Adjustment Factors for Conductors with Load Diversity

Status
Not open for further replies.
P

pdr

Member
Location
Travel to Jobs. Home in Michigan, now in Arkansas
Annex B,Table B.310.11 "Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable with Load Diversity"

10-24 conductors........70% adjustment*

*These factors include the effects of a load diversity of 50 percent.

The calculation examples shown do not define if the 14 AWG THHN are being installed with existing 14 AWG THHN's.

Questions are;

1. Define "load diversity". (not in the definitions).

2. Clarify if the calculation can be used with conductors of the same insulation type, but with different size conductors.


Note; Table 310.15(B)(2)(a)
10-20 conductors........50% adjustment
 
Re: Adjustment Factors for Conductors with Load Diversity

If you do a search of this forum for "load diversity", you will find that the term is not defined in the NEC. As a result, some say you can't use these adjustment factors. I think you will have to form your own opinion as to what you think is allowed, and safe.

Steve

[ April 28, 2005, 01:06 PM: Message edited by: steve66 ]
 
Re: Adjustment Factors for Conductors with Load Diversity

The real reason that table is now in the Annex is because no one really knows what load diversity means. It was in the code itself until they tried to define diversity.
Don
 
Re: Adjustment Factors for Conductors with Load Diversity

Not only is the phrase ?load diversity? not defined in the NEC, but it is also not defined in engineering text books. The closest standard term is ?diversity factor.? As Steve suggested, you can do a word search on this forum, and find a definition of that phrase. I will add, however, in answer to your second question, that there is good news and bad news.

The good news is that the factor would apply to each conductor individually, so that having different size wires does not matter. You adjust the ampacity of each wire from its starting point (310.16), using any and all factors that apply.

The bad news is that it would take a formal calculation, signed and sealed by a licensed professional engineer, before anyone could take credit for the higher ampacity that could be gained from ?load diversity.?
 
Re: Adjustment Factors for Conductors with Load Diversity

So Annex B, Table B.310.11, cannot help me get from 50% to 70% for an adjustment / derating factor, without a signed approval from and engineer, and even then he may choose not to because "load diversity" is not defined.

Is this correct?
 
Re: Adjustment Factors for Conductors with Load Diversity

That is my view of the situation, yes. And I am not trying to drum up business for myself or for other PEs. I don't think I would like to take on a project that required the use of that Annex. I would need more information than is generally available, and even with that information it is tough to get a diversity factor below 50%. In other words, even if you tried, even if you paid a PE to do a calculation, you might not get the results that you desire. :(
 
Re: Adjustment Factors for Conductors with Load Diversity

Charlie B:

May I throw out a what if?? Say we are going to power workstations in a large open office. We know that article 220 requires 1VA per square foot. Now assume that we provide branch circuits from a panel totaling 5VA per square foot. (That's not unrealistic if we do 1 circuit per 3 workstations). Don't you think that could qualify as 50% diversity no matter how you look at it?
 
Re: Adjustment Factors for Conductors with Load Diversity

Steve, you have not been in my office. We have scanners, printers, coffee cup heaters, radios, a heater in the winter, all kinds of chargers, electric stapler's, a television in one cubicle (can't miss Judge Judy at lunch), under the counter refrigerators (there are two in the office), under the binder bin lighting, computers, etc. Granted, this is not all in the same cubicle but you get the idea. We also had the foresight to run a 20 ampere circuit to each cubicle and none of them were multi-wire branch circuits. :D
 
Re: Adjustment Factors for Conductors with Load Diversity

Originally posted by charlie:
Steve, you have not been in my office. We have scanners, printers, coffee cup heaters, radios, a heater in the winter, all kinds of chargers, electric stapler's, a television in one cubicle (can't miss Judge Judy at lunch), under the counter refrigerators (there are two in the office), under the binder bin lighting, computers, etc.
Click to expand...
Okay, we get it, you get free power, you work for the power company. Jeez. :D :D
 
Re: Adjustment Factors for Conductors with Load Diversity

With all those luxuries, you must work in the marketing dept. Charlie!! I remember a paticular "Dilbert" cartoon strip had a doorway with a sign over it that read "Marketing Department - Two Drink Minimum" :)


The thing that bugs me is that I only have to provide 1VA per square foot. So I design all the conduits and everything so I don't have to derate the wire. Now, if I add more circuits (maybe just because I don't want Charlie's heater to trip the breaker to George's computer), I may be forcing myself to also use larger wire.

I guess I just don't think derating should be a deterent to installing more branch circuits.

Steve

[ April 29, 2005, 09:22 AM: Message edited by: steve66 ]
 
Re: Adjustment Factors for Conductors with Load Diversity

Several years and editions ago when load diversity was a part of 310-15 we used a load diversity of 50% when; not more than 50% of the conductors in a given raceway or cable were current carrying at the same time.

If the number of current carrying conductors is greater than 50% of the total number of conductors, the formula in B.310.11 must be used.

If the number of current carrying conductors is less than 50%, the formula may be used.

If the number of current carrying conductors is 50% of the total the table may be used.

We used this when we had a lot of interlocked loads that prevented conductors being energized.
 
Re: Adjustment Factors for Conductors with Load Diversity

If, for example, you have 8 wires in a conduit (i.e., two sets of A, B, C, N), and if there is a control mechanism (i.e., an interlock) that allows only 3 of the wires (plus its associated neutral) to carry current at a time, then I believe you can simply say that the number of current-carrying conductors is 3, and not 6. You would not have to correct per Table 310.15(B)(2)(a). But that has nothing to do with ?load diversity.?

The phrase ?diversity factor? (ASIDE: Is that the same thing as ?load diversity?? I do not know.) 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.? If anyone can figure out how to apply this definition in a useful way (i.e., to get more ampacity out of an installation), then I would love to hear about the method.

To Steve66: Regarding your ?what if?? situation: The difference between the number (or total rating) of the circuits you install, as compared to the VA per square foot allowance that 220 requires us to include, is unrelated to ?diversity factor,? as defined above. But no knowing what is meant by ?load diversity,? I can?t comment on your ?what if.?
 
Re: Adjustment Factors for Conductors with Load Diversity

I was fortunate enough to be the staff liaison for Code Panel 6 when all these new rules and Neher-McGrath calculations were introduce in the Code. Load Diversity is defined in the IEEE Dictionary as the difference between the sum of the maximum of two or more individual loads and the coincident or combined maximum load over a specific period of time. Diversity as used in the Code, means the percentage of energized conductors in a raceway or cable. For example, there is a 50 % diversity when 10 or more conductors are in a cable or raceway. Diversity here means the percent of energized conductors in the cable or raceway.
 
Re: Adjustment Factors for Conductors with Load Diversity

Originally posted by john m. caloggero: Load Diversity is defined in the IEEE Dictionary as the difference between the sum of the maximum of two or more individual loads and the coincident or combined maximum load over a specific period of time.
Click to expand...
That is essentially the same as the definition I quoted above for ?diversity factor.? A distinction between your definition and mine is in the use of the word ?difference,? as opposed to ?ratio.? But I think the intent is the same for both cases.
Diversity as used in the Code, means the percentage of energized conductors in a raceway or cable.
Click to expand...
If that is true, then it is not related to the definitions that you and I have quoted. ?Load Diversity? and Diversity Factor? are related to measured, maximum currents, not to whether or not a conductor is energized (or whether or not it is carrying current).

But your description of the intended use of ?Load Diversity? does not make sense to me. A cable can be ?energized,? and still not carry current. If there are no devices plugged into any receptacle outlets throughout a house, the conductors that serve those outlets are still ?energized.? On the other hand, if you meant to say ?carrying current,? instead of the conversational shorthand ?energized,? that would still not clarify the issue. That is because the code does not distinguish between conductors carrying a little current and those carrying their maximum current, when it addresses ampacities.

I think I understand now what Don said about a lack of agreement on the meaning of the phrase.
 
Re: Adjustment Factors for Conductors with Load Diversity

Table 310.15(B)(2)(a) is applicable to three or more current-carrying conductors in a cable or raceway where all of the conductors are carrying current simultaneously. The 50 percent load diversity addressed pertains to Table B 310.11 in annex B for 10 or more current-carrying conductors where only 50 percent of the conductors are carrying current simultaneously. An example might help: Assume 30 conductors with a table ampacity of 20 amps installed in a raceway and assume 1 ohm resistance. The percentage from Table B.310.11 is 60%, therefore, 0.6 x 20A = 12A. Fifty percent diversity means that only 15 are carrying current. I^2R(15)=watts.
12^2 x 1 x 15 = 2160 watts of heat. The calculated value can not be exceeded by any other combination of conductors or conductor ampacities. If 25 of the 30 conductors are carrying current at 50% of their ampacity 50% of 20A = 10A. Therefore, 10^2 x 1 x 25 = 2500W which is higher than the allowable heat of 2160W.
 
Status
Not open for further replies.
Top