diversity factor with receptacle circuits

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steve66

steve66

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Illinois
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Engineer
I am designing a new office space. Receptacle circuits will be fed with an underfloor wireway. Per Article 376 and 310.15(B)(2)(A), I have to derate for over 30 conductors. The fine print note refers to table B.310.11 in Annex B (adjustment factors for load diversity).

Question #1: Can I claim I have a 50% load diversity on these receptacle circuits?

My basis would be that it is a 45000 sq ft office; at 1VA/sq ft, I need 23 branch circuits, and I plan to provide over 100 branch circuits.

I could also use the fact I will have 6 duplexes per branch circuit. And at 180VA per duplex, thats only 720VA per 20A circuit.

Say I can claim 50% diversity and I use table B310.11. I place 42 conductors in a wireway, and use the 60% derating factor. Can I still use #12 wire with 20A breakers?

The 90 deg. ampacity of #12 is 30 Amps. 30 Amps *0.6= 18 Amps. So I can protect the wire with the next size breaker, or a 20A. Yes, or no?

Steve
 
Re: diversity factor with receptacle circuits

Originally posted by steve66: Question #1: Can I claim I have a 50% load diversity on these receptacle circuits?
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Sorry, Steve, but I think not. My reason starts with the fact that the NEC never defines ?a load diversity.? That means we have to go with industry standard definitions. Regrettably, that particular phrase is not a standard industry term. The closest standard term is ?Diversity Factor.? It 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.
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The problem is that this ratio can never be smaller than 1. There is no such thing as a ?50% Diversity Factor.? So I am left with no way to take advantage of Table B310.11.

I think I will post a suggested code change in the new Proposals for 2008 topic area. We need to clear up the meaning of this ?load diversity? term.
 
Re: diversity factor with receptacle circuits

Originally posted by wirenut1980:
Does the permission to upsize OCPD to next standard size (for under 800 Amps) only apply to services?
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No. It applies to branch circuits, feeders and services.
 
Re: diversity factor with receptacle circuits

It applies to branch circuits, feeders and services.
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But it does not apply to branch circuit conductors that serve multiple outlets for cord and plug connected loads.
Don
 
Re: diversity factor with receptacle circuits

Thats correct Don, I should have specified that.

I also should have specified that it does not apply where the code directly prohibits the practice, such as for tap conductors and transformer secondaries, as stated in 240.21.

[ October 27, 2004, 01:23 PM: Message edited by: ryan_618 ]
 
Re: diversity factor with receptacle circuits

Charlie:

I don't agree with your reasoning. I think the term "50% load diversity" has to mean one of following:
1. Half the wires are loaded to their full ampacity, and half of the wires have no current. Or:
2. All the wires are loaded to half of their ampacity.
3. A combination of the above (1/4 of the conductors loaded at 1/2 ampacity, 1/4 at full load, and 1/4 at no load).

Since the NEC is mute on which of the above applies, I believe I should be allowed to use any of the three.

It is kind of a mute point, since Don pointed out that 250.4 prohibits exactly what I wanted to do. I guess I am stuck with 30 conductors per wireway compartment.

STeve

[ October 27, 2004, 03:17 PM: Message edited by: steve66 ]
 
Re: diversity factor with receptacle circuits

But Steve, if they meant any or all of those three concepts, they would have use the term "Demand Factor." That term is defined in the NEC, and your three concepts are in keeping with that definition.
 
Re: diversity factor with receptacle circuits

Charlie,
As I recall the reason that the information in Table B.310.11 is no longer in the code body is because no one could agree on what the term "load diversity" means.
Don
 
Re: diversity factor with receptacle circuits

Originally posted by steve66:
Charlie:

I don't agree with your reasoning. I think the term "50% load diversity" has to mean one of following:
1. Half the wires are loaded to their full ampacity, and half of the wires have no current. Or:
2. All the wires are loaded to half of their ampacity.
3. A combination of the above (1/4 of the conductors loaded at 1/2 ampacity, 1/4 at full load, and 1/4 at no load).

Since the NEC is mute on which of the above applies, I believe I should be allowed to use any of the three.
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the problem with this theory is that each of the conditions you describe results in different amounts of heat being generated, so obviously you can't use them interchangably.

if the number of wires is X and I is max ampacity of the conductor:

case 1: heat = (X/2)(I^2)(R)
case 2: heat = (X)(I/2)^2(R)
case 3: heat = (X/4)(I/2)^2(R) + (X/4)(I^2)(R)

factoring out common terms

case 1: heat = (0.5)XR(I^2)
case 2: heat = (0.25)XR(I^2)
case 3: heat = (.3125)XR(I^2)
 
Re: diversity factor with receptacle circuits

Originally posted by don_resqcapt19: As I recall the reason that the information in Table B.310.11 is no longer in the code body is because no one could agree on what the term "load diversity" means.
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Thanks for that bit of history. But I think the NFPA should take this one step further, to remove the phrase ?load diversity? entirely. I will open a new thread in the 2008 Proposals area to continue that discussion.
 
Re: diversity factor with receptacle circuits

To me, it seems like a minor point that they are using the term diversity instead of demand. If I know all the wires won't be fully loaded, it would be reasonable to allow the derating factors to be adjusted. Maybe something like the "Engineering Supervision" in 310.15(C) should be allowed for derating factors.

Posted by Petersonra:

each of the conditions you describe results in different amounts of heat being generated, so obviously you can't use them interchangably.
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Sure we can use them interchangably. Case 1 is the worst case. So if we use demand factors suitable for Case #1, they should work for the other two cases also.

Steve
 
Re: diversity factor with receptacle circuits

Originally posted by steve66: If I know all the wires won't be fully loaded, it would be reasonable to allow the derating factors to be adjusted.
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I agree.
Sure we can use them interchangeably.
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Here I do not agree. In a Private Message, another Moderator told me a bit of the history of Table B310.11. He said that it used to be within the main body of the code. But it was moved to the Annex (i.e., the unenforceable part of the Code) because the Code Committees could not agree on what was meant by ?load diversity.? If the intended meaning had been the same as that of ?demand factor,? then the Code Committees would have simply changed the one phrase to the other, and left the word ?diversity? out of the book forever.

They did not choose that path. I infer that the intended meaning of the two phrases are not the same, that the two are not interchangeable.
 
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