

charlie b

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Retired Electrical Engineer
Re: Load Diversity

Originally posted by steve66: I have to disagree with Charlie on this one. . . . Since the code doesn't define "load diversity", I think engineers are allowed to define load diversity as necessary to apply to your situation.
I have to disagree right back. Or to soften the statement, I have to split a hair on this one. I agree that if the NEC does not prohibit something, then it is permissible. However, if a term is not defined in the NEC, we are to use the "industry standard" definition. The presumption is that if the term is well enough understood in the industry, then the NEC does not need to give its own definition.

But that is not the same thing as saying that we are free to define a term in any way that suits our needs and our situations. We have to use the industry standard definition, not our own definition. The problem here is that there is no industry standard definition for "load diversity." The closest term I have found in the definitions section of any of my textbooks is the term "diversity factor." I do not know if this is the same term that the authors of the NEC used, when they developed Table B310.11. Therefore I do not know how to use that table, and I have not yet encountered a project in which I had to apply that table.

I do agree that if an engineer is willing to perform a calculation and to apply his or her PE seal and signature, then the AHJ should consider that an acceptable substitute for any numbers that might be extracted from any NEC Table. That being said, I do not think the AHJ has to accept any definition of "load diversity" that we might choose to apply.

My key point is that the authors of the NEC had in mind a concept when they developed Table B310.11, and they called that concept "load diversity." If we are going to use their table, then we cannot use any definition of that term other than their intended definition of that term. I just wish one of them would step forward and give us their intended definition. But from earlier discussions on this Forum, I know that won't happen. This Table is in Annex B instead of in the enforceable sections of the NEC because the authors were not able to agree on their intended definition.
 

don_resqcapt19

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Illinois
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retired electrician
Re: Load Diversity

Charlie,
My key point is that the authors of the NEC had in mind a concept when they developed Table B310.11, and they called that concept "load diversity."
I don't think that they did. As I recall these tables appeared in Article 310 for one code cycle with an effective date of 3 years later. They were moved to the annex, in the next code cycle because no one could agree on what the term "load divesity" means.
Don
 
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