Load Diversity

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charlie b

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Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Proposal:

Add a definition under Article 100 for ?Load Diversity.? As an alternative, replace the phrase ?load diversity? in the five locations at which it presently appears in the NEC, by substituting a phrase that more accurately and clearly conveys the intended meaning.

Substantiation:

The phrase "load diversity" appears in five locations, and the word "diversity" appears nowhere else. Two of the locations are within Table B310.11. Two of the locations are in Fine Print Notes ? associated with 310.15(B)(2)(a) and 400.5 ? that refer the reader to Table B310.11. The fifth location is in Table 520.44. In all five locations, the reader is led to believe that a higher value of ampacity can be justified if the ?load diversity? is 50% or lower.

The NEC does not define ?load diversity.? It is understood that if a term or phrase is not explicitly defined in the NEC, then the reader is to interpret that term or phrase in accordance with common industry usage. However, the phrase ?load diversity? does not possess a common industry usage for which a calculated result could be expressed as a fraction less than one.

There are two commonly used definitions of ?diversity,? as applied in connection with electrical load.
?Load Diversity? is defined as, ?The difference between the sum of the maximum of two or more individual loads and the coincident or combined maximum load, usually measured in kilowatts.?
?Diversity Factor? is defined as, ?The ratio of the sum of the individual maximum demands of the various subdivisions of a system to the maximum demand of the whole system.?
The first of these terms cannot be expressed as a dimensionless ratio. It is measured in units of power. The second of these terms is a ratio that must necessarily be greater than or equal to 1.0. Therefore, the concept of ?a load factor of 50%? is meaningless.

There are two problems with leaving this term in the NEC without a useful definition: First, we have what appears to be an available alternative for calculating ampacity, but the alternative is useless and meaningless. This leads to the second problem: that someone might try to take advantage of this alternative, but do it incorrectly, and thereby install a system that is undersized for its intended loading conditions.

I can present no evidence that an incorrect application of the allowance for ?load diversity? has ever led to the installation of an unsafe system. But it is clear that present wording, and the present lack of a clear definition, (1) cannot lead a person to do it right, and (2) can lead a person to do it wrong.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Load Diversity

Pretty Cool Charlie.

I have never tried to use 'load diversity', never knew what it meant, still don't. :(

That said IMO you have a very well written proposal.

I would only ask why you did not state a source for the quotes?

Bob
 

charlie

Senior Member
Location
Indianapolis
Re: Load Diversity

Charlie, Panel 1 does not take to adding new definitions easily. My recommendation is to make a separate proposal for the definition in each location where the term is used. Additionally, make a proposal to insert a definition in Article 100 but to list where you have made the other proposals. If Panel 1 rejects the proposal, you have five more chances of the definition getting into the Code. If panel 1 accepts the proposal, the TCC will be able to direct the other panels that a new definition is in Article 100 and that they can then reference to it.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: Load Diversity

Originally posted by iwire: Pretty Cool Charlie . . . IMO you have a very well written proposal.
Many thanks.
I have never tried to use 'load diversity', never knew what it meant, still don't.
Neither do I. Or at least, I do not know how to take advantage of the NEC articles that use that phrase.
I would only ask why you did not state a source for the quotes?
The first quote I found in several locations by an Internet search. That was good enough for posting in this Forum, but I do not believe that is an acceptable source for a formal proposal. I would find a more authoritative source, before I would submit a code change.

The second quote is from Electric Power Distribution System Engineering, by Turan Gonen, published by MeGraw-Hill Book Company, NY, 1986. It was the textbook from a class in my Masters Degree program.
 

rbalex

Moderator
Staff member
Location
Mission Viejo, CA
Occupation
Professional Electrical Engineer
Re: Load Diversity

Originally posted by charlie b:
[QB] Proposal:
Add a definition under Article 100 for ?Load Diversity.? As an alternative, replace the phrase ?load diversity? in the five locations at which it presently appears in the NEC, by substituting a phrase that more accurately and clearly conveys the intended meaning.
I'd echo the "other" Charlie's strategy here. I suggest you propose a specific "workable" definition since neither of the "common" ones fit. Where several CMPs are involved, its also a good idea to let them all in on an idea as early in the process as possible with concurrent proposals. (And note they are concurrent). The TCC does an yeoman's job but they still miss coordination issues occasionally. This also increases the opportunities to get accepted at the ROC stage.

I wish CMP1 were as reluctant to accept some definition changes as Charlie said. We spent well over two of our five allotted days at the ROP stage on the 70E-2004 TC adjusting to the implications of two NEC definition changes("Energized" and "Live Parts"). Its one of several reasons 70E was reformatted and delayed being issued. (It should have been 70E-2003)
 
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